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

TL;DR

This study reports new Chandra X-ray observations of the black hole binary MAXI J1659-152 in quiescence, revealing its low luminosity, spectral softening, and implications for binary evolution models.

Contribution

It provides the first detailed X-ray spectral and luminosity analysis of MAXI J1659-152 in quiescence, highlighting spectral softening and potential minimum luminosity related to orbital period.

Findings

Detected the source at the lowest flux level yet observed.

Observed spectral softening from index 1.55 to 2.5 in quiescence.

Suggests a possible minimum quiescent luminosity for black hole binaries.

Abstract

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)e-15 erg/cm^2/s, which corresponds to a luminosity of ~1.2e31 (d/ 6 kpc)^2 erg/s. 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 fractional Eddington luminosities of (0.18-6.2)e-5 (d/ 6 kpc)^2 (M/ 8 Msun), which is consistent with the ranges inferred for other sources.