Osse monitoring of the Galactic center region

The Oriented Scintillation Spectrometer Experiment (OSSE) on board the Compton Gamma-Ray Observatory (CGRO) has observed the Galactic center and Galactic plane for more than 400 days over the past 5 years. In the Galactic center region alone, we have over 200 days of observations with better than or equal to 40% of the maximum OSSE response. These observations provide unprecedented sensitivity to transient sources in the 50 keV to 2 MeV range. Analysis of the data yields evidence for daily variability below 150 keV, consistent with previous measurements of the hard X-ray sky. However, above 150 keV, no daily variability is seen to a sensitivity of about 0.01 photons cm^-2 s^-1 MeV^-1 or better. Our results call into question previous claims of such variability, and, in particular, reports of variable ∼0.5 MeV bumps. Such features, if they exist, will require further observations to be confirmed. It is important to verify or rule out the existence of these features as they bear directly on models of black hole accretion disks as well as on models which explain the variability of narrow lines in the vicinity of dense molecular clouds.