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

摘要

We have observed the quasi-persistent neutron-star X-ray transient andeclipsing binary MXB 1659-29 in quiescence on three occasions with Chandra. Thepurpose of our observations was to monitor the quiescent behavior of the sourceafter its last prolonged (~2.5 years) outburst which ended in September 2001.The X-ray spectra of the source are consistent with thermal radiation from theneutron-star surface. We found that the bolometric flux of the source decreasedby a factor of 7-9 over the time-span of 1.5 years between our first and lastChandra observations. The effective temperature also decreased, by a factor of1.6-1.7. The decrease in time of the bolometric flux and effective temperaturecan be described using exponential decay functions, with e-folding times of 0.7and ~3 years, respectively. Our results are consistent with the hypothesis thatwe observed a cooling neutron-star crust which was heated considerably duringthe prolonged accretion event and which is still out of thermal equilibriumwith the neutron-star core. We could only determine upper-limits for anyluminosity contribution due to the thermal state of the neutron-star core. Therapid cooling of the neutron-star crust implies that it has a large thermalconductivity. Our results also suggest that enhanced cooling processes arepresent in the neutron-star core.