Gamma-ray pulsars: the Compton observatory contribution to the study of isolated neutron stars

The four instruments on the Compton Gamma Ray Observatory have identified at least seven isolated neutron stars by their pulsed gamma-ray emission. For all of these, the gamma radiation represents the largest observable fraction of the spin-down luminosity, making the gamma rays important diagnostics of particle acceleration and interaction in the neutron star magnetospheres. Several other "candidate" pulsars have tentative identifications based on pulsed radiation or positional association. The ensemble of CGRO pulsar detections, possible detections, and upper limits yields a number of patterns, most of which have exceptions. Some examples: (1) All except PSR B1509-58 have light curves consistent with double pulses; (2) All except the Crab have radio pulses out of phase with the gamma-ray pulses (the relative phase of the recently-reported radio pulse from Geminga is not known as of this writing); (3) Except possibly for PSR B1509-58 and PSR B0656+14, the gamma-ray energy spectra flatten with increasing pulsar age; (4) All except PSR B1951+32 show evidence of having high-energy spectral cutoffs in the CGRO energy band; (5) The pulsed flux from all the pulsars is relatively constant with time; (6) There is a trend for the pulsar luminosity to increase with increasing polar-cap current (or open field line voltage, or E, all of which are similar). The diversity in the phenomenology of gamma-ray pulsars presents challenges to pulsar models. For example, double-pole models, often invoked by radio astronomers, now seem problematic. Most models have evolved significantly in response to the CGRO discoveries, and no comprehensive explanation for all the observations is yet in hand.