Monitoring Chandra observations of the quasi-persistent neutron star X-ray transient MXB 1659-29 in quiescence: The cooling curve of the heated neutron star crust

We have observed the quasi-persistent neutron star X-ray transient and eclipsing binary MXB 1659 - 29 in quiescence on three occasions with Chandra. The purpose of our observations was to monitor the quiescent behavior of the source after its last prolonged (similar to 2.5 yr) outburst that ended in 2001 September. The X-ray spectra of the source are consistent with thermal radiation from the neutron star surface. We found that the bolometric flux of the source decreased by a factor of 7 - 9 over the timespan of 1.5 yr between our first and last Chandra observations. The effective temperature also decreased, by a factor of 1.6 - 1.7. The decrease in time of the bolometric flux and effective temperature can be described using exponential decay functions, with e-folding times of similar to0.7 and similar to3 yr, respectively. Our results are consistent with the hypothesis that we observed a cooling neutron star crust that was heated considerably during the prolonged accretion event and that is still out of thermal equilibrium with the neutron star core. We could only determine upper limits for any luminosity contribution because of the thermal state of the neutron star core. The rapid cooling of the neutron star crust implies that it has a large thermal conductivity. Our results also suggest that enhanced cooling processes are present in the neutron star core.