NEW TYPE OF ASTRONOMICAL OBJECT DISCOVERED IN OUR GALAXY
Don Savage, Steve Roy, Donna Drelick
NASA astronomers have discovered a new type of object
towards the center of our Milky Way galaxy exhibiting a
combination of behaviors never before seen in the 35-year
history of gamma-ray astronomy.
During the first day it was observed, the source produced
over 140 powerful bursts of gamma-rays; since then, it has
settled down to a daily rate of about twenty bursts, and it is
currently the brightest source of hard X-ray/gamma-rays in the
The discovery will be announced tomorrow in a paper
published in the scientific journal “Nature” by scientists
from NASA’s Marshall Space Flight Center, Huntsville, AL; the
University of Alabama in Huntsville; the Massachusetts
Institute of Technology in Cambridge, MA; and the University
of Amsterdam in the Netherlands.
The unusual object in the southern sky was discovered in
early December 1995 by researchers using an instrument known
as the Burst and Transient Source Experiment, aboard NASA’s
Compton Gamma Ray Observatory spacecraft. Since December 2,
the new burster has produced more than 1,000 hard X-ray
“We’re particularly excited about the discovery of a new
X-ray source,” said NASA Marshall astrophysicist Dr. Gerald
Fishman. “The object’s strange behavior is one of the major
discoveries in X-ray astronomy in the past decade.”
Apparently the sky had more surprises in store for the
observers. In mid-December, the NASA scientists discovered an
additional source of steady radiation that seemed to reside at
the same position in the sky with the burster. This new object
further surprised scientists when it was observed to
continuously emit pulses at a rate of about twice per second.
It was now classified as a pulsar, and the question that the
observers faced was “what was the relation, if any, between
the two objects?” said Dr. Chryssa Kouveliotou of the
Universities Space Research Association at the Marshall
The answer soon came back: the burster and the pulsar were
one and the same source.
“The properties of this X-ray source are unlike those of
any we know,” explained Dr. Kouveliotou. “The burst repetition
rate makes this phenomenon very different from gamma ray
bursts that we have observed several thousand times from
throughout the universe. Also, the longer duration and
persistent bursting makes the object very different from so-
called Soft Gamma Ray Repeaters, which have been observed to
burst in short, isolated episodes separated by several years.”
“What’s unique about this object is that it does so many
different things all at once,” said Fred Lamb, an
astrophysicist at the University of Illinois at Urbana-
Champaign. “We’ve seen some sources that play the drums, some
that crash cymbals, and a few that play the trumpet, but this
source is a one-man band.”
This bursting pulsar was later found by Dr. Mark Finger of
the Universities Space Research Association at NASA Marshall
to be a member of a binary system, performing one full
revolution around its low-mass companion every 12 days. “The
most likely explanation at this time is that the bursts of X-
ray energy may result when the lighter of the pair of stars
loses its material by gravitational or magnetic forces to the
neutron star,” said Kouveliotou.
A neutron star is an exotic star with a mass greater than
the Sun and a diameter of only about 10 miles. “The discovery
of the new X-ray source may lead to a better understanding of
how neutron stars form and evolve,” Kouveliotou said.
The source was discovered shortly before the recent launch
of NASA’s Rossi X-ray Timing Explorer (RXTE) spacecraft, which
carries the largest collecting area of X-ray detectors ever
flown in space. “Our highest scientific priority, after
evaluating the operation of the satellite and X-ray
instruments, was observing this transient source” said Frank
Marshall, Director of RXTE’s Science Operations Center.
“With better measurements, we should be able to pin
downthe theoretical model,” says Jean Swank, RXTE Project
Scientist. As soon as RXTE could observe the source, its
detectors were pointed to obtain detailed information about
the X-ray spectrum and its variations.
The two large instruments on the spacecraft, provided by
teams led by Swank of NASA’s Goddard Space Flight Center,
Greenbelt, MD, and Richard Rothschild of the University of
California at San Diego, quickly found the source to be very
bright across the X-ray band from 2 to 60 keV, with strong
persistent emission as well as numerous bursts.
“First, matter is accelerated to half the speed of light
because of the neutron star’s enormous gravitational force.
Then, it crashes into the surface of the neutron star and is
heated to nearly one billion degrees,” Lamb explained.
“Because it is so hot, it radiates almost entirely in X-rays
rather than visible light, in this case with a power
comparable to 1 million times the power of the Sun originating
from an area about the size of the National Mall in
RXTE made repeated scans across the source to determine
the position of the source accurately enough to allow
astronomers to search for radio or visible light from it.
Within the past ten days, a radio source and a very faint
visible star have been identified in the direction of the X-
ray source. Scientists are working furiously to see if the
radio and visible light are coming from this object.
The bursting pulsar is a transient X-ray star that is
expected to die out fairly soon, within a few weeks to, at
most, a few months. Therefore, scientists are working
feverishly to try to unravel its mysteries while it still
The Compton Gamma Ray Observatory, which was launched in
1991, is managed by NASA’s Goddard Space Flight Center,
Greenbelt, MD, and the Burst and Transient Source Experiment
is managed by NASA Marshall. The Rossi X-Ray Timing Explorer,
launched on December 30, 1995, is managed by NASA Goddard.
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