High‐Resolution X‐Ray Spectroscopy of the Ultracompact LMXB Pulsar 4U 1626−67

[abridged] We report results from four recent observations of theultracompact LMXB pulsar 4U 1626-67. All the observations obtainedhigh-resolution X-ray spectra of the system, two from the Chandra X-rayObservatory using the HETGS, and two from the XMM-Newton Observatory using theRGS as well as the EPIC PN and MOS. These data allow us to study in detail theprominent Ne and O emission line complexes which make 4U 1626-67 unique amongLMXBs. The observations were spaced over a period of 3 years for a totalobserving time of 238 ks, allowing us to monitor the line regions as well asthe overall source flux, continuum spectrum, and timing properties. Thestructure of the emission lines and the ratios of the components of thehelium-like Ne IX and O VII triplets support the hypothesis that they areformed in the high-density environment of the accretion disk. We do not findany significant changes in the line widths or ratios over this time period,though we note that the line equivalent widths decrease. We are able to placeconstraints on the strengths of the Ne K, Fe L, and O K photoelectricabsorption edges, and find that the data do not require an overabundance of Neor O in the system relative to the expected ISM values. We find that the pulsaris still spinning down, and note that the pulse profile has changedsignificantly from what was found prior to the torque reversal in 1990,suggesting that this event may be linked to a change in the geometry of theaccretion column. The flux of 4U 1626-67 continues to decrease, in keeping withthe trend of the last approximately 30 years over which it has been observed.Taking into consideration current theory on disk stability, we expect that 4U1626-67 will enter a period of quiescence in 2-15 years.Comment: 10 pages, 7 figures, submitted to Ap