For Release: Jan. 11, 1995

Donald Savage
Headquarters, Washington, D.C.
(Phone: 202/358-1727)

Steve Roy
Marshall Space Flight Center
Huntsville, Ala.
(Phone: 205/544-6535)

RELEASE NO: 95-002

NEW X-RAY SOURCE MAJOR SURPRISE FOR WORLD-WIDE ASTRONOMERS

A puzzling, extremely energetic object in space, discovered by a NASA orbiting gamma-ray-sensing instrument, has now further surprised scientists, who have found that the object is producing massive, jet-like radio plasma emissions.

The unusual object, which is in the southern-sky constellation Scorpius, was discovered last summer by researchers using the instrument known as the Burst and Transient Source Experiment, part of NASA's Compton Gamma Ray Observatory launched in 1991. Since that discovery, the object has produced numerous radio wavelength jet-like emissions that are exploding from the central core of the object at nearly the speed of light. Astronomers worldwide have been studying the object with a variety of ground-based and space-based telescopes.

"We are very excited about the discovery of the new X-ray source and the way the Burst and Transient Source Experiment helped other scientists study this very rare astrophysical event," said Colleen Wilson-Hodge of NASA's Marshall Space Flight Center in Huntsville, Ala. "The source may provide us with a way to better understand what happens near exotic objects like black holes and neutron stars."

The Burst and Transient Source Experiment includes, as part of its normal operations, an all-sky monitor which constantly watches the sky for bright outbursts of X-ray novae.

Known as GRO J1655-40, the object became one of the brightest X-ray objects in the sky, and continued to be visible in X-ray and radio wavelengths through late December, emitting massive outbursts of energy. GRO J1655-40 was observed by radio and optical astronomers soon after its initial discovery.

"Usually, after the initial flare-up of such an object, the source gradually fades in all wavelength bands," explained Burst and Transient Source Experiment co-investigator Dr. William Paciesas of the University of Alabama in Huntsville. "However, in this case it suddenly began to brighten rapidly in radio wavelengths and eject material which could be observed in very high resolution radio images. The radio emissions are thought to result from the deceleration of particles in a plasma jet being emitted from one of two stars in a binary star system." Many astronomers believe one of the stars is a black hole; others believe it is a neutron star.

The jet phenomenon is primarily observed to occur near the centers of some active galaxies many millions of light years away. To find such an object only about 10,000 light years distant is a major surprise for astronomers. Only a few months ago, another transient X-ray source called GRS 1915+105, which was discovered in 1992 by the Russian/French spacecraft GRANAT, was seen to have radio jets as does GRO J1655-40. It also is within our own galaxy. "The relative closeness of these objects is an opportunity to perhaps understand the mechanisms which cause the radio, jet-like emissions in active galaxies," said Paciesas.

Astronomers have no way of knowing how long the X-ray source will continue to be active. Optical observers, who can "measure" the mass of the neutron star or black hole, will not be able to view the source for several months, when it has moved away from the Sun and once again entered the night sky.

Analysis of the object's behavior showed that it probably is of a class of objects called X-ray novae. An X-ray nova is an object that suddenly brightens to become one of the brightest sources of X-ray energy in the sky. Only one or two of these sources, which are thought to be binary or double-star systems, are discovered per year. The sudden outbursts of X-ray energy may be a result of the process of the smaller of the pair of stars losing its material from gravitational forces to the larger star. The object may also brighten in the visible, radio, infrared and/or gamma ray energy wavelengths.

The recent X-ray measurements of the rare object GRO J1655-40 are to be presented at the American Astronomical Society Meeting in Tucson, Ariz., this week by Wilson-Hodge and by Paciesas. Dr. Robert Hjellming of the National Radio Astronomy Observatory will present findings based on radio measurements of GRO J1655-40.

The Compton Gamma Ray Observatory is managed by NASA's Goddard Space Flight Center in Greenbelt, Md. and the Burst and Transient Source Experiment is managed by the Marshall Center.