First hints on cometary X-rays

Some people at MPE have followed the ROSAT observations of Comet Hyakutake with a lot of interest. During the times when ROSAT passes over the ground station in Weilheim / Germany there is a possibility to watch the X-ray photons as they arrive at the detector. This usually happens five to six times a day; the typical duration of each contact is about 10 minutes.

Fortunately, during six of the nine observations of Comet Hyakutake such a quicklook was possible:

Position of comet C/1996 B2 (Hyakutake) during all Weilheim contacts
and its distance from the center of the HRI field of view
1) 26 March 1996 (Tuesday):     12:04:35 - 12:14:49 UTC
ROSAT pointing direction :      13:29:16.00   80:28:15.6        off-axis angle
comet 1996.03.26 / 12:04:35 :   13:30:27.38   80:21:52.2  --->  7.05 arcmin
comet 1996.03.26 / 12:14:49 :   13:29:27.92   80:28:13.0  --->  0.50 arcmin

2) 26 March 1996 (Tuesday):     15:24:26 - 15:34:40 UTC
ROSAT pointing direction :      13:05:51.00   82:23:31.2        off-axis angle
comet 1996.03.26 / 15:24:26 :   13:06:28.10   82:22:27.3  --->  2.17 arcmin
comet 1996.03.26 / 15:34:40 :   13:04:55.12   82:28:23.5  --->  5.01 arcmin

3) 26 March 1996 (Tuesday):     17:04:04 - 17:13:34 UTC
ROSAT pointing direction :      12:49:59.80   83:17:49.2        off-axis angle
comet 1996.03.26 / 17:04:04 :   12:49:42.49   83:18:33.7  --->  0.90 arcmin
comet 1996.03.26 / 17:13:34 :   12:47:52.74   83:23:44.7  --->  6.98 arcmin
4) 27 March 1996 (Wednesday):   13:37:28 - 13:47:27 UTC
ROSAT pointing direction :      04:52:59.88   83:01:58.8        off-axis angle
comet 1996.03.27 / 13:37:28 :   04:53:44.73   83:04:21.7  --->  2.74 arcmin
comet 1996.03.27 / 13:47:27 :   04:52:21.63   82:59:59.2  --->  2.31 arcmin

5) 27 March 1996 (Wednesday):   16:56:54 - 17:06:00 UTC
ROSAT pointing direction :      04:31:08.76   81:37:01.2        off-axis angle
comet 1996.03.27 / 16:56:54 :   04:31:15.61   81:37:18.0  --->  0.38 arcmin
comet 1996.03.27 / 17:06:00 :   04:30:26.20   81:33:19.2  --->  4.01 arcmin

6) 28 March 1996 (Thursday):    10:10:35 - 10:20:46 UTC
ROSAT pointing direction :      03:44:48.12   74:34:01.2        off-axis angle
comet 1996.03.28 / 10:10:35 :   03:44:50.69   74:34:54.7  --->  0.91 arcmin
comet 1996.03.28 / 10:20:46 :   03:44:37.58   74:31:10.1  --->  2.94 arcmin
The coordinates of the comet have been calculated by linear spherical
interpolation (ASTROLIB) from the ephemeris of Don Yeomans - JPL (Mar. 22,
1996) which takes orbital perturbations and light time corrections into
account. They are geocentric coordinates and refer to the mean equator
and equinox of J2000.

Following these contacts was extremely thrilling:

  • During the first contact (26 March 1996, 12:04:35 - 12:14:49 UTC) a signal was only marginally detectable, and it was not sure at all whether X-rays from the comet would be visible during the Weilheim contacts.
  • The second contact (26 March 1996, 15:24:26 - 15:34:40 UTC) showed some indication for a subtle enhancement on the right-hand half of the HRI FOV (our quicklook screen displays detector coordinates) which could be interpreted as evidence for X-ray emission associated with the comet, although there were concerns that this slight excess might have been caused by inhomogeneities of the detector or by the contribution of extrasolar X-ray sources (though comparison with the ROSAT all-sky survey did not show evidence for such gradients. At this time, some people were already convinced about the successful X-ray detection of a comet (and nick-named this observation 'Hyakutake - second light').
  • The third contact (26 March 1996, 17:04:04 - 17:13:34 UTC), however, seemed to support the doubts on the existence of X-ray emission from the comet, as the excess signal was fainter and almost at the same position on the detector. At this time we had not yet calculated the position of the (fast moving) comet with respect to the HRI pointing direction for the time of the Weilheim contact, and expected to see the excess emission at some other position of the detector.
  • To clarify the situation, we calculated the position of the comet for the Weilheim contacts and its distance from the center of the HRI field of view (this is the 'off-axis angle' information in the table above), and found that the comet should be located at similar off-axis angles for all contacts. Therefore, the appearance of a faint enhancement at similar detector coordinates during the second and third contacts could well have been caused by the comet.
  • As a further check, we observed a subsequent ROSAT Weilheim contact on the next morning, where the telescope was not pointing towards the comet, but at an apparently faint X-ray source. Since no obvious signal was present in the HRI FOV, we used this as a 'flatfield' observation to estimate the HRI homogeneity. The excess observed in the two contacts before was not present in this observation - another evidence in favor of an X-ray detection of the comet in the two contacts before.
  • Therefore, for the fourth contact (27 March 1996, 13:37:28 - 13:47:27 UTC), we expected the excess emission to appear again - and there it was ! It was quite conspicuous this time, much more obvious than at any of the contacts before. The right side of the detector clearly registered more photons than the left one. To get a rough estimate on the spectrum (the HRI has some intrinsic energy resolution), we accumulated images in three different bands: one in the lowest HRI channel (I), and two in the lower (II) and upper (III) half of the remaining HRI bandwidth. In the image of the lowest channel (I) the excess emission was marginally detectable. This excludes the possibility that it is due to UV leakage of the HRI detector. In the soft band (II) it was very clear, and in the hard band (III) there was some indication of an anti-correlation with respect to the soft band. If this can be confirmed to be real and not caused by a detector effect, this could be evidence for shadowing of the diffuse X-ray background by a comet.
  • The fifth contact (27 March 1996, 16:56:54 - 17:06:00 UTC) was a nice confirmation of what we had seen during the fourth contact, both in terms of the general morphology and the spectral properties.
  • The sixth contact (28 March 1996, 10:10:35 - 10:20:46 UTC) was very similar to the contacts 4 and 5 and once again confirmed that comet Hyakutake can be observed in X-rays.

We checked the pulse height distribution of the signals and found them to be inconsistent with UV emission. Therefore the recorded emission from the comet must be X-rays!

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