SN 1054 (The supernova that created the Crab Nebula)

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SN 1054

Giant picture mosaic of the Crab Nebula, remnant of SN 1054, taken by the Hubble Space Telescope in visible light. Credit: NASA/ESA.
Other designations	SN 1054, SN 1054A, CSI+21-05315, PLX 1266, V* CM Tau
Event type	Supernova, star
Spectral class	Type II
Date	4 July 1054
Constellation	Taurus
Right ascension	5h 34.5m
Declination	+22° 01'
Epoch	?
Galactic coordinates	G.184.6–5.8
Distance	6.5 kly (2.0 kpc)
Remnant	Nebula
Host	Milky Way
Progenitor	Unknown
Progenitor type	Unknown
Colour (B-V)	Unknown
Peak apparent magnitude	−6
Preceded by	SN 1006
Followed by	SN 1181

SN 1054 is a supernova that was first observed on 4 July 1054, and remained visible for around two years. The event was recorded in contemporary Chinese astronomy, and references to it are also found in a later (13th-century) Japanese document, and in a document from the Arab world. Furthermore, there are a number of proposed, but doubtful, references from European sources recorded in the 15th century, and perhaps a pictograph associated with the Ancestral Puebloan culture found near the Peñasco Blanco site in New Mexico.

The remnant of SN 1054, which consists of debris ejected during the explosion, is known as the Crab Nebula. It is located in the sky near the star Zeta Tauri (ζ Tauri). The core of the exploding star formed a pulsar, called the Crab Pulsar (or PSR B0531+21). The nebula and the pulsar that it contains are some of the most studied astronomical objects outside the Solar System. It is one of the few Galactic supernovae where the date of the explosion is well known. The two objects are the most luminous in their respective categories. For these reasons, and because of the important role it has repeatedly played in the modern era, SN 1054 is the best known supernova in the history of astronomy.

The Crab Nebula is easily observed by amateur astronomers thanks to its brightness, and was also catalogued early on by professional astronomers, long before its true nature was understood and identified. When the French astronomer Charles Messier watched for the return of Halley's Comet in 1758, he confused the nebula for the comet, as he was unaware of the former's existence. Motivated by this error, he created his catalogue of non-cometary nebulous objects, the Messier Catalogue, to avoid such mistakes in the future. The nebula is catalogued as the first Messier object, or M1.

Identification of the supernova

The Crab Nebula was identified as the supernova remnant of SN 1054 between 1921 and 1942, at first speculatively (1920s), with some plausibility by 1939, and beyond reasonable doubt by Jan Oort in 1942. 

In 1921, Carl Otto Lampland was the first to announce that he had seen changes in the structure of the Crab Nebula. This announcement occurred at a time when the nature of the nebulas in the sky was completely unknown. Their nature, size and distance were subject to debate. Observing changes in such objects allows astronomers to determine whether their spatial extension is "small" or "large", in the sense that notable fluctuations to an object as vast as our Milky Way cannot be seen over a small time period, such as a few years, whereas such substantial changes are possible if the size of the object does not exceed a diameter of a few light-years. Lampland's comments were confirmed some weeks later by John Charles Duncan, an astronomer at the Mount Wilson Observatory. He benefited from photographic material obtained with equipment and emulsions that had not changed since 1909; as a result the comparison with older snapshots was easy and emphasized a general expansion of the cloud. The points were moving away from the centre, and did so faster as they got further from it.

Also in 1921, Knut Lundmark compiled the data for the "guest stars" mentioned in the Chinese chronicles known in the West. He based this on older works, having analysed various sources such as the Wenxian Tongkao, studied for the first time from an astronomical perspective by Jean-Baptiste Biot in the middle of the 19th century. Lundmark gives a list of 60 suspected novae, then the generic term for a stellar explosion, in fact covering what is now understood as two distinct phenomena, novae and supernovae. The nova of 1054, already mentioned by the Biots in 1843, is part of the list. It stipulates the location of this guest star in a note at the bottom of the page as being "close to NGC 1952", one of the names for the Crab Nebula, but it does not seem to create an explicit link between them. 

In 1928, Edwin Hubble was the first to note that the changing aspect of the Crab Nebula, which was growing bigger in size, suggests that it is the remains of a stellar explosion. He realised that the apparent speed of change in its size signifies that the explosion which it comes from occurred only nine centuries ago, which puts the date of the explosion in the period covered by Lundmark's compilation. He also noted that the only possible nova in the region of the Taurus constellation (where the cloud is located) is that of 1054, whose age is estimated to correspond to an explosion dating from the start of the second millennium.

Hubble therefore deduced, correctly, that this cloud was the remains of the explosion which was observed by Chinese astronomers.

Hubble's comment remained relatively unknown as the physical phenomenon of the explosion was not known at the time. Eleven years later, when the fact that supernovae are very bright phenomena was highlighted by Walter Baade and Fritz Zwicky and when their nature was suggested by Zwicky, Nicholas Mayall proposed that the star of 1054 was actually a supernova, based on the speed of expansion of the cloud, measured by spectroscopy, which allows astronomers to determine its physical size and distance, which he estimated at 5000 light-years. This was under the assumption that the velocities of expansion along the line of sight and perpendicularly to it were identical. Based on the reference to the brightness of the star which featured in the first documents discovered in 1934, he deduced that it was a supernova rather than a nova. 

This deduction was subsequently refined, which pushed Mayall and Jan Oort in 1942 to analyse historic accounts relating to the guest star more closely. These new accounts, globally and mutually concordant, confirm the initial conclusions by Mayall and Oort in 1939 and the identification of the guest star of 1054 is established beyond all reasonable doubt. Most other historical supernovas are not confirmed so conclusively: supernovas of the first millennium (SN 185, SN 386 and SN 393) are established on the basis of a single document each, and so they cannot be confirmed; in relation to the supposed historical supernova which followed the one in 1054, SN 1181, there are legitimate doubts concerning the proposed remnant (3C58) and an object of less than 1000 years of age. Other historical supernovae of which there are written accounts which precede the invention of the telescope (SN 1006, SN 1572 and SN 1604) are however established with certitude. Telescope-era supernovae are of course associated with their remnant, when one is observed, with full certitude, but none is known within the Milky Way.

Historical records

The Crab Nebula is a remnant of an exploded star. This is the Crab Nebula in various energy bands, including a hard X-ray image from the HEFT data taken during its 2005 observation run. Each image is 6′ wide.

 

The guest star reported by Chinese astronomers in 1054 is identified as SN 1054. The highlighted passages refer to the supernova.

 

SN 1054 is one of eight supernovae in the Milky Way that can be identified because written testimony describing the explosion has survived. In the nineteenth century, astronomers began to take an interest in the historic records. They compiled and examined the records as part of their research on recent novae, comets, and later, the supernovae. 

The first people to attempt a systematic compilation of records from China were the father and son Biot. In 1843, the sinologist Édouard Biot translated for his father, the astronomer Jean-Baptiste Biot, passages from the astronomical treatise of the 348-volume Chinese encyclopaedia, the Wenxian Tongkao. 

Almost 80 years later, in 1921, Knut Lundmark undertook a similar effort based on a greater number of sources. In 1942, Jan Oort, convinced that the Crab Nebula was the "guest star" of 1054 described by the Chinese, asked sinologist J.J.L. Duyvendak to help him compile new evidence on the observation of the event.

Chinese astronomy

Simulated image of supernova SN 1054 at the position of modern Crab Nebula, as presumably would have been observed from capital of Song Dynasty at Kaifeng, China during the morning of July 4th, 1054.

 

Stars that appeared temporarily in the sky were generically called "guest stars" (kè xīng 客星) by Chinese astronomers. The guest star of 1054 occurred during the reign of the Emperor Renzong of the Song dynasty (960–1279). The relevant year is recorded in Chinese documents as "the first year of the Zhihe era". Zhihe was an era name used during the reign of Emperor Renzong, and corresponds to the years 1054–1056 C.E., so the first year of the Zhihe era corresponds to the year 1054 C.E. 

Some of the Chinese accounts are well preserved and detailed. The oldest and most detailed accounts are from Song Huiyao and Song Shi, historiographical works of which the extant text was redacted perhaps within a few decades of the event. There are also some later records, redacted in the 13th century, which are not necessarily independent of the older ones. Three accounts are apparently related because they describe the angular distance from the guest star to Zeta Tauri as "perhaps several inches away", but they are in apparent disagreement about the date of appearance of the star. The older two mention the day jichou 己丑, but the third, the Xu Zizhi Tongjian Changbian, the day yichou 乙丑. These terms refer to the Chinese sexagenary cycle, corresponding to numbers 26 and 2 of the cycle, which corresponds, in the context where they are cited, respectively, to 4 July and 10 June. As the redaction of the third source is of considerably later date (1280) and the two characters are similar, this is easily explained as a transcription error, the historical date being jichou 己丑, 4 July. 

The description of the guest star's location as "to the south-east of Tianguan, perhaps several inches away" has perplexed modern astronomers, because the Crab Nebula is not situated in the south-east, but to the north-west of Zeta Tauri. 

The duration of visibility is explicitly mentioned in chapter 12 of Song Shi, and slightly less accurately, in the Song Huiyao. The last sighting was on 6 April 1056, after a total period of visibility of 642 days. This duration is supported by the Song Shi. According to the Song Huiyao the visibility of the guest star was for only 23 days, but this is after mentioning visibility during daylight. This period of 23 days applies in all likelihood solely to visibility during the day.

