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) x 10(-15) erg s(-1) cm(-2), which corresponds to a luminosity of similar to 1.2 x 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 similar to 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 (similar to 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 similar to 2-4 times longer. This could imply that a minimum quiescent luminosity may exist for black hole X-ray binaries, around orbital periods of similar to 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) x 10(-5) (d/6 kpc)(2) (M/8 M-circle dot) L-Edd, which is consistent with the ranges inferred for other sources.