Phase‐dependent Spectral Variability in 4U 1907+09

Abstract

We report on ASCA, RXTE, and archival observations of the high-mass X-ray binary pulsar 4U 1907+09. Spectral measurements of the absorption and flux were made at all phases of the X-ray pulsar orbit, including the first spectral measurements of an extended period of low flux during two of the ASCA observations. We find that a simple spherical wind model can fit the time-averaged light curve as measured by the RXTE All-Sky Monitor but does not fit the observed changes in the absorption column or account for the existence of the phase-locked secondary flare. An additional model component consisting of a trailing stream can account for the variations in column depth. However, these models favor a high inclination angle for the system, suggesting a companion mass more consistent with an identification as a Be star. In this case, an equatorially enhanced wind and inclined neutron star orbit may be a more appropriate interpretation of the data.