2006ApJ...636..971C

We report the results of observations of the black hole binaries XTE J1550-564 and H1743-322 in their quiescent state using the Chandra X-Ray Observatory. Both sources are detected at their faintest level of X-ray emission ever observed with a 0.5-10 keV unabsorbed luminosity of 2x10³² (d/5 kpc)² ergs/s for XTE J1550-564 and 9x10³¹ (d/8 kpc)² ergs/s for H1743-322. These luminosities are in the upper range compared to the faintest levels observed in other black hole systems, possibly related to residual accretion for these sources with frequent outbursts. For XTE J1550-564, the Chandra observations also constrain the X-ray spectrum, as a fit with an absorbed power-law model yields a photon index of 2.25±0.08, clearly indicating a softening of the X-ray spectrum at lower luminosities compared to the standard hard state. Similar softening at low luminosity is seen for several black hole transients with orbital periods less than 60 hr. Most of the current models of accreting black holes are able to reproduce such softening in quiescence. In contrast, we find that systems with orbital periods longer than 60 hr appear to have hard spectra in quiescence, and their behavior may be consistent with hardening in quiescence.