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Astronomers Capture Galactic Light Show With X-Ray Satellite

June 3, 1999

Chicago -- Forget about summer fireworks and rock concerts. The best light-show around is from a couple dozen stars in the center of the Milky Way galaxy that randomly flare, disappear and flare again in X-ray light, sometimes lasting a few minutes, sometimes lasting for months.

NASA astronomers scanning a section of the galactic center twice weekly since February 1999 with the Rossi X-ray Timing Explorer (RXTE) recorded a vibrant patch of universe rich with black holes and neutron stars that seem to appear then disappear. They present a census from this first-of-its-kind galactic sweep, as well as a 20-second time-elapsed movie of the flickering light sources, at the Centennial Meeting of the American Astronomical Society in Chicago on June 3.

Of the 38 X-ray sources counted in the census so far, at least 21 are neutron stars and four are probable black hole systems. The movie conveniently pieces together months of actual RXTE observations, providing a first-ever glimpse of a large collection of flickering X-ray transients in action. X-ray transients are star systems that turn on and off in X-ray radiation.

The purpose of the ongoing galactic center scan, according to Dr. Craig Markwardt, co-investigator for the study at NASA's Goddard Space Flight Center in Greenbelt, Md., is to find one elusive type of X-ray transient -- an accreting millisecond X-ray pulsar. This is a collapsed star that has the mass of the Sun compressed into a city-sized sphere, spinning over a thousand times in one second as it sucks matter off of a neighboring star and releases X-ray energy.

Astronomers saw their first and only such pulsar last April in the center of the Milky Way, assigning it the technical name SAX J1808.4-3658. They suspect there are many more in the same region.

"If you want to see the lights, you head downtown," said Markwardt. " Just like a city glows the brightest downtown, the galactic center has the highest concentration of stars. So it's here that we have the best chance of finding a millisecond X-ray pulsar."

Markwardt said SAX J1808.4-3658 is interesting because it is a cross between old isolated millisecond pulsars, of which many are known, and slower-spinning pulsars in binary (two-star) systems. Scientists theorize that millisecond pulsars attain their fast spins by collecting gas from a neighboring star, a process called accretion. They then escape this binary system and travel alone through space. SAX J1808.4-3658 is the first millisecond pulsar to be seen in an accreting binary system, and may thus be the missing link between the two populations.

Markwardt hasn't yet found a second millisecond X-ray pulsar, but he did stumble upon seven new X-ray sources that are "highly variable". This means they might increase as much as tenfold in X-ray luminosity and then fade out of sight as quickly as in a few minutes to a few days. SAX J1808.4-3658 itself is highly variable, and it is in fact presently not observable, presumed to be dormant until this November, based on past behavior.

While waiting for the sudden appearance of another millisecond pulsar, Markwardt wanted to illustrate how some star systems irregularly flare up. With sensitive and regularly sampled images of the galactic center region, Markwardt was able to string individual observations into a time-elapsed movie, visualizing how the stars have turned on and off in X-ray radiation over the past four months.

Far from just a pretty light show, Markwardt's movie of X-ray data depicts the physics of extreme gravity and temperature. The X-ray sources observed here are primarily black holes and neutron stars, the core remains of massive star explosions, in binary orbits around normal companion stars. Both exert extreme gravitational force that pulls gas away from their neighboring stars. The X-ray glow that astronomers see comes from this gas heated to millions of degrees under the force of gravity.

As the neighboring star gradually revolves closer to its extreme gravity companion, Markwardt said, it releases more gas to the grip of gravity. This results in brighter X-ray glows. When the gas stops flowing in, the glow stops. Scientists believe the tidal ebb and flow of gas and the positioning of the stars' eclipsing orbits cause the observed flickering of X-ray radiation.

"One goal of X-ray astronomy is to better understand the binary systems that are X-ray transients," said Dr. Jean Swank, a co-investigator for the study at Goddard and Project Scientist for RXTE. "Dr. Markwardt's animation is a visualization of the firefly-like character of their X-ray brightness.

"Our observations will monitor all the significant outbursts that occur in this year in the central region of our galaxy," Swank said. "We will be learning where they are and how often they are bright, and we will keep a close watch for special systems like SAX J1808.4-3658, which are hidden treasures." The galactic center scan is expected to continue until March, 2000.

Other X-ray sources in the RXTE census include 12 unclassified objects, four of which are "highly variable" sources Markwardt discovered. Markwardt said a further refined spectroscopic analysis might help determine the nature of these sources and thus lead to a more accurate knowledge of the evolution of massive objects in our galaxy.

NASA's RXTE was launched in December 1995 to observe fast-changing, energetic and rapidly spinning objects, such as supermassive black holes, active galactic nuclei, neutron stars and millisecond pulsars. Dr. Craig Markwardt is a Resident Research Associate in NASA/Goddard's X-ray Branch, working for NRC.

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