GAMMA-RAYS FROM NEUTRON-STARS

The number of Galactic neutron stars is significant, N similar to 10(8-9), but radiation from their surfaces is hard to detect. The nearest isolated neutron stars could be as close as similar to 10 pc (the estimate sensitively depends on assumptions about initial velocities), but would probably be too old, and thus too cold, for optical detections. That gamma-ray emission is a useful alternative window for neutron star studies was revealed through observations of pulsed gamma-ray emission from the Crab and Vela pulsars (e.g. Bignami 1987). A wealth of recent gamma-ray observations of neutron stars is provided by sophisticated experiments aboard the COMPTON Gamma Ray Observatory (CGRO) and several other spacecraft. We summarize the current status of the Galactic population of gamma-ray pulsars. Gamma ray bursts are also believed to be associated with neutron stars. Although data from BATSE aboard CGRO have recently challenged the standard paradigm that gamma-ray bursts (GRBs) originate on or near neutron stars in the Galactic disk, the possibility of bursts from an extended Galactic halo of high velocity neutron stars is still under consideration. Furthermore, many cosmological scenarios also invoke neutron stars as the energy source. We thus include GRBs in this review. A subset of GRBs, the soft gamma repeaters, has recently been associated with Galactic supernova remnants. Their properties and counterparts are discussed. In addition, we briefly describe gamma-ray emission from slowly accreting neutron stars, and the gamma-ray line afterglow resulting from production of radioactive isotopes during their birth.