The x-ray properties of the black hole transient MAXI J1659-152 in quiescence

We present new Chandra X-ray observations of the transient black hole X-ray binary MAXI J1659-152 in quiescence. These observations were made more than one year after the end of the source's 2010-2011 outburst. We detect the source at a 0.5-10 keV flux of 2.8(8) × 10^-15 erg s^-1 cm ^-2, which corresponds to a luminosity of ∼1.2 × 10 ³¹ (d/6 kpc)² erg s^-1. This level, while being the lowest at which the source has been detected, is within factors of ∼2 of the levels seen at the end of the initial decay of the outburst and soon after a major reflare of the source. The quiescent luminosity of MAXI J1659-152, which is the shortest-orbital-period black hole X-ray binary (∼2.4 hr), is lower than that of neutron-star X-ray binaries with similar periods. However, it is higher than the quiescent luminosities found for black hole X-ray binaries with orbital periods ∼2-4 times longer. This could imply that a minimum quiescent luminosity may exist for black hole X-ray binaries, around orbital periods of ∼5-10 hr, as predicted by binary-evolution models for the mass transfer rate. Compared to the hard state, we see a clear softening of the power-law spectrum in quiescence, from an index of 1.55(4) to an index of 2.5(4). We constrain the luminosity range in which this softening starts to (0.18-6.2) × 10^-5 (d/6 kpc)² (M/8 M _⊙) L Edd, which is consistent with the ranges inferred for other sources.