Star-Eating Mass Found Near Center of Milky Way

20 September 1999

A brilliant X-ray outburst in the direction of the galactic center flared to prominence last Wednesday, turning a relatively sedate energy source into the brightest X-ray object in the sky.

Instruments aboard at least two X-ray observing spacecraft detected the flash, which was most likely produced by a huge mass of material being swallowed by a black hole or neutron star.The event has been tracked down to an area near a visible star called GM Sagittarii in the constellation Sagittarius.

That star is known to vary in brightness when observed at visible light, said Mike McCollough, a staff scientist at NASA's Marshall Space Flight Center. The existence of a high-energy X-ray source next to it means that the star is actually part of a system called an X-ray binary, a pair in which a star and an extremely dense object such as a neutron star or a black hole orbit each other, he said.

Last week's unusually energetic activity was first noticed Wednesday, said McCollough, who works on a team that analyzes data from the Burst and Transient Source Experiment (BATSE) instrument aboard the Compton Gamma Ray Observatory. That NASA spacecraft is designed to observe sources of high-energy radiation.

When McCollough heard that other instruments had detected the X-ray burst, he looked at information from the Compton instruments and noticed a brief, but huge spike in emissions from GM Sagittarii on Tuesday. The object -- which is typically some 30 times dimmer than the pulsar in the Crab nebula -- jumped to about five times brighter than that object before it dimmed again.

The Crab pulsar is usually the brightest X-ray object in the sky. It is the standard against which X-ray luminosity is commonly measured.

Sifting through data for Wednesday, McCollough found nothing for the first half of the day, he said.

"Then about 10 hours into the day we started seeing it go off. Literally within about six hours it got to about eight times the Crab," he said. "It essentially wasn't there and then it just went through the roof on us."

Meanwhile, scientists watching data from the Rossi X-ray Explorer (another spacecraft that observes X-rays) were witnessing the intense flare in X-ray emissions from Sagittarius, but at slightly less energetic X-ray wavelengths than BATSE picks up.

Within eight hours on Wednesday, Rossi scientists saw GM Sagittarii flare to more than 12 times brighter than the Crab nebula then drop just as dramatically, said William Heindl, a research scientist at the Center for Astrophysics and Space Science at the University of California at San Diego, who works with data from the Rossi observer.

The spacecraft's sky scanner measured most the activity, Heindl said, but scientists aimed the instrument's telescope into view just in time to see the object's last violent sputter -- the object had dimmed, then within a period of about 15 minutes, it flashed to about twice the brightness of the Crab before Earth obscured the spacecraft's view.

When Rossi next glimpsed GM Sagittarii, the object was quiet and dark, betraying no hint of its violent outburst, Heindl said.

The jolts in X-ray emissions both in the low-energy "soft" X-rays, measured by Rossi, and the higher-energy "hard" X-rays detected by BATSE mean that they were produced as material falls into either by a black hole or neutron star, McCollough said. The source of that material is probably the companion star that is feeding the black hole or neutron star.

Based on the new observations, the system will need to be reclassified as an X-ray binary, McCollough said. The system will likely attract a lot more attention in the coming year as scientists try to learn more about the star, its mass, and just what the high-density object beside it is, he said.


[ Important note: Due to a mix-up among astronomers, the star in this article is misidentified as GM Sagittarii. The star associated with the brilliant X-ray burst of September 15, 1999 is now called V 4641 Sagittarii. For two decades astronomers have been incorrectly calling V4641 by the name GM Sagittarii, a mistake which was recently discovered and announced on October 13, 1999. ]