Hiding in plain sight: Chandra observations of the quiescent neutron star 4U 2129+47 in eclipse

During a previous outburst phase, the neutron star 4U 2129+47 exhibited evidence for a spatially extended accretion disk corona via broad, partial X-ray eclipses occurring periodically on the 5.24 hr orbit. Since 1983, however, 4U 2129+47 has entered a quiescent state several orders of magnitude fainter. We have performed a 37 ks (≈two binary orbits) Chandra observation of 4U 2129+47 to determine whether an extended coronal structure also exists in quiescence. Total eclipses are found, and the rapidity of the eclipse ingress and egress is used to place upper limits on the size of the X-ray source. The spectrum is found to be comprised of a soft component plus a fairly hard power-law tail. The former is seen to be sinusoidally modulated over the orbital period in a manner consistent with neutral column variations, possibly due to the interaction of an accretion stream with a (small) disk about the neutron star. We fit realistic neutron star atmosphere models to the soft spectra and comment on the consistency of the spectra with cooling neutron star models. It has been previously suggested that the 4U 2129+47 system is a hierarchical triple, with the outer body being an F star. We use differential astrometry to show that the X-ray point source and F star are spatially coincident to within O″.1. We further compare newly determined upper limits for the extrinsic neutral column to the reddening of the F star. Finally, we discuss how future X-ray observations can further constrain models of quiescent neutron star emission as well as directly verify the triple hypothesis.