4U 1626-67 and the character of highly compact binary X-ray sources

We present the results of new observations of the 7.7 5 X-ray pulsar 4U 1626 - 67 with SAS 3. We confirm the presence of quasi-periodic oscillations in the X-ray intensity and demonstrate that these oscillations have a preferred time scale of ∼1 × 103 s but are not strictly periodic. We also place stringent new upper limits on orbital motion of the X-ray star for orbital periods between 10 5 and 7 hr. This evidence lends further support to a model for this source composed of a neutron star that is accreting matter from a low-mass main-sequence dwarf or degenerate dwarf companion in a highly compact binary system. If this picture is correct, it is likely that the time scale of the quasi-periodic oscillations is directly related to the orbital period. We discuss other properties of this model, which may be applicable to other galactic X-ray sources. In particular, we analyze the temporal evolution of X-ray luminosity, Lx, from such a system under the assumption that the mass transfer is driven by the decay of the orbit due to gravitational radiation, and we show that high luminosities (Lx ≳ 1036 ergs s-1) may well be attained during some phases of the binary evolution.