X-ray emission from comets - discovered by ROSAT
In 1996 the X-ray sky was enhanced with an unexpected class of objects: for
the very first time X-ray emission from comets was detected. This surprising
discovery was made with the ROSAT satellite, first on comet C/1996 B2
(Hyakutake) during its close approach to Earth, then a few weeks later on
other comets, observed by chance during the ROSAT all-sky survey in 1990/91.
These comets are C/1990 K1 (Levy), C/1990 N1 (Tsuchiya-Kiuchi),
45P/Honda-Mrkos-Pajdusakova, and C/1991 A2 (Arai). The last two were
optically more than 10000 times fainter than Hyakutake. In the case of comet
C/1991 A2 (Arai), the X-ray observations took place six weeks before the comet
Comet Tsuchiya-Kiuchi was in the field of view on two
and Levy on three occasions.
All comets observed within 2 AU from the sun and brighter
than 12 mag were detected with ROSAT.
Thus, comets represent a new class of X-ray sources.
The spectral resolution of the ROSAT PSPC allows specific models to be tested,
and the unlimited field of view provided by the all-sky survey
makes it possible to trace the full extent of the X-ray emission.
The ongoing research activities indicate that charge exchange between
highly charged heavy ions (e.g.
in the solar wind and cometary neutrals is the dominant process for the
X-ray emission. Comets may thus be utilized as probes for monitoring
the heavy ion content of the solar wind.
Dennerl, K., J. Englhauser, J. Trümper, Science 277,
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© X-Ray Group at MPE (group)
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