Sources

The Song Huiyao (literally "Collected important documents of the Song dynasty") covers the period 960–1220. Huiyao is a traditional form of history books in China which aimed mainly to preserve primary sources, and as such are important sources supplementing the official Twenty-Four Histories. The Song dynasty had a specific government department dedicated to compiling the Huiyao, and some 2,200 volumes were published in ten batches during the Song dynasty. However, most of these documents were lost by the time of the Qing Dynasty except for the synopsis and a relatively small portion preserved as part of the imperial Yongle Encyclopedia. In 1809, the portion preserved in the Yongle Encyclopedia was extracted and re-published as the Song Huiyao Jigao (the "draft extract of the Song Huiyao"). Subsequent scholars have worked on the project further and the current edition dates from 1936. 

This document recounts the observation of the guest star, focusing on the astrological aspect but also giving important information on the visibility of the star, by day and by night.

Zhihe era, first year, seventh lunar month, 22nd day. [...] Yang Weide declared: "I humbly observe that a guest star has appeared; above the star there is a feeble yellow glimmer. If one examines the divination regarding the Emperor, the interpretation [of the presence of this guest star] is the following: The fact that the star has not overrun Bi and that its brightness must represent a person of great value. I demand that the Office of Historiography is informed of this." All officials congratulated the Emperor, who ordered his congratulations be [back] forwarded to the Office of Historiography. First year of the era of Jiayou, third lunar month, the director of the Astronomical Office said "The guest star has disappeared, which means the departure of the host [that it represents]." Previously, during the first year of the Zhihe era, during the fifth lunar month, it had appeared at dawn, in the direction of the east, under the watch of Tiānguān (天關, Zeta Tauri). It had been seen in daylight, like Venus. It had rays stemming in all directions, and its colour was reddish white. Altogether visible for 23 days.

The Song Shi is the official annals of the Song dynasty. Chapter 12 mentions the guest star, not its appearance but rather the moment of its disappearance. The corresponding entry dated 6 April 1056 indicates:

Jiayou era, first year, third lunar month, xinwei day, the director of the Office of Astronomy reported during the fifth lunar month of the first year of the Zhihe era, a guest star had appeared at dawn, in the direction of the east, under the watch of Tianguan. Now it has disappeared.

In chapter 56 ("Astronomical treaty") of the same document, the guest star is again mentioned in a chapter dedicated to this type of phenomenon, this time focusing on its appearance,

Zhihe era of the reign, first year, fifth lunar month, jichou day. A guest star has appeared to the south-east of Tianguan, perhaps several inches away. After a year or more, it gradually disappeared.

The Xu Zizhi Tongjian Changbian ("Long compilation of the continuation of the Zizhi Tongjian"), a book covering the period of 960–1126 and written 40 years or so later by Li Tao (1114–1183), contains the oldest Chinese testimonies relating to the observation of the star. It was rediscovered in 1970 by the specialist in Chinese civilisations Ho Peng Yoke and collaborators. It is relatively imprecise in the case of the explosion of SN 1054. A loose translation of what was stated:

First year of the Zhihe era, fifth lunar month, ji-chou day. A guest star has appeared to the south-east of Tianguan, perhaps several inches away [of this star]. (The star disappeared in the third lunar month of the first year of the Jiayou era.)

There is an account of the star from the Liao Dynasty, which ruled in the area around northeast China from 907–1125. The book in question, the Qidan Guo Zhi, was compiled by Ye Longli in 1247. It includes various astronomical notes, some of which are clearly copied from the Song Shi. This entry referring to the star of 1054 seems unique:

Chongxi era of the reign of [King Xingzong], twenty-third year eighth lunar moon, the ruler of the realm is dead. It happened before a solar eclipse at noon, and a guest star appeared. The highest office at the Office of History, Liu Yishou had said "These are omens of the death of the King." This prediction has been realised.

The account of Qidan Guo Zhi alluded to the notable astronomical events that preceded the death of King Xingzong. Various historical documents allow us to establish the date of death of the Emperor Xingzong as 28 August 1055, during the eighth lunar month of the twenty-fourth (and not twenty-third) year of his reign. The dates of the two astronomical events mentioned (the eclipse and the appearance of the guest star) are not specified, but were probably before the obituary (2 or 3 years at most). Two solar eclipses were visible shortly before that date in the Khitan kingdom, on 13 November 1053 and 10 May 1054. Of these, only one occurred around noon, that of 13 November; it seems likely that this is what the document mentions. As for the guest star, only a rough estimate of location is given, corresponding to the moon mansion Mao. This mansion is situated just east of where the star appeared, as mentioned in the other testimonies. Since no other known significant astronomical event occurred in this region of the sky during the two years that preceded the death of Xingzong, it seems likely that the text is actually referring to the star of 1054. 

The Wenxian Tongkao is the first East Asian source that came to the attention of Western astronomers; it was translated by Édouard Biot in 1843. This source, compiled by Ma Duanlin in 1280, is relatively brief. The text is very close to that of the Song Shi:

Zhihe era of the reign, first year, fifth lunar month, ji-chou day. A guest star has appeared to the south-east of Tiānguān, perhaps several inches away. After a year or more, it gradually disappeared.

Identity of Tianguan

The asterisms (or "constellations") of Chinese astronomy were catalogued around the 2nd century BC. The asterisms with the brightest stars in the sky were compiled in a work called Shi Shi, which also includes Tianguan. Identification of Tianguan is comparatively easy, as it is indicated that it is located at the foot of the Five Chariots asterism, the nature of which is left in hardly any doubt by representation on maps of the Chinese sky: it consists of a large pentagon containing the bright stars of the Auriga. As Tianguan is also represented to the north of the Three Stars asterism, the composition of which is well known, corresponding to the bright stars of Orion, its possible localisation is strongly restricted to the immediate proximity of the star ζ Tauri, located between "Five Chariots" and "Three Stars". This star, of medium brightness (apparent magnitude of 3.3), is the only star of its level of brightness in this area of the sky (there is no other star that is brighter than an apparent magnitude of 4.5 within 7 degrees of ζ Tauri), and therefore the only one likely to figure among the asterisms of "Shi Shi". All of these elements, along with some others, allow "Tianguan" to be confirmed beyond reasonable doubt as corresponding to the star ζ Tauri. 

Northeast region of the Taurus constellation, with ζ and β Tauri stars and the location of the supernova of 1054 between them (M1).

Position relative to Tianguan

Three Chinese documents indicate that the guest star was located "perhaps a few inches" South-East of Tianguan. Song Shi and Song Huiyao stipulate that it "was standing guard" for the asterism, corresponding to the star ζ Tauri. The "South-East" orientation has a simple astronomical meaning, the celestial sphere having, like the Earth's globe, both north and south celestial poles, the "South-East" direction thus corresponding to a "bottom-left" location in relation to the reference object (in this case, the star ζ Tauri) when it appears at the South. However, this "South-East" direction has long left modern astronomers perplexed in the context of this event: the logical remnant of the supernova corresponding to the guest star is the Crab Nebula, but it is not situated to the southeast of ζ Tauri, rather in the opposite direction, to the northwest. 

The term "perhaps a few inches" (ke shu cun in the Latin transliteration) is relatively uncommon in Chinese astronomical documents. The first term, ke, is translated as "approximately" or "perhaps", the latter being currently preferred. The second term, shu, means "several", and more specifically any number between 3 and 9 (limits included). Finally, cun resembles a unit of measurement for angles translated by the term "inch". It is part of a group of three angular units, zhang (also written chang), chi ("foot") and cun ("inch"). Different astronomical documents indicate without much possible discussion that a zhang corresponds to ten chi, and that one chi corresponds to ten cun. The angular units are not the ones used to determine stars' coordinates, which are given in terms of du, an angular unit corresponding to the average angular distance travelled by the sun per day, which corresponds to around 360/365.25 degrees, in other words almost one degree. The use of different angular units can be surprising, but it is similar to the current situation in modern astronomy, where the angular unit used to measure angular distances between two points is certainly the same as for declination (the degree), but is different for right ascension (which is expressed in angular hours; an angular hour corresponds to exactly 15 degrees. In Chinese astronomy, right ascension and declination have the same unit, which is not the one used for other angular distances. The reason for this choice to use different units in the Chinese world is not well known.

Meaning of units

However, the exact value of these new units (zhang, chi and cun) was never stipulated, but can be deduced by the context in which they are used. For example, the spectacular passing of Halley's comet in 837 indicates that the tail of the comet measured 8 zhang. Even if it is not possible to know the angular size of the comet at the time it passed, it is certain that 8 zhang correspond to 180 degrees at the most (maximum visible angle on the celestial sphere), which means that one zhang can hardly exceed 20 degrees, and therefore one cun cannot exceed 0.2 degrees. A more rigorous estimation was made from 1972 on the basis of references of minimal separations expressed in chi or cun between two stars in the case of various conjunctions. The results suggest that one cun is between 0.1 and 0.2 degrees and that one chi is between 0.44 and 2.8 degrees, a range which is compatible with the estimations for one cun. A more solid estimation error is that it is generally accepted that one chi is in the order of one degree (or one du), and that one cun is in the order of one tenth of a degree. The expression “perhaps a few inches” therefore suggests an angular distance in the order of one degree or less.

