RELEASE NO: 99-003 (98-223)

THE ONLY KNOWN BURSTING PULSAR PROVIDES CLUES TO PULSAR NATURE

Astronomers observing the unique "Bursting Pulsar" -- the only pulsar known to generate both regular pulses and frequent enormous bursts of radiation -- have characterized the relationship between the two phenomena. This may shed light on the pulsing mechanism behind other pulsars and how it is affected by magnetic fields, companion stars and occasional bursts of radiation.

Dr. Michael Stark, an astrophysicist with NASA's Goddard Space Flight Center (GSFC) and the University of Maryland, College Park, and Dr. Keith Jahoda, astrophysicist in the X-ray branch at GSFC, present their findings today at the 193rd American Astronomical Society Meeting in Austin, Texas.

A pulsar is a rapidly spinning neutron star that emits very regular pulses of radiation -- so regular, in fact, that Dr. Stark has spent the past two years trying to figure out why the Bursting Pulsar, named GRO J1744-28, is a mere 20 milliseconds late in its pulse cycle.

"We discovered in 1996 that large bursts of radiation emitted by GRO J1744-28 caused delays in its pulse signals," said Dr. Stark. "Other pulsars also have ‘glitches,’ sudden changes in a neutron star’s normally regular pulsation cycle probably caused by internal changes in the neutron star. Understanding this interaction could help us answer lingering questions about what causes glitches in other circumstances and exactly what causes a neutron star to emit large, short bursts of radiation."

Dr. Stark explained the bursts in GRO J1744-28 to be enormous flashes of X-rays, lasting only a few seconds, which the star emits once or twice an hour in addition to its emitting regular pulses of X-rays twice per second.

"The curious thing," said Dr. Stark, "was that the delay in the regular pulsation didn't go away when the bursts went away. This is significant because it suggests that the bursts trigger the delay, but some other process causes the delay itself."

What that other process is remains a mystery, but Dr. Stark and others are beginning to make some progress in understanding it. "We are now at the point of understanding the fuse without understanding the bomb. I'd like to understand the bomb."

Early on it appeared that the cause of the delay was within the neutron star, as is the case with the apparent cause of glitches in other pulsars. Dr. Stark now believes the source of the pulse delay is external to the neutron star. For example, the X-rays emitted by the neutron star may be bent by material surrounding the star or it may leave the surface of the star in a different direction than usual.

A neutron star was once a star several times larger than our Sun. After consuming all of its nuclear fuel, it exploded in a supernova, leaving behind a compact sphere only 10-15 km in diameter but still somewhat more massive than our Sun and with a very strong magnetic field. Its compact size allows it to spin rapidly, and its extreme magnetic field can cause hot spots at its magnetic poles. We see regular pulses of radiation as these hot spots spin around with the star, similar to the beam from a lighthouse as it spins its lantern.

The lighthouse analogy can be extended in the case of GRO J1744-28. When the pulse signal is delayed, it is either because something moved the lighthouse or because the beam of light is being bent after it is emitted from the lighthouse by some kind of "mirror" suspended nearby.

Early analysis of GRO J1744-28 favored the motion of the hot spot -- that is, something moving the lighthouse. But the analysis presented today appears to rule that out in favor of the bending of the X-rays -- or, the magically suspended mirror nearby.

"We still haven't found what it is that is acting like a mirror redirecting these X-rays, but at least now we know we are looking for one," said Dr. Stark.

Although the nature of the mirror is a mystery, there are many possibilities. GRO J1744-28 is in a binary system with another star that provides fuel for its X-ray emission. On the way to the neutron star, this fuel forms a disk around it, which may have the ability to bend the X-rays. Work is still needed to understand how the disk would accomplish this.

Drs. Stark and Jahoda observed the bursting pulsar with the Proportional Counter Array on the Rossi X-ray Timing Explorer (RXTE). The bursts were first identified by detecting low-energy gamma rays in December 1995, with the Compton Gamma Ray Observatory, a few weeks before the launch of RXTE. The bursts appeared twice, in the early months of 1996 and 1997, and have not been observed since.

Note to editors: Images to support this story may be found on the Internet at:

http://www.gsfc.nasa.gov/ftp/newsmedia/AAS/PULSAR