A Chandra Observation of the Neutron Star X-Ray Transient and Eclipsing Binary MXB 1659-29 in Quiescence

After almost 2.5 yr of actively accreting, the neutron star X-ray transient and eclipsing binary MXB 1659-29 returned to quiescence in 2001 September. We report on a Chandra observation of this source taken a little over a month after this transition. The source was detected at an unabsorbed 0.5-10 keV flux of only (2.7-3.6)×10^-13 ergs cm^-2 s^-1, which implies a 0.5-10 keV X-ray luminosity of approximately (3.2-4.3)×10³³(d/10kpc)² ergs s^-1, with d the distance to the source in kiloparsecs. Its spectrum had a thermal shape and could be well fitted by either a blackbody with a temperature kT of ~0.3 keV or a neutron star atmosphere model with a kT of ~0.1 keV. The luminosity and spectral shape of MXB 1659-29 are very similar to those observed of the other neutron star X-ray transients when they are in their quiescent state. The source was variable during our observation, exhibiting a complete eclipse of the inner part of the system by the companion star. Dipping behavior was observed before the eclipse, likely due to obscuration by an extended feature in the outer part of a residual accretion disk. We discuss our observation in the context of the cooling neutron star model proposed to explain the quiescent properties of neutron star X-ray transients.