Problems with description

If all the available elements strongly suggest that the star of 1054 was a supernova, and that in the area next to where the star was seen, there is a remnant of a supernova which has all of the characteristics expected of an object that is around 1000 years old, a major problem arises: the new star is described as being to the South-East of Tianguan, while the Crab Nebula is to the North-East. This problem has been known since the 1940s and has long been unsolved. In 1972 for example, Ho Peng Yoke and his colleagues suggested that the Crab Nebula was not the product of the explosion of 1054, but that the true remnant was to the South-East, as indicated in several Chinese sources. For this, they envisaged that the angular unit cun corresponds to a considerable angle of 1 or 2 degrees, meaning that the distance from the remnant to ζ Tauri was therefore considerable. Aside from the fact that this theory does not account for the large angular sizes of certain comets, expressed in zhang, it comes up against the fact that there it does not make sense to measure the gap between a guest star and a star located so far away from it, when there are closer asterisms that could be used.

In their controversial article (see above) Collins and his colleagues make another suggestion: on the morning of 4 July, the star ζ Tauri was not bright enough and too low on the horizon to be visible. If the guest star, which was located close to it, was visible, it is only because its brightness was comparable to Venus. However, there was another star, brighter and higher on the horizon, which was possibly visible, for reference: Beta Tauri (β Tauri). This star is located at around 8 degrees north-north-west of ζ Tauri. The Crab Nebula is south-south-east of β Tauri. Collins et al. suggest therefore that at the time of its discovery, the star was seen to the south-east of β Tauri, and that as the days passed and visibility improved, astronomers were able to see that it was in fact a lot closer to ζ Tauri, but that the direction "south-east" used for the first star was kept in error.

The solution to this problem was suggested (without proof) by A. Breen and D. McCarthy in 1995. and proved very convincingly by D. A. Green et F. R. Stephenson (2003) The term "stand on guard" obviously signifies a proximity between the two stars, but also means a general orientation: a guest star "standing on guard" for a fixed star is systematically located below it. In order to support this theory, Green and Stephenson investigated other entries in Song Shi, which also includes reference to "standing on guard". They selected entries relating to conjunctions betweens the stars identified and planets, of which the trajectory can be calculated without difficulty and with great precision on the indicated dates. Of the 18 conjunctions analysed, spreading from 1172 (the Jupiter–Regulus conjunction on 5 December) to 1245 (the Saturn–Gamma Virginis conjunction on 17 May), the planet was more to the north (in the sense of a lower declination) in 15 cases, and in the three remaining cases, it was never in the south quadrant of the star.

In addition, Stephenson and Clark (1977) had already highlighted such an inversion of direction in a planetary conjunction: on 13 September 1253, an entry in the astronomical report Koryo-sa indicated that Mars had hidden the star to the south-east of the twenty-eight mansions sign Ghost (Chinese constellation) (Delta Cancri), while in reality, it approached the star north-west of the asterism (Eta Cancri).

Meigetsuki (Japan)

The oldest and most detailed record from Japan is in the Meigetsuki, the diary of Fujiwara no Teika (1162–1241), a poet and courtier. There are two other Japanese documents, presumably dependent on the Meigetsuki:

• The 14th century Ichidai Yoki: The description is very similar to the Meigetsuki, omitting several details (hour of apparition, and possibly erroneous parts of the lunar month). The short text also contains many typographical errors.
• The 17th-century Dainihonshi, containing very little information. The brevity contrasts with the more detailed descriptions of "guest stars" (supernovas) of 1006 and 1181.

The Meigetsuki places the event in the fourth lunar moon, one month earlier than the Chinese texts. Whatever the exact date during this month, there seems to be a contradiction between this period and the observation of the guest star: the star was close to the sun, making daytime and nighttime observation impossible. The visibility in daylight as described by the Chinese texts is thus validated by the Japanese documents, and is consistent with a period of moderate visibility, which implies that the star's period of diurnal visibility was very short. In contrast, the day of the cycle given in the Chinese documents is compatible with the months that they state, reinforcing the idea that the month on the Japanese document is incorrect. The study of other medieval supernovas (SN 1006 and SN 1181) reveals a proximity in the dates of discovery of a guest star in China and Japan, although clearly based on different sources.

Fujiwara no Teika's interest in the guest star seems to have come accidentally whilst observing a comet in December 1230, which prompted him to search for evidence of past guest stars, among those SN 1054 (as well as SN 1006 and SN 1181, the two other historic supernovas from the early second millennium). The entry relating to SN 1054 can be translated as:

Tengi era of the emperor Go-Reizei, second year, fourth lunar month, after the middle period of ten days. At chou [a Chinese term for 1–3am], a guest star appeared in the degrees of the moon mansions of Zuixi and Shen. It has been viewed in the direction of the East and has emerged from the Tianguan star. It was as big as Jupiter.

The source used by Fujiwara no Teika is the records of Yasutoshi Abe (Onmyōdō doctor), but it seems to have been based, for all the astronomical events he has recorded, on documents of Japanese origin. The date he gives is prior to the third part of ten days of the lunar month mentioned, which corresponds to the period of between 30 May and 8 June 1054 of the Julian calendar, which is around one month earlier than Chinese documentation. This difference is usually attributed to an error in the lunar months (fourth place and fifth place). The location of the guest star, clearly straddling the moon mansions Shen and Zuixi, corresponds to what would be expected of a star appearing in the immediate vicinity of Tianguan.

Ibn Butlan (Iraq)

While SN 1006, which was significantly brighter than SN 1054, was mentioned by several Arab chroniclers, there exist no Arabic reports relating to the rather faint SN 1181. Only one Arabic account has been found concerning SN 1054, whose brightness is between those of the last two stars mentioned. This account, discovered in 1978, is that of a Nestorian Christian doctor, Ibn Butlan, transcribed in the Uyun al-Anba, a book on detailed biographies of physicians in the Islamic World compiled by Ibn Abi Usaybi'a (1194–1270) in the mid-thirteenth century. This is a translation of the passage in question:

I copied the following hand written testimony [that of Ibn Butlan]. He stated: "One of the famous epidemics of our time has occurred when a spectacular star appeared in [the zodiac star] Gemini, of the year 446 [of the Muslim calendar]. In the autumn of that year, fourteen thousand people were buried in Constantinople. Thereafter, in the middle of the summer of 447, the majority of the Fostat people [Le Caire] and all foreigners died". He [Ibn Butlan] continues: "While this spectacular star appeared in the sign of Gemini [...] it caused the epidemic of the Fostat by the Nile being low when it appeared in 445 [sic]."

The three years cited (AH 445, 446, 447) refer, respectively, to: 23 April 1053 – 11 April 1054, 12 April 1054 – 1 April 1055, and 2 April 1055 – 20 March 1056. There is an apparent inconsistency in the year of occurrence of the star, first announced as 446, then 445. This problem is solved by reading other entries in the book, which quite explicitly specify that the Nile was low at 446. This year of the Muslim calendar ran from 12 April 1054 to 1 April 1055, which is compatible with the appearance of the star in July 1054, as its location (admittedly rather vague), is in the astrological sign of Gemini (which, due to axial precession, covers the eastern part of the Constellation Taurus). The date of the event in 446 is harder to determine, but the reference to the level of the Nile refers to the period preceding its annual flood, which happens during the summer.

Suggested European sightings

Henry before Tivoli pointing up at a new star.

 

Since 1980, several European documents have been identified as possible observations of the supernova.

The first such suggestion was made in 1980 by Umberto Dall'Olmo (1925–1980). The following passage which reports an astronomical sighting is taken from an account compiled by Jacobus Malvecius in the 15th century:

And in those days a star of immense brightness appeared within the circle of the Moon a few days after its separation from the Sun.

The date this passage refers to is not explicit, however, and by means of a reference to an earthquake in Brescia 11 April 1064, it would seem ten years too late, attributed by Dall'Olmo to a transcription error. Another candidate is the Cronaca Rampona, proposed in 1981, which however also indicates a date several years after the event, in 1058 instead of 1054. 

The European candidate documents are all very imprecise, and remain unconvincing from an astronomical perspective even when collated; they would be impossible to interpret in the sense of an observation of a supernova if no information had been preserved from the Chinese accounts.

Conversely, the lack of accounts from European chroniclers has long raised questions. In fact, it is known that the supernova of 1006 was recorded in a large number of European documents, albeit not in astronomical terms. Among the proposed explanations for the lack of European accounts of SN 1054, its concurrence with the East-West Schism is prominent. In fact, the date of the excommunication of the Patriarch of Constantinople Michael I Cerularius (16 July) corresponds to the star reaching its maximum brightness and being visible in the daytime. Among the six proposed European documents, one does not seem to correspond to the year of the supernova (the chronicle of Jacobus Malvecius). Another (the Cronaca Rampona) has large dating and internal coherence problems. The four others are relatively precisely dated, but contradict the Chinese documents: they date from Spring and not Summer 1054, that is to say before the conjunction between the supernova and the sun. Three of the documents (the chronicle of Jacobus Malvecius, the Cronaca Rampona and the Armenian chronicle) make reference relatively explicitly to conjunctions between the moon and stars, of which one is identified (Jupiter, in the Armenian chronicle). The three other documents are very unclear and have almost unusable astronomical content. 

