OSSE and RXTE observations of GRS 1915+105: Evidence for nonthermal comptonization

GRS 1915+105 was observed by the Oriented Scintillation Spectroscopy Experi ment (OSSE) aboard the Compton Gamma Ray Observatory nine times in 1995-200 0, and eight of those observations were simultaneous with those by the Ross i X-Ray Timing Explorer (RXTE). We present an analysis of all of the OSSE d ata and of two RXTE-OSSE spectra with the lowest and highest X-ray fluxes. The OSSE data show a power-law-like spectrum extending up to greater than o r similar to 600 keV without any break. We interpret this emission as stron g evidence for the presence of nonthermal electrons in the source. The broa dband spectra cannot be described by either thermal or bulk-motion Comptoni zation, whereas they are well described by Comptonization in hybrid thermal /nonthermal plasmas.