1996A&AS..120C.269B

The X-ray burster 4U 1915-05 was observed from January 24, 1995 through February 14, 1995 by the OSSE experiment aboard the Compton Gamma-Ray Observatory. While previous 1-30keV observations suggested that 4U 1915-05 could be easily detectable by OSSE, no hard X-ray emission was detected, and stringent upper limits have been derived. The upper limits are 1.4x10^–4photons/cm²/s and 6.5 10^–5photons/cm²/s in the 50-98keV and 98-158keV bands, respectively. This translates into luminosity limits of 1.4x10³⁵erg/s and 1.7x10³⁵erg/s assuming a source distance of 9.3kpc. Based on the existing data, we define three distinct X-ray spectral states associated with three distinct intensity states of the source: high/very soft state, an intermediate/hard state and a low/(possibly) soft state with 1-37keV X-ray luminosities of 1.2x10³⁷, 6-7x10³⁶ and 3x10³⁶erg/s, respectively. For these three states we have converted the OSSE upper limits into upper limits on any energy cutoff present in the X-ray to hard X-ray source spectrum. These upper limits range from ∼10keV up to 50keV depending on source the intensity and spectral shape assumed. We found that they are consistent with the source being either in the high state or low state at the time of the OSSE observations. For the high state, the OSSE upper limits imply that less than ∼4% of the total luminosity is radiated in the hard X-ray band. On the other hand, the source should have been detected in its intermediate state if the cutoff energy, as inferred from Ginga observations, is indeed larger than ∼50keV.