The enigmatic black-hole candidate and X-ray transient IGR J17091-3624 in its quiescent state as seen with XMM-Newton

Abstract: We report on two short XMM-Newton observations performed in August 2006 and February 2007 during the quiescence state of the enigmatic black hole candidate system IGR J17091-3624. During both observations the source was clearly detected. Although the errors on the estimated fluxes are large, the source appears to be brighter by several tens of percents during the February 2007 observation compared to the August 2006 observation. During both observations the 2-10 keV luminosity of the source was close to ~10^{33} erg/s for an assumed distance of 10 kpc. However, we note that the distance to this source is not well constrained and it has been suggested it might be as far as 35 kpc which would result in an order of magnitude higher luminosities. If the empirically found relation between the orbital period and the quiescence luminosity of black hole transients is also valid for IGR J17091-3624, then we can estimate an orbital period of >100 hours (>4 days) for a distance of 10 kpc but it could be as large as tens of days if the source is truly much further away. Such a large orbital period would be similar to GRS 1915+105 which has an orbital period of ~34 days. Orbital periods this large could possibly be connected to the fact that both sources exhibit the same very violent and extreme rapid X-ray variability which has so far not yet been seen from any other black hole system. Alternatively the orbital period of IGR J17091-3624 might be more in line with the other systems (<100 hours) but we happened to have observed the source in an episode of elevated accretion which was significantly higher than its true quiescent accretion rate. In that case, the absence or presence of extreme short-term variability properties as is seen for IGR J17091-3624 and GRS 1915+105 is not related to the orbital periods of these black hole systems.