On the origin of black hole X-ray emission in quiescence: Chandra observations of XTE J1550-564 and H 1743-322

We report the results of observations of the black hole binaries XTE J1550-564 and H 1743-322 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 2 $\times$ 10$^{32}$ (d/5 kpc)$^2$ erg s$^{-1}$ for XTE J1550-564 and 9 $\times$ 10$^{31}$ (d/8 kpc)$^2$ erg s$^{-1}$ for H 1743-322. 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 $\pm$ 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 hours. 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 hours appear to have hard spectra in quiescence and their behaviour may be consistent with hardening in quiescence.