Comet Hyakutake – First X-Ray Detected Comet – by Albert Lim (2000)

Comet Hyakutake was the first comet to have been detected in X-rays. The surprising discovery was made by the German X-ray observatory ROSAT on the 27th March 1996 when the comet was less than 10 million km from Earth. This discovery surprised astronomers because the level of X-ray energies detected was two magnitude (100 times) stronger than predicted.

Fig 01 : First X-ray image of Comet Hyakutake through  ROSAT’s
High Resolution Imager. No X-rays were detected from nucleus (+).

Prior to this discovery, scientist had not expected comets to emit X-rays. Another surprising find was the unexpected changes in brightness of the X-rays over short periods of a few hours. ROSAT made first contact with Comet Hykutake at 12:14:49 UTC on the 26th March 1996 ; when the first signs of X-rays were marginally detectable. On the second contact at 15:24:26 UTC the same day, suspicion of X-ray emission associated with Hykutake appeared but was inconclusive. At third contact at 17:04:04 later that day, cast doubts as the X-ray signals had became fainter. The X-rays also appeared at almost the same position on the detector. In March 96, Hyakutake was moving across the sky at a rate of about a degree an hour ; as a result, it literally raced across the field of view of ROSAT’s X-ray detector during each of the 2,000 second exposures. The forth, fifth and sixth contact starting at 13:47:27 on the 27^th March 96 finally pinned down the evidence confirming that Hyakutake was indeed shinning at X-ray frequencies.

To date, 8 comets, including the famous Comet Hale-Bopp have been observed in X-rays. Unlike Comet Hale Bopp however, Comet Hyakutake was formed in interstellar space - evidence for this was provided from ultraviolet and millimeter-wave observations confirming that isotope ratios for Hyakutake as consistent with that of interstellar origin. Hyakutake’s current entry into the inner solar system also changed it’s orbital period to about 8,600 years.
Various theories abound to attempt to explain the unexpected X-ray from comets. One suggest that X-ray emission from the Sun are absorbed by the cometary cloud at the nucleus containing gaseous water molecules - X-rays are then re-emitted by these molecules as fluorescence. The cloud is so thick that it absorbs all the solar X-ray striking it and therefore none reach the remaining parts of the comet. This was proposed as an attempt to explain why X-ray emission is a crescent shape only at the nucleus. A second theory propose that X-rays are emitted due to violent collision of the cometary materials with the extremely high velocity “winds” of plasma and particles streaming from the sun. A third theory, proposed by Sir Fred Hoyle and his student Wickramasinghe, suggest that the X-rays are actually solar X-rays scattered by grains of carbon-based material a few nanometers in diameter. This is also consistent with their theory that comets shed the common flu virus and others onto Earth, thus seeding planets with life. Although Hoyle and his student have been propounding this for many years now, it is only more recently that Secker, Weeson and Lepock have shown that viruses and bacteria can indeed be ejected from red giant stars by radiative pressures and can survive if they are shielded from cosmic ultraviolet radiation by thin carbon layers.

It is even more interesting to note that following the first X-ray detection of Comet Hyakutake, astronomers searched archives of ROSAT’s X-ray data and succeeded in finding an earlier comet called Comet Tsuchiya-Kiuchi in X-rays. Comet Tsuchiya-Kiuchi was too faint to be visible with the naked eye and ROSAT’s X-ray telescope was not even looking at it. In fact, ROSAT was involved with the all-sky survey then. Astronomers were surprised that  X-rays were also detected from comet Tsuchiya-Kiuchi, even though the scanning mode of Rosat’s X-ray telescope permitted the comet to be in the instrument’s field of view for only a mere 30 seconds! The main scientific impact of this discovery was the fact that while Hyakutake was imaged with Rosat’s    X-ray / EVU detectors, comet Tsuchiya-Kiuchi was detected with ROSAT’s Positional Sensitive Proportional Counter (PSPC). The X-ray / EUV did not have enough intrinsic energy resolution, whereas the PSPC was capable of producing for the first time, X-ray spectral information of comet Tsuchiya-Kiuchi.