First X-Rays from a Comet Discovered
team of U.S. and German astrophysicists have made the
first ever detection of
X-rays coming from a comet. Their 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) in 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 of the X-ray emission from Comet Hyakutake took the astronomers
by surprise, and they are also puzzled by the rapid changes in their
intensity.
"We had no clear expectation that comets shine in X-rays," said
Dr. Michael J. Mumma of NASA's
Goddard Space Flight Center (GSFC) in Greenbelt, MD, but the
opportunity to search for this radiation in a comet coming so close to the
earth was too good to miss. "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 detected repeatedly during March 26-27 as it swept across the
sky at a rate equivalent to crossing twice the diameter of the full Moon in an
hour. As a result, the comet actually raced through the field of view of
ROSAT's X-ray telescope during each 2,000-second exposure. The German
scientists were able to correct for the comet's motion during each observation,
and produce accurate images with the aid of a computer.
The difficult observations of the swift comet were made possible because of
special arrangements made between MPE and the German mission control center in
Oberpfaffenhofen, Germany. The comet observations were scheduled to take place
exactly when ROSAT would be in real-time contact with the ground station at
Weilheim, Germany. Also, the usual two-week period for processing of the ROSAT
data was expedited for the comet program, so that the data were transmitted to
members of the science team shortly after they were acquired. "It's a
great example of international cooperation in science," said the
U.S. Project Scientist, Dr. Robert Petre (GSFC). "We're very grateful to
our German colleagues for the opportunity to work with them on a historic
discovery."
X-rays were never 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. Unlike
the visible light
from the comet, the nucleus or solid body is not only not the brightest point
in the comet. In fact, the scientists say, the nucleus does not show up in the
X-ray image from ROSAT.
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, 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 GSFC, 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."
The research was conducted by GSFC planetary scientists Lisse and Mumma,
along with Drs. Dennerl, Jakob Englhauser, and Jürgen Schmitt of the MPE, and
the German and U.S. ROSAT Project Scientists, Professor Joachim Trümper of MPE
and Dr. Robert Petre (GSFC), respectively. Comet Hyakutake was discovered on
January 30, 1996, and was subsequently approved as a Target of Opportunity for
ROSAT, which means that the regular schedule of
satellite
operations was reorganized to enable the observations of the comet.
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