Evidence for dust-related X-ray emission from comet C/1995 O1 (Hale-Bopp)

We report the discovery of X-ray emission from comet C/1995 O1 (Hale-Bopp) by the LECS instrument on board BeppoSAX on 1996 September 10-11. The 0.1-2.0 keV luminosity decayed by a factor of 2 on a timescale of similar to 10 hr with a mean value of 5 x 10(16) ergs s(-1). The spectrum is well fitted by a thermal bremsstrahlung model with a temperature of 0.29 +/- 0.06 keV or a power law with a photon index of 3.1(-0.2)(+0.6). The lack of detected C and O line emission places severe constraints on many models for cometary X-ray emission, especially those that involve X-ray production in cometary gas. The luminosity is a factor of at least 3.4 greater than measured by the Extreme Ultraviolet Explorer (EUVE) 4 days later. This difference may be related to the emergence from the nucleus on 1996 September 9 of a dust-rich cloud. Over the next few days, the cloud continued to expand, becoming increasingly tenuous until it had reached an extent of similar to 3 x 10(5) km (or similar to 2') by the start of the EWE observation. We speculate that the observed reduction in X-ray intensity is evidence for dust fragmentation. These results support the view that cometary X-ray emission arises from the interaction between solar X-rays and cometary dust.