NRL OSSE Instrument Observes Unique New X-Ray Source

4/30/1996 - 53-96r
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Unlike the peaceful nighttime sky we see with our eyes, the cosmos is violent and variable at X-ray and gamma ray energies. One such high-energy eruption that occurred during the last few months has been monitored by the Naval Research Laboratory's (NRL's) Oriented Scintillation Spectrometer Experiment (OSSE) on NASA's Compton Gamma Ray Observatory (CGRO).

OSSE observations of this totally new X-ray source -- believed to be located near the center of our galaxy -- show that it is pulsating at half second intervals, similar to a flashing lighthouse beam. It bursts at rates ranging from 20 bursts per hour soon after its discovery to one or two per hour now. Pulsation and bursting behaviors have been previously associated with X-ray sources, but never before from the same object. The new bursting X-ray pulsar has been officially named GROJ1744-28 after the discovering mission and its celestial coordinates.

Pulsed X-rays from such high-energy eruptions are thought to be produced when a neutron star pulls matter from its binary companion star. A neutron star is the remnant of the collapse of an ordinary star. Outbursts of X-ray emission from a neutron star occur as matter flows from a companion star onto the surface of a neutron star and heats to such high temperatures, on the order of one hundred million degrees, that it glows in X-rays. A magnetic field a trillion times stronger than that found at the surface of the Earth is needed to funnel the matter onto hot spots at the poles of the neutron star.

The discovery of this unusual object was made by the Burst and Transient Source Experiment (BATSE) on CGRO, which continuously monitors the sky at X-ray and soft gamma-ray energies. When BATSE first detected this object in December, 1995 the instrument was "seeing" bursting rates of roughly 20 per hour, each lasting between 5 and 20 seconds. After several days, the bursting rate slowed considerably, but the source then began producing bright, persistent emission and became one of the brightest X-ray stars in the sky. At this point, CGRO was reoriented for other instruments, including OSSE, to begin observations.

If this object is near the Galactic Center, as the OSSE observations suggest, then the X-ray power of this source exceeds the total energy output of our Sun by a factor of nearly one million, with the bursts being even brighter. This would be equivalent to packing the mass of the Sun and the radiant power of one million suns into a region not more than 20 miles
across. With this much power, most astronomers think that matter would be blown away by the force of the radiation, so that the X-ray fuel source would be shut off. Thus, say scientists,either its direction is a coincidence, or our understanding of extremely luminous sources in incomplete.

OSSE has observed GROJ1774-28 on three separate occasions, making several interesting discoveries. For example, the phase of the pulsed flux during an outburst lags that of the flux during steady emission by 20%. This is a phenomenon never before seen in X-ray pulsars and may indicate that the location of the X-ray emitting region moves from one point of the neutron star surface to another between steady emission and outburst.

Analysis of the OSSE data from this source showed that the spectra of the steady and burst emissions were not significantly different, suggesting that the emission mechanisms for the two intervals are the same. This mechanism is thought to be the conversion of gravitational energy into X-rays when falling matter hits the surface of the neutron star. This is significantly different from standard X-ray burst sources, which are thought to undergo thermonuclear explosions during outbursts.

From the OSSE observations, the NRL researchers have concluded that GROJ1744-28 has a magnetic field about 10 times smaller than most X ray pulsars and about 100 times larger than most X-ray bursters. By having a field just in the middle of these regimes, the bursting X-ray pulsar shares properties of both types of objects. The scientists, now studying different models for the bursts, hope to solve this problem before it fades away completely, which could happen within the next few weeks.

OSSE was developed by the Gamma and Cosmic Ray Astrophysics Branch of NRL's Space Science Division, under the leadership of principal investigator, Dr. James Kurfess. OSSE team members studying GROJ1744 28 include, Jim Kurfess, Mark Strickman, Chuck Dermer, Eric Grove, Neil Johnson and Gerry Share of NRL; Greg Jung and Bernard Philips of Universities Space Research Associates; Danny Messina of SFA, Inc.; Steve Matz of Northwestern University; and Steve Sturner, NRC/NRL research associate.

OSSE is one of four instruments launched on CGRO in 1991 to provide continuous coverage of the most energetic phenomena in nature and can respond to targets of opportunity such as this bursting X-ray pulsar. The instrument was designed to make comprehensive observations of astrophysical sources in hard X-ray and gamma ray ranges and uses four identical detector systems to observe the high-energy processes occurring in compact sources in our galaxy such as pulsars, neutron stars, and suspected black holes, and explosive phenomena such as supernovae, novae and gamma ray bursts.

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