In 1999, George W. Collins and his colleagues defended the plausibility of European sighting of SN 1054. They argue that the records suggest that European sightings even predate Chinese and Japanese reports by more than two months (April 1054). These authors emphasize the problems associated with the Chinese reports, especially the position of the supernova relative to Zeta Tauri. They also adduce a Khitan document which they suggest might establish observation of the supernova at the time of the solar eclipse of 10 May 1054 (which would corrobate the "late" date of Chinese observation of the event). Conversely, they interpret the European documents, taken in conjunction, as plausibly establishing that an unusual astronomical phenomenon was visible in Europe in the spring of 1054, i.e. even before the Sun's conjunction with Zeta Tauri. They also surmise that the correct year in the report by Ibn Butlan is AH 445 (23 April 1053 – 11 April 1054) rather than AH 446 (12 April 1054 – 1 April 1055). 

The publication by Collins et al. was criticised by Stephenson and Green (2003). These authors insist that the problems with the Chinese and Japanese documents can easily be resolved philologically (as common copyists' mistakes) and need not indicate unreliability of the Chinese observations. Stephenson and Green condemn attempts at uncovering European sightings of the supernova as it were at any cost as suffering from confirmation bias, "anxious to ensure that this event was recorded by Europeans". They also reject the idea of the Khitan document referring to the supernova as a mistake based in a translation of the document, and as inconsistent with astronomical reality. Green and Stephenson (2003) thus argue for the standing majority consensus established by 1995, to the effect that the European documents do not offer themselves to an interpretation as sightings of SN 1054. The thesis of Collins et al. upon publication was reviewed in the magazine Ciel & Espace with some enthusiasm but it has not received much attention since its rejection by Stephenson and Green (2003).

The Cronaca Rampona

The account of a supernova sighting which is considered the most feasible comes from a medieval chronicle from the region of Bologna, the Cronaca Rampona. This text, a subject of astronomers' attention since 1972, was interpreted as a possible sighting of the supernova in 1981, and again in 1999. The part of the chronicle that was highlighted translates to:

In AD 1058, Pope Stephen IX has come to the throne [...] Also in this year of Christ 1058, Henry III reigned [or "lived"] for 49 years. He went to Rome for the first time in the month of May. At this time, famine and death was upon the whole world. He stayed in the province of Tibur for three days in the month of June [...] At that time, a very brightly-shining star (stella clarissima) entered into the circle [or the circuit] of the new moon, in the thirteenth calends at the beginning of the night.

Without even discussing the last, astronomical part of the passage, skeptics point out at least two discrepancies in the following: Pope Stephen IX became Pope in 1057, not 1058, and Emperor Henry III who is mentioned, actually Henry III, Holy Roman Emperor, was born in 1017, 39 and not 49 years before 1058, his reign having started in 1039 (King of the Romans, then as emperor of the Romans from 1046 after being consecrated by Pope Clement II during the course of his brief pontificate). Henry III, therefore, was dead in 1056, and his reign could not have coincided with that of Stephen IX. It seems more likely that the text was the subject of various alterations, as the date format (for example, the number 1058 is written as Ml8, with a mix of Roman and Arab characters, common in the period when the Cronaca Rampona was written (15th century) but not in the 11th century. Furthermore, associating the event described with the sighting of a supernova in 1054 would require the supposition that the Cronaca Rampona entry was in the wrong place in relation to the rest of the document, as the different entries are in chronological order and several previous entries are later than 1054 (in order, the previous entries refer to 1046, 1049, 1051, 1055, 1056, written in a mix of Arab and Roman characters, namely Mxl6, Mxl9, Mli, Mlv and Ml6). Additionally, there is a discrepancy with the date of the new moon. The term calends, which refers to the Roman calendar, can be written in the ordinary form of the Gregorian calendar, and the phase of the moon can be calculated from it. It is clear that the new moon did not occur on the thirteenth day of the Calends in any month in 1054. All of this is in strong opposition to the precision of the dates of references to eclipses in medieval European chronicles: a study of 48 partial or total solar eclipses from 733 to 1544, reveals that 42 dates out of 48 are correct, and of the six remaining, three are incorrect by one of two days and the three others give the correct day and month, but not the year.

Finally, even considering that the stated event corresponds to May or June 1054 nevertheless, and describes a conjunction between the already visible supernova and the moon, another problem arises: during those months, the moon did not pass very close to the location of the supernova. Therefore, it is possible that the account describes an approach or a concealment of a planet by the moon, contemporary to the suggested date (1058). This scenario is corroborated by two contemporary documents which are perfectly dated and describe a conjunction and a planetary concealment by the moon in relatively similar terms. These two documents, unearthed by Robert Russell Newton, are taken from the Annales Cavenses, Latin chronicles from la Trinità della Cava (Province of Salerno). They mention "a bright star that entered into the circle of the (new) moon" for both 17 February 1086 ([Martii incipiente nocte] stella clarissima in circulum lunae primae ingressa est) and for 6 August 1096 (stella clarissima venit in circulum lunae). The first event can be verified as Venus being eclipsed by the moon, the second as the Moon passing Jupiter at a distance of less than one degree after a lunar eclipse which was also mentioned in the chronicle.

Hayton of Corycus

The Cronaca Rampona account is apparently also reflected in the Armenian chronicle of Hayton of Corycus (written before 1307). The relevant passage translated from the Armenian manuscript reads:

AD 1048. There was the 5th year, 2nd month, 6th day of Pope Leo in Rome. Robert Kijart arrived in Rome and sieged the Tiburtina town. There was starvation over the whole world. That year a bright star appeared within the circle of the Moon, the Moon was new, on May 14th, in the first part of the night.

Vahe Gurzadyan's proposal connecting the Hayton of Corycus's chronicle with Cronaca Rampona and SN 1054 dates to 2012.

Other

View of the sky at dusk on the day of the death of Pope Leo IX. The three planets Mercury, Mars, and Venus are seen together on the West-South-West horizon (at the bottom-right of the image), with Jupiter the furthest away (top right), all next to the constellation of Orion (centre-bottom) and its bright peripheral stars (notably Sirius, bottom-left, and Capella, top right). They could be "countless lamps" caused by Albertus cited in De Obitus Leonis, that the supernova had been present or not.

 

In a work entitled De Obitus Leonis ("On the Death of [Pope] Leo") by one subdeacon Libuinus, there is a report of an unusual celestial phenomenon. A certain Albertus, leading a group of pilgrims in the region of Todi, Umbria, reportedly confirmed having seen, on the day that Pope Leo IX died, a phenomenon described as

quasi stratam palliis fulgentibus adornatam at innumeris coruscantem lampadibus.
[Translation:] like a road decorated with wonderful adornments and shining with innumerable lamps

Guidoboni et al. (1994), proposed that this may relate to SN 1054, and was endorsed by Collins et al. (1999).

Guidoboni et al. (1994) also proposed a Flemish text as an account of a sighting of the supernova. The text, from Saint Paul's church—no longer extant—in the Flemish town of Oudenburg, describes the death of Pope Leo IX in Spring 1054 (the date described corresponds to 14 April 1054).

On the eighteenth calends of May, on the second day of the week at around midday, the soul [of Pope Leo IX] departed. At the moment it left his body, in Rome, where he rests, but "also everywhere on earth, a circle of extraordinary brightness appeared in the sky for half an hour."

McCarthy and Breen (1997) proposed an extract from an Irish chronicle as a possible European sighting of the supernova. This chronicle indicates the following for 1054:

A round circle of fire was seen at Ros Ela on the Sunday of the feast day of Saint George over five hours during the day, and countless black birds passed before it, in the centre of which there was a larger bird [...]

The date of the event corresponds to 24 April: (Saint George's Day is 23 April and fell on a Saturday in 1054. Thus the mention of the "Sunday of Saint George's Day" corresponds to the next day, 24 April) long before the sighting noted by the Chinese. The astronomical nature of the account remains very uncertain, and interpretation as a solar halo or aurora seems at least as probable.

Suggested records in North American petroglyphs

The sky on the morning of 5 July, showing the conjunction between the supernova (blue square) and the moon. If the orientation of the phenomenon does not correspond to the petroglyph, the relative orientation of the crescent moon in relation to the star corresponds, along with the order of size of the angular distance between the two stars.

 

Two Native American paintings in Arizona show a crescent moon located next to a circle that could represent a star. It has been proposed that this represents a conjunction between the moon and the supernova, made possible by the fact that, seen from the Earth, the supernova occurred in the path of the Ecliptic. This theory is compatible with the datings in these paintings. In fact, on the morning of 5 July, the moon was located in the immediate proximity of the supernova, which could reinforce the idea that it was this proximity that had been represented in these paintings. This interpretation cannot be confirmed. The dating of the paintings is extremely imprecise (between the 10th and 12th century), and only one of them shows the crescent moon with the correct orientation in relation to the supernova. Moreover, this type of drawing could well represent a proximity of the moon with Venus or Jupiter.

Another, better known document was updated during the 1970s at the Chaco Canyon site (New Mexico), occupied around 1000 AD by the Ancestral Pueblo Peoples. On the flat underside of an overhang, it represents a hand, below which there is a crescent moon facing a star at the bottom-left. On the wall underneath the petroglyph there is a drawing which could be the core and tail of a comet. Apart from the petroglyph, which could represent the configuration of the moon and supernova on the morning of 5 July 1054, this period corresponds to the apogee of the Anasazi civilisation. It seems possible to propose an interpretation of the other petroglyph, which, if it is more recent than the other one, could possibly correspond to the passing of Halley's Comet in 1066. Although plausible, this interpretation is impossible to confirm and does not explain why it was the supernova of 1054 that was represented, rather than the supernova of 1006, which was brighter and also visible to this civilisation.

