NEW TYPE OF ASTRONOMICAL OBJECT DISCOVERED IN OUR GALAXY

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

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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