Multitude of iron lines including a Compton-scattered component in OAO 1657 – 415 detected with<i>Chandra</i>

Abstract

We present a high-resolution X-ray spectrum of the accreting X-ray pulsar OAO 1657 -415 obtained with HETG + ACIS-S onboard Chandra, revealing the presence of a broad line component around 6.3 keV associated with the neutral iron K line at 6.4 keV. This is interpreted as a Compton shoulder arising from the Compton-scattering of the 6.4-keV fluorescence photons, making OAO 1657-415 the second accreting neutron star in which such a feature has been detected. A Compton shoulder reveals the presence of dense matter surrounding the X-ray source. We did not detect any periodicity in the light-curve and we obtained an upper limit of 2 per cent for the pulse fraction during this observation. This could be a result of the smearing of the pulses when X-ray photons are scattered from a large region around the neutron star. In addition to the Fe K , Fe K and Ni K lines already reported for this source, we report for the first time the presence of He-like and H-like iron emission lines at 6.7 and 6.97 keV in the first-order HETG spectrum. The detection of such ionized lines, indicative of a highly ionized surrounding medium, is rare in X-ray binaries.