http://arxiv.org/abs/1812.06108
We present a high resolution X-ray spectrum of the accreting X-ray pulsar, OAO 1657–415 with HETG+ACIS-S onboard \emph{Chandra},revealing the presence of a broad line component around $\sim$ 6.3 keV associated with the neutral iron K$\alpha$ line at 6.4 \rm{keV}. This is interpreted as Compton shoulder arising from the Compton scattering of the 6.4 \rm{keV} fluorescence photons making OAO 1657–415 the second accreting neutron star where such a feature is detected. A Compton shoulder reveals the presence of dense matter surrounding the X-ray source. We did not detect any periodicity in the lightcurve and obtained an upper limit of $\sim$ 2\% for the pulse fraction during this observation. This could be due to 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$\alpha$, Fe $K_\beta$ and Ni $K_\alpha$ lines already reported for this source, we also report for the first time, the presence of He-like and H-like iron emission lines at 6.7 and 6.97 \rm{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.
P. Pradhan, G. Raman and B. Paul
Tue, 18 Dec 18
91/91
Comments: 16 pages, 5 figures, accepted for publication in MNRAS
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