2007ApJ...660..605K

We report results from four recent observations of the ultracompact LMXB pulsar 4U 1626-67. These observations obtained high-resolution X-ray spectra with Chandra HETGS and XMM RGS, allowing us to study in detail the prominent Ne and O emission-line complexes. The observations were spaced over a period of 3 yr, enabling us to monitor the line regions as well as the overall spectral and timing properties. The structure of the emission lines and the helium-like Ne IX and O VII triplets support the hypothesis that they are formed in the high-density environment of an accretion disk. We do not find any significant variability in the line widths or ratios, though we note that the equivalent widths decrease. Using the most recent calibration products, we are able to place constraints on the strengths of the Ne K, Fe L, and O K photoelectric absorption edges. In contrast to our earlier analysis, the data do not require an overabundance of Ne or O in the system relative to the expected ISM values. We find that the pulsar is still spinning down but note that the pulse profile has changed significantly from what was found prior to the torque reversal in 1990, suggesting a change in the geometry of the accretion column. The flux of 4U 1626-67 continues to decrease, following the trend of the last ~30 yr over which it has been observed. Taking into consideration current theory on disk stability, we expect that 4U 1626-67 will enter a period of quiescence in 2-15 yr.