EGRET observations of X-ray binaries

Several known low- and high-mass X-ray binaries with orbital periods between tens of minutes and several days were examined in EGRET data. Instrument thresholds for orbital modulation were calculated, and appropriate observations were analyzed with new software which uses maximum likelihood methods for signal detection, fully accounting for variations in EGRET exposure, as well as the structured gamma-ray background and the energy-dependent point-spread function. While some sources were not detected by EGRET at all, several were marginally detected with less than 5 σ significance. However, due to significant background and low statistics, the observations were unable to constrain the modulation fraction of any of the sources.