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 \sim 10 hr with a mean value of 5 \times 10 ^ { 16 } erg s ^ { -1 } .
The spectrum is well fit by a thermal bremsstrahlung model with a temperature of 0.29 \pm 0.06 keV , or a power-law with a photon index of 3.1 \pm ^ { 0.6 } _ { 0.2 } .
The lack of detected C and O line emission places severe constraints on many models for cometary X-ray emission , especially those which involve X-ray production in cometary gas .
The luminosity is a factor of at least 3.4 greater than measured by 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 \sim 3 \times 10 ^ { 5 } km ( or \sim 2 ^ { \prime } ) by the start of EUVE 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 .