Suggested records in Aboriginal oral tradition

The Aboriginal people of the region around Ooldea have passed in oral tradition a detailed account of their mythology of the constellation Orion and the Pleiades. The anthropologist Daisy Bates was the first to attempt to compile records of this story. Work done by her and others has shown that all of the protagonists of the story of Nyeeruna and the Yugarilya correspond to individual stars covering the region around Orion and the Pleiades, with the exception of Baba, the father dingo, which is a major protagonist of the story and of the yearly re-enactments of the myth by the local people:

Again Nyeeruna's magic comes back in great force and brightness, and when Kambugudha sees the strong magic in arm and body, she calls to a father dingo (horn of the Bull) to come and humiliate Nyeeruna and Babba the Dingo rushes over to Nyeeruna and shakes and swings him east and west by his middle and Kambugudha points at him and laughs but her frightened little sisters hide their heads under their little mountain devil neck humps until Babba loosens his hold and returns to his place again.

It has been suggested by Leaman and Hamacher that the location usually assigned to Baba by the locals (recorded by Bates as being at the "horn of the bull") is more likely to correspond to SN 1054 than to a faint star of that region such as β or ζ Tauri. This is motivated by the reference to Babba "returning to his place again" after attacking Nyeeruna which could refer to a transient star, as well as the fact that important characters of the myth are associated with bright stars. However, Leaman and Hamacher clarify there is no solid evidence to support this interpretation, which remains speculative. Hamacher demonstrates the extreme difficulty in identifying supernovae in indigenous oral traditions. 

Other elements of the story which have been found to correspond to astronomical elements by these authors include: awareness by the Aboriginal people of the different colors of the stars, possible awareness of the variability of Betelgeuse, observations of meteors in the Orionid meteor shower and the possibility that the rite associated with the myth is held at a time of astronomical significance, corresponding to the few days in the year when due to the Sun’s proximity to Orion, it is unseen throughout the night, but is always in the sky during the daytime.

Media references

The supernova is mentioned in Ayreon's song To the Quasar, from the album Universal Migrator Part 2: Flight of the Migrator, and is also the subject of Elen Cora's song Astronomers in China.

X-ray astronomy

From Wikipedia, the free encyclopedia

 

X-rays start at ~0.008 nm and extend across the electromagnetic spectrum to ~8 nm, over which the Earth's atmosphere is opaque.

 

X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and satellites. X-ray astronomy is the space science related to a type of space telescope that can see farther than standard light-absorption telescopes, such as the Mauna Kea Observatories, via x-ray radiation. 

X-ray emission is expected from astronomical objects that contain extremely hot gases at temperatures from about a million kelvin (K) to hundreds of millions of kelvin (MK). Moreover, the maintenance of the E-layer of ionized gas high in the Earth's Thermosphere also suggested a strong extraterrestrial source of X-rays. Although theory predicted that the Sun and the stars would be prominent X-ray sources, there was no way to verify this because Earth's atmosphere blocks most extraterrestrial X-rays. It was not until ways of sending instrument packages to high altitude were developed that these X-ray sources could be studied. 

The existence of solar X-rays was confirmed early in the rocket age by V-2s converted to sounding rocket purpose, and the detection of extraterrestrial X-rays has been the primary or secondary mission of multiple satellites since 1958. The first cosmic (beyond the solar system) X-ray source was discovered by a sounding rocket in 1962. Called Scorpius X-1 (Sco X-1) (the first X-ray source found in the constellation Scorpius), the X-ray emission of Scorpius X-1 is 10,000 times greater than its visual emission, whereas that of the Sun is about a million times less. In addition, the energy output in X-rays is 100,000 times greater than the total emission of the Sun in all wavelengths. 

Many thousands of X-ray sources have since been discovered. In addition, the space between galaxies in galaxy clusters is filled with a very hot, but very dilute gas at a temperature between 10 and 100 megakelvins (MK). The total amount of hot gas is five to ten times the total mass in the visible galaxies.

Sounding rocket flights

The first sounding rocket flights for X-ray research were accomplished at the White Sands Missile Range in New Mexico with a V-2 rocket on January 28, 1949. A detector was placed in the nose cone section and the rocket was launched in a suborbital flight to an altitude just above the atmosphere. 

X-rays from the Sun were detected by the U.S. Naval Research Laboratory Blossom experiment on board. An Aerobee 150 rocket was launched on June 12, 1962 and it detected the first X-rays from other celestial sources (Scorpius X-1). It is now known that such X-ray sources as Sco X-1 are compact stars, such as neutron stars or black holes. Material falling into a black hole may emit X-rays, but the black hole itself does not. The energy source for the X-ray emission is gravity. Infalling gas and dust is heated by the strong gravitational fields of these and other celestial objects. Based on discoveries in this new field of X-ray astronomy, starting with Scorpius X-1, Riccardo Giacconi received the Nobel Prize in Physics in 2002.

The largest drawback to rocket flights is their very short duration (just a few minutes above the atmosphere before the rocket falls back to Earth) and their limited field of view. A rocket launched from the United States will not be able to see sources in the southern sky; a rocket launched from Australia will not be able to see sources in the northern sky.

X-ray Quantum Calorimeter (XQC) project

A launch of the Black Brant 8 Microcalorimeter (XQC-2) at the turn of the century is a part of the joint undertaking by the University of Wisconsin–Madison and NASA's Goddard Space Flight Center known as the X-ray Quantum Calorimeter (XQC) project.

 

In astronomy, the interstellar medium (or ISM) is the gas and cosmic dust that pervade interstellar space: the matter that exists between the star systems within a galaxy. It fills interstellar space and blends smoothly into the surrounding intergalactic medium. The interstellar medium consists of an extremely dilute (by terrestrial standards) mixture of ions, atoms, molecules, larger dust grains, cosmic rays, and (galactic) magnetic fields. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field. 

Of interest is the hot ionized medium (HIM) consisting of a coronal cloud ejection from star surfaces at 10⁶-10⁷ K which emits X-rays. The ISM is turbulent and full of structure on all spatial scales. Stars are born deep inside large complexes of molecular clouds, typically a few parsecs in size. During their lives and deaths, stars interact physically with the ISM. Stellar winds from young clusters of stars (often with giant or supergiant HII regions surrounding them) and shock waves created by supernovae inject enormous amounts of energy into their surroundings, which leads to hypersonic turbulence. The resultant structures are stellar wind bubbles and superbubbles of hot gas. The Sun is currently traveling through the Local Interstellar Cloud, a denser region in the low-density Local Bubble. 

To measure the spectrum of the diffuse X-ray emission from the interstellar medium over the energy range 0.07 to 1 keV, NASA launched a Black Brant 9 from White Sands Missile Range, New Mexico on May 1, 2008. The Principal Investigator for the mission is Dr. Dan McCammon of the University of Wisconsin–Madison.

Balloons

Balloon flights can carry instruments to altitudes of up to 40 km above sea level, where they are above as much as 99.997% of the Earth's atmosphere. Unlike a rocket where data are collected during a brief few minutes, balloons are able to stay aloft for much longer. However, even at such altitudes, much of the X-ray spectrum is still absorbed. X-rays with energies less than 35 keV (5,600 aJ) cannot reach balloons. On July 21, 1964, the Crab Nebula supernova remnant was discovered to be a hard X-ray (15–60 keV) source by a scintillation counter flown on a balloon launched from Palestine, Texas, United States. This was likely the first balloon-based detection of X-rays from a discrete cosmic X-ray source.

High-energy focusing telescope

The Crab Nebula is a remnant of an exploded star. This image shows the Crab Nebula in various energy bands, including a hard X-ray image from the HEFT data taken during its 2005 observation run. Each image is 6′ wide.

 

The high-energy focusing telescope (HEFT) is a balloon-borne experiment to image astrophysical sources in the hard X-ray (20–100 keV) band. Its maiden flight took place in May 2005 from Fort Sumner, New Mexico, USA. The angular resolution of HEFT is c. 1.5'. Rather than using a grazing-angle X-ray telescope, HEFT makes use of a novel tungsten-silicon multilayer coatings to extend the reflectivity of nested grazing-incidence mirrors beyond 10 keV. HEFT has an energy resolution of 1.0 keV full width at half maximum at 60 keV. HEFT was launched for a 25-hour balloon flight in May 2005. The instrument performed within specification and observed Tau X-1, the Crab Nebula.

High-resolution gamma-ray and hard X-ray spectrometer (HIREGS)

A balloon-borne experiment called the High-resolution gamma-ray and hard X-ray spectrometer (HIREGS) observed X-ray and gamma-rays emissions from the Sun and other astronomical objects. It was launched from McMurdo Station, Antarctica in December 1991 and 1992. Steady winds carried the balloon on a circumpolar flight lasting about two weeks each time.

Rockoons

Navy Deacon rockoon photographed just after a shipboard launch in July 1956.

 

The rockoon (a portmanteau of rocket and balloon) was a solid fuel rocket that, rather than being immediately lit while on the ground, was first carried into the upper atmosphere by a gas-filled balloon. Then, once separated from the balloon at its maximum height, the rocket was automatically ignited. This achieved a higher altitude, since the rocket did not have to move through the lower thicker air layers that would have required much more chemical fuel. 

The original concept of "rockoons" was developed by Cmdr. Lee Lewis, Cmdr. G. Halvorson, S. F. Singer, and James A. Van Allen during the Aerobee rocket firing cruise of the USS Norton Sound on March 1, 1949.

