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 .