High Resolution Spectra of the Galactic Binary Pulsar 4U 1907+09

The accreting pulsar 4U1907-09 is one of ~35 high-mass X-ray binaries with known high magnetic field strengths. A detected cyclotron resonance scattering feature (CRSF) at ~19 keV leads to a field strength of ~2×10¹² G. The system consists of a slowly-rotating (~440 s) X-ray pulsar accreting from the stellar wind of an O8/9 supergiant. The X-ray pulsar is in a close (~8.37 d) elliptical orbit (e ~ 0.28) around its donor. We conducted an analysis of four High Energy Transmission Grating observations with the Chandra X-ray Observatory for a total of ~140 ks and one NuSTAR observation for 78 ks, at different orbital phases, to probe the variation of the absorbing column around the orbit. We measure line fluorescence at Fe K and possibly Si K and other lower Z elements to study dense clumps in the wind of the pulsar’s companion star. The details of the NuSTAR observation are used to determine the higher energy continuum beyond 10 keV.