Suzaku observation of the transient X-ray pulsar GRO J1008-57

TL;DR

This paper presents the first broad-band spectral analysis of the transient X-ray pulsar GRO J1008-57 using Suzaku, revealing energy-dependent pulse profiles, spectral components, and absorption features during its 2007 outburst.

Contribution

It provides the first broad-band spectral characterization of GRO J1008-57, including pulse profile energy dependence and detailed spectral modeling with multiple continuum models.

Findings

Pulsations detected up to 80-100 keV with a period of 93.737 s.

Energy-dependent pulse profiles: double peaked at soft X-rays, single peaked at hard X-rays.

Broad-band spectrum well described by three models, with a soft blackbody component and iron emission line.

Abstract

We report the timing and broad-band spectral properties of the Be transient high mass X-ray binary pulsar GRO J1008-57 using a Suzaku observation in the declining phase of its 2007 November-December outburst. Pulsations with a period of 93.737 s were clearly detected in the light curves of the pulsar up to the 80-100 keV energy band. The pulse profile was found to be strongly energy dependent, a double peaked profile at soft X-ray energy bands (< 8 keV) and a single peaked smooth profile at hard X-rays. The broad-band energy spectrum of the pulsar, reported for the first instance in this paper, is well described with three different continuum models viz. (i) a high energy cut-off power-law, (ii) a Negative and Positive power-law with EXponential cut-off (NPEX), and (iii) a partial covering power-law with high energy cut-off. Inspite of large value of absorption column density in the direction of the pulsar, a blackbody component of temperature ~0.17 keV for the soft excess was required for the first two continuum models. A narrow iron K_\alpha emission line was detected in the pulsar spectrum. The partial covering model, however, is found to explain the phase averaged and phase resolved spectra well. The dip like feature in the pulse profile can be explained by the presence of an additional absorption component with high column density and covering fraction at the same pulse phase. The details of the results are described in the paper.