Variations in the pulsation and spectral characteristics of OAO 1657-415

TL;DR

This study analyzes the pulsation and spectral variability of the X-ray pulsar OAO 1657-415 using Suzaku observations, revealing significant intensity and spectral changes likely caused by a clumpy stellar wind, with no clear cyclotron line detected.

Contribution

First detailed broad-band spectral and pulsation analysis of OAO 1657-415 during a Suzaku observation highlighting variability due to stellar wind clumps.

Findings

Significant increase in count rate and hardness ratio during the observation.

High absorption and iron line equivalent widths in certain segments.

No conclusive evidence of a cyclotron line detected.

Abstract

We present broad-band pulsation and spectral characteristics of the accreting X-ray pulsar OAO 1657--415 with a 2.2 d long \emph{Suzaku} observation carried out covering its orbital phase range $\sim$ 0.12--0.34, with respect to the mid-eclipse. During the last third of the observation, the X-ray count rate in both the X-ray Imaging Spectrometer (XIS) and the HXD-PIN instruments increased by a factor of more than 10. During this observation, the hardness ratio also changed by a factor of more than 5, uncorrelated with the intensity variations. In two segments of the observation, lasting for $\sim$ 30--50 ks, the hardness ratio is very high. In these segments, the spectrum shows a large absorption column density and correspondingly large equivalent widths of the iron fluorescence lines. We found no conclusive evidence for the presence of a cyclotron line in the broad-band X-ray spectrum with \emph{Suzaku}. The pulse profile, especially in the XIS energy band shows evolution with time but not so with energy. We discuss the nature of the intensity variations, and variations of the absorption column density and emission lines during the duration of the observation as would be expected due to a clumpy stellar wind of the supergiant companion star. These results indicate that OAO 1657--415 has characteristics intermediate to the normal supergiant systems and the systems that show fast X-ray transient phenomena.