Spectral States of the X-Ray Binary IGR J17091–3624 Observed by INTEGRAL and RXTE

IGR J17091-3624 was discovered in 2003 April by IBIS, the gamma-ray imager on board the INTEGRAL satellite, during its Galactic Center Deep Exposure program. The source was initially detectable only in the 40-100 keV range but after 2 days was also detected in the 15-40 keV range. Its flux had by then increased to 40 and 25 mcrab in the 15-40 and 40-100 keV bands, respectively. RXTE observed the source simultaneously on 2003 April 20, with an effective exposure of 2 ks. We report here the spectral and temporal evolution of the source, which shows a transition between the hard and soft states. We analyze in detail the RXTE/INTEGRAL Comptonized spectrum of the hard state, as well as the JEM-X detection of a blackbody component during the source softening. Even though the source spectral behavior and time variability show a similarity with the outburst of the black hole candidate IGR J17464-3213 (=H1743-322), observed by INTEGRAL in 2003, the nature of its compact object (BH vs. NS) remains controversial.