From July 17 to July 27, 1956, the Naval Research Laboratory (NRL) shipboard launched eight Deacon rockoons for solar ultraviolet and X-ray observations at ~30° N ~121.6° W, southwest of San Clemente Island, apogee: 120 km.

X-ray astronomy satellite

X-ray astronomy satellites study X-ray emissions from celestial objects. Satellites, which can detect and transmit data about the X-ray emissions are deployed as part of branch of space science known as X-ray astronomy. Satellites are needed because X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and satellites.

X-ray telescopes and mirrors

The Swift Gamma-Ray Burst Mission contains a grazing incidence Wolter I telescope (XRT) to focus X-rays onto a state-of-the-art CCD.

 

X-ray telescopes (XRTs) have varying directionality or imaging ability based on glancing angle reflection rather than refraction or large deviation reflection. This limits them to much narrower fields of view than visible or UV telescopes. The mirrors can be made of ceramic or metal foil.

The first X-ray telescope in astronomy was used to observe the Sun. The first X-ray picture (taken with a grazing incidence telescope) of the Sun was taken in 1963, by a rocket-borne telescope. On April 19, 1960 the very first X-ray image of the sun was taken using a pinhole camera on an Aerobee-Hi rocket.

The utilization of X-ray mirrors for extrasolar X-ray astronomy simultaneously requires:

• the ability to determine the location at the arrival of an X-ray photon in two dimensions and
• a reasonable detection efficiency.

X-ray astronomy detectors

Proportional Counter Array on the Rossi X-ray Timing Explorer (RXTE) satellite.

 

X-ray astronomy detectors have been designed and configured primarily for energy and occasionally for wavelength detection using a variety of techniques usually limited to the technology of the time.

X-ray detectors collect individual X-rays (photons of X-ray electromagnetic radiation) and count the number of photons collected (intensity), the energy (0.12 to 120 keV) of the photons collected, wavelength (c. 0.008–8 nm), or how fast the photons are detected (counts per hour), to tell us about the object that is emitting them.

Astrophysical sources of X-rays

Andromeda Galaxy – in high-energy X-ray and ultraviolet light (released 5 January 2016).

 

This light curve of Her X-1 shows long term and medium term variability. Each pair of vertical lines delineate the eclipse of the compact object behind its companion star. In this case, the companion is a two solar-mass star with a radius of nearly four times that of our Sun. This eclipse shows us the orbital period of the system, 1.7 days.

 

Several types of astrophysical objects emit, fluoresce, or reflect X-rays, from galaxy clusters, through black holes in active galactic nuclei (AGN) to galactic objects such as supernova remnants, stars, and binary stars containing a white dwarf (cataclysmic variable stars and super soft X-ray sources), neutron star or black hole (X-ray binaries). Some solar system bodies emit X-rays, the most notable being the Moon, although most of the X-ray brightness of the Moon arises from reflected solar X-rays. A combination of many unresolved X-ray sources is thought to produce the observed X-ray background. The X-ray continuum can arise from bremsstrahlung, black-body radiation, synchrotron radiation, or what is called inverse Compton scattering of lower-energy photons by relativistic electrons, knock-on collisions of fast protons with atomic electrons, and atomic recombination, with or without additional electron transitions.

An intermediate-mass X-ray binary (IMXB) is a binary star system where one of the components is a neutron star or a black hole. The other component is an intermediate mass star.

Hercules X-1 is composed of a neutron star accreting matter from a normal star (HZ Herculis) probably due to Roche lobe overflow. X-1 is the prototype for the massive X-ray binaries although it falls on the borderline, ~2 M_☉, between high- and low-mass X-ray binaries.

Celestial X-ray sources

The celestial sphere has been divided into 88 constellations. The International Astronomical Union (IAU) constellations are areas of the sky. Each of these contains remarkable X-ray sources. Some of them have been identified from astrophysical modeling to be galaxies or black holes at the centers of galaxies. Some are pulsars. As with sources already successfully modeled by X-ray astrophysics, striving to understand the generation of X-rays by the apparent source helps to understand the Sun, the universe as a whole, and how these affect us on Earth. Constellations are an astronomical device for handling observation and precision independent of current physical theory or interpretation. Astronomy has been around for a long time. Physical theory changes with time. With respect to celestial X-ray sources, X-ray astrophysics tends to focus on the physical reason for X-ray brightness, whereas X-ray astronomy tends to focus on their classification, order of discovery, variability, resolvability, and their relationship with nearby sources in other constellations.

This ROSAT PSPC false-color image is of a portion of a nearby stellar wind superbubble (the Orion-Eridanus Superbubble) stretching across Eridanus and Orion.

 

Within the constellations Orion and Eridanus and stretching across them is a soft X-ray "hot spot" known as the Orion-Eridanus Superbubble, the Eridanus Soft X-ray Enhancement, or simply the Eridanus Bubble, a 25° area of interlocking arcs of Hα emitting filaments. Soft X-rays are emitted by hot gas (T ~ 2–3 MK) in the interior of the superbubble. This bright object forms the background for the "shadow" of a filament of gas and dust. The filament is shown by the overlaid contours, which represent 100 micrometre emission from dust at a temperature of about 30 K as measured by IRAS. Here the filament absorbs soft X-rays between 100 and 300 eV, indicating that the hot gas is located behind the filament. This filament may be part of a shell of neutral gas that surrounds the hot bubble. Its interior is energized by ultraviolet (UV) light and stellar winds from hot stars in the Orion OB1 association. These stars energize a superbubble about 1200 lys across which is observed in the visual (Hα) and X-ray portions of the spectrum.

Proposed (future) X-ray observatory satellites

There are several projects that are proposed for X-ray observatory satellites. See main article link above.

Explorational X-ray astronomy

Ulysses' second orbit: it arrived at Jupiter on February 8, 1992, for a swing-by maneuver that increased its inclination to the ecliptic by 80.2 degrees.

 

Usually observational astronomy is considered to occur on Earth's surface (or beneath it in neutrino astronomy). The idea of limiting observation to Earth includes orbiting the Earth. As soon as the observer leaves the cozy confines of Earth, the observer becomes a deep space explorer. Except for Explorer 1 and Explorer 3 and the earlier satellites in the series, usually if a probe is going to be a deep space explorer it leaves the Earth or an orbit around the Earth.

For a satellite or space probe to qualify as a deep space X-ray astronomer/explorer or "astronobot"/explorer, all it needs to carry aboard is an XRT or X-ray detector and leave Earth's orbit.

Ulysses was launched October 6, 1990, and reached Jupiter for its "gravitational slingshot" in February 1992. It passed the south solar pole in June 1994 and crossed the ecliptic equator in February 1995. The solar X-ray and cosmic gamma-ray burst experiment (GRB) had 3 main objectives: study and monitor solar flares, detect and localize cosmic gamma-ray bursts, and in-situ detection of Jovian aurorae. Ulysses was the first satellite carrying a gamma burst detector which went outside the orbit of Mars. The hard X-ray detectors operated in the range 15–150 keV. The detectors consisted of 23-mm thick × 51-mm diameter CsI(Tl) crystals mounted via plastic light tubes to photomultipliers. The hard detector changed its operating mode depending on (1) measured count rate, (2) ground command, or (3) change in spacecraft telemetry mode. The trigger level was generally set for 8-sigma above background and the sensitivity is 10⁻⁶ erg/cm² (1 nJ/m²). When a burst trigger is recorded, the instrument switches to record high resolution data, recording it to a 32-kbit memory for a slow telemetry read out. Burst data consist of either 16 s of 8-ms resolution count rates or 64 s of 32-ms count rates from the sum of the 2 detectors. There were also 16 channel energy spectra from the sum of the 2 detectors (taken either in 1, 2, 4, 16, or 32 second integrations). During 'wait' mode, the data were taken either in 0.25 or 0.5 s integrations and 4 energy channels (with shortest integration time being 8 s). Again, the outputs of the 2 detectors were summed.

The Ulysses soft X-ray detectors consisted of 2.5-mm thick × 0.5 cm² area Si surface barrier detectors. A 100 mg/cm² beryllium foil front window rejected the low energy X-rays and defined a conical FOV of 75° (half-angle). These detectors were passively cooled and operate in the temperature range −35 to −55 °C. This detector had 6 energy channels, covering the range 5–20 keV.

X-Rays from Pluto

Theoretical X-ray astronomy

Theoretical X-ray astronomy is a branch of theoretical astronomy that deals with the theoretical astrophysics and theoretical astrochemistry of X-ray generation, emission, and detection as applied to astronomical objects.

Like theoretical astrophysics, theoretical X-ray astronomy uses a wide variety of tools which include analytical models to approximate the behavior of a possible X-ray source and computational numerical simulations to approximate the observational data. Once potential observational consequences are available they can be compared with experimental observations. Observers can look for data that refutes a model or helps in choosing between several alternate or conflicting models. 

Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency, the general tendency is to try to make minimal modifications to the model to fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model. 

Most of the topics in astrophysics, astrochemistry, astrometry, and other fields that are branches of astronomy studied by theoreticians involve X-rays and X-ray sources. Many of the beginnings for a theory can be found in an Earth-based laboratory where an X-ray source is built and studied.

Dynamos

Dynamo theory describes the process through which a rotating, convecting, and electrically conducting fluid acts to maintain a magnetic field. This theory is used to explain the presence of anomalously long-lived magnetic fields in astrophysical bodies. If some of the stellar magnetic fields are really induced by dynamos, then field strength might be associated with rotation rate.

