Monitoring the black hole binary GRS 1758–258 with INTEGRAL and RXTE

Abstract

The microquasar GRS 1758−258 is one of only three persistent black hole binaries that spend most of their time in the hard spectral state. It therefore provides the rare opportunity for an extensive long term study of this state. INTEGRAL has been monitoring the source since 2003 spring during two ∼ 3 months long Galactic Center viewing epochs each year, currently amounting to 11 epochs, including 2008 spring. Quasi-simultaneous RXTE observations are available as well. We present an analysis of the epoch averaged broad band spectra. Results include the observation of clearly anti-correlated variations of the 3–300 keV flux and spectral softness, and of the occurrence a faint soft state, i.e., an extreme example of hysteretic behavior. The hard source spectrum and long exposures allow us to extend the analysis of the average hard state spectrum to ∼ 800 keV, resulting in the possible detection of a non-thermal Comptonization component. We also discuss the peculiar hardness-intensity diagram of this source.