# Multitude of iron lines including a Compton scattered component in OAO 1657--415 detected with Chandra

## TL;DR

This paper presents a high-resolution Chandra spectrum of OAO 1657--415, revealing a Compton shoulder and multiple iron lines, indicating dense and highly ionized surrounding matter, with no detected pulsations during observation.

## Contribution

First detection of a Compton shoulder in an accreting neutron star, along with new ionized iron lines, providing insights into the dense and ionized environment around the pulsar.

## Key findings

- Detection of a broad Fe Kα line with a Compton shoulder.
- First observation of He-like and H-like iron lines in this source.
- No periodicity detected, suggesting pulse smearing due to scattering.

## Abstract

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.