AChandraObservation of the Neutron Star X‐Ray Transient and Eclipsing Binary MXB 1659−29 in Quiescence

After almost 2.5 years of actively accreting, the neutron star X-raytransient and eclipsing binary MXB 1659-29 returned to quiescence in 2001September. We report on a Chandra observation of this source taken a littleover a month after this transition. The source was detected at an unabsorbed0.5-10 keV flux of only (2.7 - 3.6) x 10^{-13} ergs/s/cm/cm, which implies a0.5-10 keV X-ray luminosity of approximately (3.2 - 4.3) x 10^{33} (d/10 kpc)^2erg/s, with d the distance to the source in kpc. Its spectrum had a thermalshape and could be well fitted by either a blackbody with a temperature kT of0.3 keV or a neutron star atmosphere model with a kT of ~0.1 keV. Theluminosity and spectral shape of MXB 1659-29 are very similar to those observedof the other neutron star X-ray transients when they are in their quiescentstate. The source was variable during our observation, exhibiting a completeeclipse of the inner part of the system by the companion star. Dipping behaviorwas observed before the eclipse, likely due to obscuration by an extendedfeature in the outer part of a residual accretion disk. We discuss ourobservation in the context of the cooling neutron star model proposed toexplain the quiescent properties of neutron star X-ray transients.Comment: Accepted for publication in ApJ. It will be published in the 10 September 2003 issu