AstroSat observations of eclipsing high mass X-ray binary pulsar OAO 1657-415

TL;DR

This study analyzes AstroSat observations of the high mass X-ray binary pulsar OAO 1657-415, revealing pulsation behavior, spectral features, and orbital phase-dependent variability, contributing to understanding accretion processes in such systems.

Contribution

First detection of pulsations during specific orbital phases and detailed spectral analysis of OAO 1657-415 using AstroSat data.

Findings

Pulsations detected at 37.0375 s during July observation.

Pulse profile shows a broad peak with a dip, varying with energy.

Spectral model includes absorbed power-law, iron line, and blackbody.

Abstract

We present the results obtained from analysis of two AstroSat observations of the high mass X-ray binary pulsar OAO 1657-415. The observations covered 0.681-0.818 and 0.808-0.968 phases of the $\sim$10.4 day orbital period of the system, in March and July 2019, respectively. Despite being outside the eclipsing regime, the power density spectrum from the first observation lacks any signature of pulsation or quasi-periodic oscillations. However, during July observation, X-ray pulsations at a period of 37.0375 s were clearly detected in the light curves. The pulse profiles from the second observation consist of a broad single peak with a dip-like structure in the middle across the observed energy range. We explored evolution of the pulse profile in narrow time and energy segments. We detected pulsations in the light curves obtained from 0.808--0.92 orbital phase range, which is absent in the remaining part of the observation. The spectrum of OAO 1657-415 can be described by an absorbed power-law model along with an iron fluorescent emission line and a blackbody component for out-of-eclipse phase of the observation. Our findings are discussed in the frame of stellar wind accretion and accretion wake at late orbital phases of the binary.