XMM-Newton observations of two black hole X-ray transients in quiescence

We report on XMM-Newton observations of GRO J1655-40 and GRS 1009-45, which are two black hole X-ray transients currently in their quiescent phase. GRO J1655-40 was detected with a 0.5-10 keV luminosity of 5.9 x 10³¹ erg s^-1 with 20% accuracy, taking into account both statistical errors and uncertainty on the distance. This luminosity is comparable to a previous Chandra measurement, but ten times lower than the 1996 ASCA value, most likely obtained when the source was not yet in a true quiescent state. Unfortunately, XMM-Newton failed to detect GRS 1009-45. A stringent upper limit of 8.9 x 10³⁰ erg s^-1 was derived by combining data from the EPIC-MOS and PN cameras.

The X-ray spectrum of GRO J1655-40 is very hard as it can be fitted with a power law model of photon index ~ 1.3+/- 0.4. Similarly hard spectra have been observed from other systems; these rule out coronal emission from the secondary or disk flares as the origin of the observed X-rays. On the other hand, our observations are consistent with the predictions of the disc instability model in the case that the accretion flow forms an advection dominated accretion flow (ADAF) at distances less than a fraction ( ~ 0.1-0.3) of the circularization radius. This distance corresponds to the greatest extent of the ADAF that is thought to be possible.