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NASA Press Release on GRO 1744-28

Don Savage
Headquarters, Washington, DC     February 28, 1996
(Phone:  202/358-1547)

Steve Roy
Marshall Space Flight Center, Huntsville, AL
(Phone:  205/544-0034)

Donna Drelick
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301/286-7995)



RELEASE: 96-37

NEW TYPE OF ASTRONOMICAL OBJECT DISCOVERED IN OUR GALAXY

    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 
sky. 

    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 
bursts.  

    "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 
Center.   

    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 
Washington, DC."

    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 
shines.

    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.


                        -end-


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