Headquarters, Washington, DC              April 4, 1996
(Phone:  202/358-1547)

Jim Sahli
Goddard Space Flight Center, Greenbelt, MD
(Phone:  301/286-0697)

RELEASE:  96-66 

FIRST X-RAYS FROM A COMET DISCOVERED 

     A team of U.S. and German astrophysicists have made the 
first ever detection of X-rays coming from a comet. 

      The discovery of a strong radiation signal -- about 
100 times brighter than even the most optimistic predictions 
-- was made March 27, 1996, during observations of comet 
Hyakutake using Germany's orbiting ROSAT satellite.  

     "It was a thrilling moment when the X-rays from the 
comet appeared on our screen at the ROSAT ground station," 
said Dr. Konrad Dennerl of the Max Planck Institute for 
Extraterrestrial Physics (MPE), Garching, Germany.  
Following the initial detection, the team reported repeated 
X-ray emissions from the comet over the next 24 hours.  The 
comet was near its closest approach to the Earth at a 
distance of less than 10 million miles when it was first 
detected by ROSAT.

     The strength and rapid changes in intensity of the 
comet's X-ray emission both surprised and puzzled 
astronomers.

     "We had no clear expectation that comets shine in X-rays,"
said Dr. Michael J. Mumma of NASA's Goddard Space Flight Center, 
Greenbelt, MD.  "Now we have our work cut out for us in 
explaining these data, but that's the kind of problem you 
love to have."  

     The comet was examined repeatedly during March 26-27 as 
it swept across the sky.  The German scientists were able to 
correct satellite attitude for the comet's motion during 
each observation, and produce accurate images with the aid 
of a computer. 

     X-rays have never been found from a comet before, and 
scientists had optimistically predicted an intensity that 
turned out to be about 100 times weaker than the radiation 
actually detected by ROSAT.  Strong changes in the 
brightness of the X-rays were another surprise.  There were 
pronounced increases and decreases in the X-ray brightness 
from one ROSAT observation to another, typically over a time 
difference of a few hours.  

     Still another puzzle is the nature of the physical 
process that generates the X-rays, but the ROSAT image may 
contain clues to this process.  In the image, the X-rays 
from the comet seem to come from a crescent-shaped region on 
the sunward side of Comet Hyakutake. 

     Explaining the unexpected bright X-ray emission is the 
next major task for the science team.  One preliminary 
theory is that X-ray emission from the Sun was absorbed by a 
cloud of gaseous water molecules surrounding the nucleus of 
the comet, and then were re-emitted by the molecules in a 
process physicists call "fluorescence."

     According to this idea, the cloud is so thick that its 
sunward side absorbs nearly all the incoming solar X-rays, 
so that none reach the remainder of the cloud.  This could 
explain why the cometary X-ray emission has the form of a 
crescent, rather than that of a sphere around the nucleus.  

     A second possible explanation is that the X-rays are 
produced from the violent collision between the comet 
material and the supersonic "wind" of plasma and particles 
streaming away from the Sun.

     "We always learn something new when we study an object 
at different wavelengths," commented Dr. Carey M. Lisse of 
Goddard, the leader of the X-ray investigation.  "Now we 
have to determine why the comet is so bright in X-rays, and 
see what we can learn about its structure and composition 
from these unique images."

                    - end -


The image also is available on the Internet at URLs: