Evidence for an accretion stream in 4U 1907+09

The X-ray binary system 4U 1907+09 contains an accretion-powered X-ray pulsar in an eccentric orbit around a massive early-type companion star. It has been previously suggested that a trailing gas stream along with strong stellar winds could describe the orbital variations in X-ray flux. In this work, 12 yr of X-ray observations from the Rossi X-ray Timing Explorer/All-Sky Monitor are analysed. We study both the X-ray flux and column density dependence on the orbital phase. The dynamics of the wind and stream are calculated for various companion star radii, system inclinations and eccentricities; the wind and stream model is parametrized by companion star rotation, stellar wind velocity, stream width and density. These parameters are taken as inputs to compute the expected accretion luminosity which is then compared to the observed flux versus the orbital phase. The column density along the line of sight to the neutron star is calculated using the best-fitting results from the flux analysis and verifies the high extinction in the direction of 4U 1907+09. We find that the stream component of accretion contributes significantly to describing the orbital variations in X-ray flux observed in 4U 1907+09. The best-fitting free parameters of our wind and stream model such as mass-loss rate, terminal wind velocity, companion star rotation speed and interstellar extinction are found to be consistent with literature values for 4U 1907+09. The best fit to the data was found at higher values of eccentricity (e = 0.31) and lower companion star radii (R(c) = 16-21 R(circle dot)).