Astronomical models

Images released to celebrate the International Year of Light 2015
(Chandra X-Ray Observatory).

 

From the observed X-ray spectrum, combined with spectral emission results for other wavelength ranges, an astronomical model addressing the likely source of X-ray emission can be constructed. For example, with Scorpius X-1 the X-ray spectrum steeply drops off as X-ray energy increases up to 20 keV, which is likely for a thermal-plasma mechanism. In addition, there is no radio emission, and the visible continuum is roughly what would be expected from a hot plasma fitting the observed X-ray flux. The plasma could be a coronal cloud of a central object or a transient plasma, where the energy source is unknown, but could be related to the idea of a close binary.

In the Crab Nebula X-ray spectrum there are three features that differ greatly from Scorpius X-1: its spectrum is much harder, its source diameter is in light-years (ly)s, not astronomical units (AU), and its radio and optical synchrotron emission are strong. Its overall X-ray luminosity rivals the optical emission and could be that of a nonthermal plasma. However, the Crab Nebula appears as an X-ray source that is a central freely expanding ball of dilute plasma, where the energy content is 100 times the total energy content of the large visible and radio portion, obtained from the unknown source.

The "Dividing Line" as giant stars evolve to become red giants also coincides with the Wind and Coronal Dividing Lines. To explain the drop in X-ray emission across these dividing lines, a number of models have been proposed:

1. low transition region densities, leading to low emission in coronae,
2. high-density wind extinction of coronal emission,
3. only cool coronal loops become stable,
4. changes in a magnetic field structure to that an open topology, leading to a decrease of magnetically confined plasma, or
5. changes in the magnetic dynamo character, leading to the disappearance of stellar fields leaving only small-scale, turbulence-generated fields among red giants.

Analytical X-ray astronomy

High-mass X-ray binaries (HMXBs) are composed of OB supergiant companion stars and compact objects, usually neutron stars (NS) or black holes (BH). Supergiant X-ray binaries (SGXBs) are HMXBs in which the compact objects orbit massive companions with orbital periods of a few days (3–15 d), and in circular (or slightly eccentric) orbits. SGXBs show typical the hard X-ray spectra of accreting pulsars and most show strong absorption as obscured HMXBs. X-ray luminosity (L_x) increases up to 10³⁶ erg·s⁻¹ (10²⁹ watts).

The mechanism triggering the different temporal behavior observed between the classical SGXBs and the recently discovered supergiant fast X-ray transients (SFXT)s is still debated.

Stellar X-ray astronomy

Stellar X-ray astronomy is said to have started on April 5, 1974, with the detection of X-rays from Capella. A rocket flight on that date briefly calibrated its attitude control system when a star sensor pointed the payload axis at Capella (α Aur). During this period, X-rays in the range 0.2–1.6 keV were detected by an X-ray reflector system co-aligned with the star sensor. The X-ray luminosity of L_x = 10³¹ erg·s⁻¹ (10²⁴ W) is four orders of magnitude above the Sun's X-ray luminosity.

Stellar coronae

Coronal stars, or stars within a coronal cloud, are ubiquitous among the stars in the cool half of the Hertzsprung-Russell diagram. Experiments with instruments aboard Skylab and Copernicus have been used to search for soft X-ray emission in the energy range ~0.14–0.284 keV from stellar coronae. The experiments aboard ANS succeeded in finding X-ray signals from Capella and Sirius (α CMa). X-ray emission from an enhanced solar-like corona was proposed for the first time. The high temperature of Capella's corona as obtained from the first coronal X-ray spectrum of Capella using HEAO 1 required magnetic confinement unless it was a free-flowing coronal wind.

In 1977 Proxima Centauri is discovered to be emitting high-energy radiation in the XUV. In 1978, α Cen was identified as a low-activity coronal source. With the operation of the Einstein observatory, X-ray emission was recognized as a characteristic feature common to a wide range of stars covering essentially the whole Hertzsprung-Russell diagram. The Einstein initial survey led to significant insights:

• X-ray sources abound among all types of stars, across the Hertzsprung-Russell diagram and across most stages of evolution,
• the X-ray luminosities and their distribution along the main sequence were not in agreement with the long-favored acoustic heating theories, but were now interpreted as the effect of magnetic coronal heating, and
• stars that are otherwise similar reveal large differences in their X-ray output if their rotation period is different.

To fit the medium-resolution spectrum of UX Ari, subsolar abundances were required.

Stellar X-ray astronomy is contributing toward a deeper understanding of

• magnetic fields in magnetohydrodynamic dynamos,
• the release of energy in tenuous astrophysical plasmas through various plasma-physical processes, and
• the interactions of high-energy radiation with the stellar environment.

Current wisdom has it that the massive coronal main sequence stars are late-A or early F stars, a conjecture that is supported both by observation and by theory.

Young, low-mass stars

A Chandra X-ray image of the Cluster of newly formed stars in the Orion Nebula.

 

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to 10⁵ times stronger than for main-sequence stars of similar masses.

X-ray emission for pre–main-sequence stars was discovered by the Einstein Observatory. This X-ray emission is primarily produced by magnetic reconnection flares in the stellar coronae, with many small flares contributing to the "quiescent" X-ray emission from these stars. Pre–main sequence stars have large convection zones, which in turn drive strong dynamos, producing strong surface magnetic fields. This leads to the high X-ray emission from these stars, which lie in the saturated X-ray regime, unlike main-sequence stars that show rotational modulation of X-ray emission. Other sources of X-ray emission include accretion hotspots and collimated outflows.

X-ray emission as an indicator of stellar youth is important for studies of star-forming regions. Most star-forming regions in the Milky Way Galaxy are projected on Galactic-Plane fields with numerous unrelated field stars. It is often impossible to distinguish members of a young stellar cluster from field-star contaminants using optical and infrared images alone. X-ray emission can easily penetrate moderate absorption from molecular clouds, and can be used to identify candidate cluster members.

Unstable winds

Given the lack of a significant outer convection zone, theory predicts the absence of a magnetic dynamo in earlier A stars. In early stars of spectral type O and B, shocks developing in unstable winds are the likely source of X-rays.

Coolest M dwarfs

Beyond spectral type M5, the classical αω dynamo can no longer operate as the internal structure of dwarf stars changes significantly: they become fully convective. As a distributed (or α²) dynamo may become relevant, both the magnetic flux on the surface and the topology of the magnetic fields in the corona should systematically change across this transition, perhaps resulting in some discontinuities in the X-ray characteristics around spectral class dM5. However, observations do not seem to support this picture: long-time lowest-mass X-ray detection, VB 8 (M7e V), has shown steady emission at levels of X-ray luminosity (L_X) ≈ 10²⁶ erg·s⁻¹ (10¹⁹ W) and flares up to an order of magnitude higher. Comparison with other late M dwarfs shows a rather continuous trend.

Strong X-ray emission from Herbig Ae/Be stars

Herbig Ae/Be stars are pre-main sequence stars. As to their X-ray emission properties, some are

• reminiscent of hot stars,
• others point to coronal activity as in cool stars, in particular the presence of flares and very high temperatures.

The nature of these strong emissions has remained controversial with models including

• unstable stellar winds,
• colliding winds,
• magnetic coronae,
• disk coronae,
• wind-fed magnetospheres,
• accretion shocks,
• the operation of a shear dynamo,
• the presence of unknown late-type companions.

K giants

The FK Com stars are giants of spectral type K with an unusually rapid rotation and signs of extreme activity. Their X-ray coronae are among the most luminous (L_X ≥ 10³² erg·s⁻¹ or 10²⁵ W) and the hottest known with dominant temperatures up to 40 MK. However, the current popular hypothesis involves a merger of a close binary system in which the orbital angular momentum of the companion is transferred to the primary.

Pollux is the brightest star in the constellation Gemini, despite its Beta designation, and the 17th brightest in the sky. Pollux is a giant orange K star that makes an interesting color contrast with its white "twin", Castor. Evidence has been found for a hot, outer, magnetically supported corona around Pollux, and the star is known to be an X-ray emitter.

Eta Carinae

Classified as a Peculiar star, Eta Carinae exhibits a superstar at its center as seen in this image from Chandra X-ray Observatory. Credit: Chandra Science Center and NASA.

 

New X-ray observations by the Chandra X-ray Observatory show three distinct structures: an outer, horseshoe-shaped ring about 2 light years in diameter, a hot inner core about 3 light-months in diameter, and a hot central source less than 1 light-month in diameter which may contain the superstar that drives the whole show. The outer ring provides evidence of another large explosion that occurred over 1,000 years ago. These three structures around Eta Carinae are thought to represent shock waves produced by matter rushing away from the superstar at supersonic speeds. The temperature of the shock-heated gas ranges from 60 MK in the central regions to 3 MK on the horseshoe-shaped outer structure. "The Chandra image contains some puzzles for existing ideas of how a star can produce such hot and intense X-rays," says Prof. Kris Davidson of the University of Minnesota. Davidson is principal investigator for the Eta Carina observations by the Hubble Space telescope. "In the most popular theory, X-rays are made by colliding gas streams from two stars so close together that they'd look like a point source to us. But what happens to gas streams that escape to farther distances? The extended hot stuff in the middle of the new image gives demanding new conditions for any theory to meet."

