Suzaku observations of the HMXB 1A 1118-61

TL;DR

This paper analyzes Suzaku observations of the Be/X-ray transient 1A 1118-61 during outburst and decay, revealing spectral features, a cyclotron line, and changes in accretion geometry and viewing angle.

Contribution

It provides the first detailed broadband spectral analysis of 1A 1118-61 during outburst and decay, confirming a cyclotron line and exploring accretion geometry changes.

Findings

Detection of a ~5 keV cyclotron line

Possible harmonic at ~110 keV

Spectral and pulse profile changes during outburst

Abstract

We present broad band analysis of the Be/X-ray transient 1A 1118-61 by Suzaku at the peak of its 3rd observed outburst in January 2009 and 2 weeks later when the source flux had decayed by an order of magnitude. The continuum was modeled with a \texttt{cutoffpl} model as well as a compTT model, with both cases requiring an additional black body component at lower energies. We confirm the detection of a cyclotron line at ~5 keV and discuss the possibility of a first harmonic at ~110 keV. Pulse profile comparisons show a change in the profile structure at lower energies, an indication for possible changes in the accretion geometry. Phase resolved spectroscopy in the outburst data show a change in the continuum throughout the pulse period. The decrease in the CRSF centroid energy also indicates that the viewing angle on the accretion column is changing throughout the pulse period.