X-Rays from Comet Tsuchiya-Kiuchi

Some facts on the X-ray emission from comet Tsuchiya-Kiuchi:

• Comet Tsuchiya-Kiuchi ( = C/1990 N1 = 1990i = 1990 XVII ) was in the field of view of the ROSAT Position Sensitive Proportional Counter (PSPC) during the ROSAT All-Sky Survey between 1990 Nov. 18.76 and 20.36 UT.
• During the observations comet Tsuchiya-Kiuchi was at a geocentric distance of Delta = 1.08 AU, and at a heliocentric distance of r = 1.38 AU. The visual magnitude was at m1 = 7.0.
• The PSPC was scanning 22 times across the comet; the scans were spaced by 96 minutes. During each scan the comet was covered by a maximum of 30 s. The total exposure was 500 s.
• The x-ray image, corrected for the comet's motion, shows an extended source at the expected position, with a radial extent of at least 10', or 470 000 km.
• The energy resolution of the PSPC (41% FWHM at 1 keV) allows, for the first time, spectroscopic studies of the x-ray radiation of a comet.
• The spectrum can be modelled by thermal bremsstrahlung emission with kT = 0.4 +/- 0.1 keV. Inclusion of the 525-eV oxygen-fluorescence line improves the confidence of the fit from 5% to 19%. Pure line emission due to oxygen and carbon fluorescence can be ruled out.
• The 0.1- to 2.4-keV x-ray luminosity was 2.5 x 10E16 erg/s, and is thus comparable to that of comet Hyakutake (cf. IAUC 6373). The optical luminosity of comet Tsuchiya-Kiuchi, however, was lower by a factor of 15 than that of comet Hyakutake.

Some background information:

We have searched archival X-ray data taken with the ROSAT satellite for cometary X-ray emission, and succeeded in finding comet Tsuchiya-Kiuchi (C/1990 N1) in the ROSAT all-sky survey data (cf. IAUC 6404). In order to detect the comet, it was necessary to correct the X-ray photons individually for its proper motion, since the comet was moving between the survey scans which were separated by 96 min.

This finding is exciting for several reasons:

While comet Hyakutake (C/1996 B2) was a spectacular case, comet Tsuchiya- Kiuchi (C/1990 N1) was too faint to be visible at all with the naked eye. Furthermore, the X-ray telescope was not pointing at the comet (as in the case of comet Hyakutake) but was scanning over the sky, so that the comet was in the field of view for not more than 30 sec per scan. In view of the optical faintness and the special observing conditions, the detection of X-rays from comet Tsuchiya-Kiuchi was a surprising event.

Besides the fact that X-rays from a second, much fainter comet have now been discovered, this detection has an additional scientific impact: it allows, for the first time, spectroscopic studies of the X-ray properties.

The observations of comet Hyakutake had been taken with X-ray/EUV detectors which had no sufficient intrinsic energy resolution, so that all the spectral information which was available so far had to be derived from the flux ratio between the X-ray and EUV bands. For the all-sky survey, however, a different detector with spectral capabilities, the Positional Sensitive Proportional Counter (PSPC), was in the focal plane of the X-ray telescope (it cannot be used regularly any more because it requires permanent gas supply, and there is not much gas left).

The spectral information now available definitely rules out the possibility of pure line emission due to oxygen and carbon fluorescence, and indicates a typical 'temperature' of kT = 0.4 keV for assumed thermal bremsstrahlung emission. Although the conditions for thermal bremsstrahlung are probably not all satisfied, we are using this model at the moment in some pragmatic approach to get a rough idea on the energetics involved; we intend to refine the spectral investigations as soon as a more adequate model will become available.