Amateur X-ray astronomy

Collectively, amateur astronomers observe a variety of celestial objects and phenomena sometimes with equipment that they build themselves. The United States Air Force Academy (USAFA) is the home of the US's only undergraduate satellite program, and has and continues to develop the FalconLaunch sounding rockets. In addition to any direct amateur efforts to put X-ray astronomy payloads into space, there are opportunities that allow student-developed experimental payloads to be put on board commercial sounding rockets as a free-of-charge ride.

There are major limitations to amateurs observing and reporting experiments in X-ray astronomy: the cost of building an amateur rocket or balloon to place a detector high enough and the cost of appropriate parts to build a suitable X-ray detector.

History of X-ray astronomy

NRL scientists J. D. Purcell, C. Y. Johnson, and Dr. F. S. Johnson are among those recovering instruments from a V-2 used for upper atmospheric research above the New Mexico desert. This is V-2 number 54, launched January 18, 1951, (photo by Dr. Richard Tousey, NRL).

 

In 1927, E.O. Hulburt of the US Naval Research Laboratory and associates Gregory Breit and Merle A. Tuve of the Carnegie Institution of Washington explored the possibility of equipping Robert H. Goddard's rockets to explore the upper atmosphere. "Two years later, he proposed an experimental program in which a rocket might be instrumented to explore the upper atmosphere, including detection of ultraviolet radiation and X-rays at high altitudes".

In the late 1930s, the presence of a very hot, tenuous gas surrounding the Sun was inferred indirectly from optical coronal lines of highly ionized species. The Sun has been known to be surrounded by a hot tenuous corona. In the mid-1940s radio observations revealed a radio corona around the Sun.

The beginning of the search for X-ray sources from above the Earth's atmosphere was on August 5, 1948 12:07 GMT. A US Army (formerly German) V-2 rocket as part of Project Hermes was launched from White Sands Proving Grounds. The first solar X-rays were recorded by T. Burnight.

Through the 1960s, 70s, 80s, and 90s, the sensitivity of detectors increased greatly during the 60 years of X-ray astronomy. In addition, the ability to focus X-rays has developed enormously—allowing the production of high-quality images of many fascinating celestial objects.

Major questions in X-ray astronomy

As X-ray astronomy uses a major spectral probe to peer into source, it is a valuable tool in efforts to understand many puzzles.

Stellar magnetic fields

Magnetic fields are ubiquitous among stars, yet we do not understand precisely why, nor have we fully understood the bewildering variety of plasma physical mechanisms that act in stellar environments. Some stars, for example, seem to have magnetic fields, fossil stellar magnetic fields left over from their period of formation, while others seem to generate the field anew frequently.

Extrasolar X-ray source astrometry

With the initial detection of an extrasolar X-ray source, the first question usually asked is "What is the source?" An extensive search is often made in other wavelengths such as visible or radio for possible coincident objects. Many of the verified X-ray locations still do not have readily discernible sources. X-ray astrometry becomes a serious concern that results in ever greater demands for finer angular resolution and spectral radiance. 

There are inherent difficulties in making X-ray/optical, X-ray/radio, and X-ray/X-ray identifications based solely on positional coincidents, especially with handicaps in making identifications, such as the large uncertainties in positional determinants made from balloons and rockets, poor source separation in the crowded region toward the galactic center, source variability, and the multiplicity of source nomenclature.

X‐ray source counterparts to stars can be identified by calculating the angular separation between source centroids and position of the star. The maximum allowable separation is a compromise between a larger value to identify as many real matches as possible and a smaller value to minimize the probability of spurious matches. "An adopted matching criterion of 40" finds nearly all possible X‐ray source matches while keeping the probability of any spurious matches in the sample to 3%."

Solar X-ray astronomy

All of the detected X-ray sources at, around, or near the Sun appear to be associated with processes in the corona, which is its outer atmosphere.

Coronal heating problem

In the area of solar X-ray astronomy, there is the coronal heating problem. The photosphere of the Sun has an effective temperature of 5,570 K yet its corona has an average temperature of 1–2 × 10⁶ K. However, the hottest regions are 8–20 × 10⁶ K. The high temperature of the corona shows that it is heated by something other than direct heat conduction from the photosphere.

It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating. The first is wave heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient gas in the form of heat. The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of large solar flares and myriad similar but smaller events—nanoflares.

Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfvén waves have been found to dissipate or refract before reaching the corona. In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms.

Coronal mass ejection

A coronal mass ejection (CME) is an ejected plasma consisting primarily of electrons and protons (in addition to small quantities of heavier elements such as helium, oxygen, and iron), plus the entraining coronal closed magnetic field regions. Evolution of these closed magnetic structures in response to various photospheric motions over different time scales (convection, differential rotation, meridional circulation) somehow leads to the CME. Small-scale energetic signatures such as plasma heating (observed as compact soft X-ray brightening) may be indicative of impending CMEs. 

The soft X-ray sigmoid (an S-shaped intensity of soft X-rays) is an observational manifestation of the connection between coronal structure and CME production. "Relating the sigmoids at X-ray (and other) wavelengths to magnetic structures and current systems in the solar atmosphere is the key to understanding their relationship to CMEs."

The first detection of a Coronal mass ejection (CME) as such was made on December 1, 1971 by R. Tousey of the US Naval Research Laboratory using OSO 7. Earlier observations of coronal transients or even phenomena observed visually during solar eclipses are now understood as essentially the same thing. 

The largest geomagnetic perturbation, resulting presumably from a "prehistoric" CME, coincided with the first-observed solar flare, in 1859. The flare was observed visually by Richard Christopher Carrington and the geomagnetic storm was observed with the recording magnetograph at Kew Gardens. The same instrument recorded a crotchet, an instantaneous perturbation of the Earth's ionosphere by ionizing soft X-rays. This could not easily be understood at the time because it predated the discovery of X-rays (by Roentgen) and the recognition of the ionosphere (by Kennelly and Heaviside).

Exotic X-ray sources

A microquasar is a smaller cousin of a quasar that is a radio emitting X-ray binary, with an often resolvable pair of radio jets. LSI+61°303 is a periodic, radio-emitting binary system that is also the gamma-ray source, CG135+01. Observations are revealing a growing number of recurrent X-ray transients, characterized by short outbursts with very fast rise times (tens of minutes) and typical durations of a few hours that are associated with OB supergiants and hence define a new class of massive X-ray binaries: Supergiant Fast X-ray Transients (SFXTs). Observations made by Chandra indicate the presence of loops and rings in the hot X-ray emitting gas that surrounds Messier 87. A magnetar is a type of neutron star with an extremely powerful magnetic field, the decay of which powers the emission of copious amounts of high-energy electromagnetic radiation, particularly X-rays and gamma rays.

X-ray dark stars

A solar cycle: a montage of ten years' worth of Yohkoh SXT images, demonstrating the variation in solar activity during a sunspot cycle, from after August 30, 1991, at the peak of cycle 22, to September 6, 2001, at the peak of cycle 23. Credit: the Yohkoh mission of Institute of Space and Astronautical Science (ISAS, Japan) and NASA (US).

During the solar cycle, as shown in the sequence of images at right, at times the Sun is almost X-ray dark, almost an X-ray variable. Betelgeuse, on the other hand, appears to be always X-ray dark. Hardly any X-rays are emitted by red giants. There is a rather abrupt onset of X-ray emission around spectral type A7-F0, with a large range of luminosities developing across spectral class F. Altair is spectral type A7V and Vega is A0V. Altair's total X-ray luminosity is at least an order of magnitude larger than the X-ray luminosity for Vega. The outer convection zone of early F stars is expected to be very shallow and absent in A-type dwarfs, yet the acoustic flux from the interior reaches a maximum for late A and early F stars provoking investigations of magnetic activity in A-type stars along three principal lines. Chemically peculiar stars of spectral type Bp or Ap are appreciable magnetic radio sources, most Bp/Ap stars remain undetected, and of those reported early on as producing X-rays only few of them can be identified as probably single stars. X-ray observations offer the possibility to detect (X-ray dark) planets as they eclipse part of the corona of their parent star while in transit. "Such methods are particularly promising for low-mass stars as a Jupiter-like planet could eclipse a rather significant coronal area."

X-ray dark planet/comet

X-ray observations offer the possibility to detect (X-ray dark) planets as they eclipse part of the corona of their parent star while in transit. "Such methods are particularly promising for low-mass stars as a Jupiter-like planet could eclipse a rather significant coronal area."

As X-ray detectors have become more sensitive, they have observed that some planets and other normally X-ray non-luminescent celestial objects under certain conditions emit, fluoresce, or reflect X-rays.

Comet Lulin

Image of Comet Lulin on 28 January 2009, when the comet was 99.5 million miles from Earth and 115.3 million miles from the Sun, from Swift. Data from Swift's Ultraviolet/Optical Telescope is shown in blue and green, and from its X-Ray Telescope in red.

Main article: Comet Lulin

 

NASA's Swift Gamma-Ray Burst Mission satellite was monitoring Comet Lulin as it closed to 63 Gm of Earth. For the first time, astronomers can see simultaneous UV and X-ray images of a comet. "The solar wind—a fast-moving stream of particles from the sun—interacts with the comet's broader cloud of atoms. This causes the solar wind to light up with X-rays, and that's what Swift's XRT sees", said Stefan Immler, of the Goddard Space Flight Center. This interaction, called charge exchange, results in X-rays from most comets when they pass within about three times Earth's distance from the Sun. Because Lulin is so active, its atomic cloud is especially dense. As a result, the X-ray-emitting region extends far sunward of the comet.