The Oriented Scintillation Spectrometer Experiment (OSSE) was launched on NASA's COMPTON Gamma Ray Observatory (CGRO) in April 1991. This 35,000-pound satellite is the most advanced mission to undertake scientific observations in high-energy astrophysics. The observatory provides continuous coverage of the most energetic phenomena in nature and can respond to targets of opportunity such as new supernovae or major solar flare activity. The mission has continued to provide high-quality data since its launch.

NASA's Compton Gamma Ray Observatory

NASA's Compton Gamma Ray Observatory

OSSE has been monitoring a new galactic X-ray source that erupted without warning in December 1996. The X rays from this object shows half-second pulsations and display irregular outbursts. Pulsations and bursting behavior have been associated with different types of X-ray sources but have never before been seen from the same object. The X rays are probably produced when a neutron star pulls matter off its binary companion star, but there are still many mysteries about this source.

If this object is actually near the Galactic Center, as the OSSE observations suggest, then the power released by this source in X rays exceeds the total energy output of our sun by a factor of nearly a million, with the bursts being even higher. At these luminosities, most astronomers think that the accreting matter would be blown away by the force of the radiation such that the steady power supply of in-falling matter would be shut off.

Besides bursting, the CGRO observations also show that this source pulses with a 0.467-second period and is in a binary system with an orbital period of 11.76 days. OSSE has now observed this object on three occasions: December 14-20, 1997, January 2-12, and January 16-30, 1998. This is a phenomenon never before seen in X-ray pulsars. It could mean that the location of the X-ray-emitting region moves from one point of the neutron star surface to another when it undergoes an outburst.

OSSE observations also show that the hard X-ray spectra from this source during and between bursts is not significantly different, suggesting that the emission mechanisms for the two intervals are the same. The X rays probably result from the conversion of gravitational energy into radiation when falling matter hits the surface of the neutron star, which is only about 20 km in diameter. This is different from standard X-ray burst sources, which are thought to undergo thermonuclear explosions during outbursts.

New maps of gamma rays from CGRO show evidence of a previously unknown and unexpected cloud of positrons, a form of antimatter, lying some 3,000 light years above the center of our galaxy. NRL researchers expected the maps to show a large cloud of antimatter near the galactic center and along the plane of the Galaxy, produced primarily by the explosions of young massive stars. The maps show this gamma-ray activity, but surprisingly they also show a second cloud of antimatter well off the galactic plane.

The OSSE instrument, ten times more sensitive than earlier gamma-ray experiments, is providing scientists with the first opportunity to undertake comprehensive observations of the distribution and variability of the sources producing the positrons in the Galaxy. To date, OSSE has spent more than a year mapping the distribution of the 511,000-electron-volt gamma rays coming from the center of our Galaxy and searching for variations in the number of gamma rays observed.