Timing and broad band spectroscopy of 1A 1118-61 with Suzaku

TL;DR

This study analyzes the timing and broad-band spectral properties of the transient Be X-ray binary pulsar 1A 1118-61 during outburst using Suzaku, revealing energy-dependent pulse profiles, a soft excess, and a cyclotron feature indicating a high magnetic field.

Contribution

First detailed pulse phase-resolved spectral analysis of 1A 1118-61 during outburst, identifying phase-dependent spectral variations and a cyclotron line at ~50 keV.

Findings

Energy-dependent pulse profiles with multiple peaks at low energies

Detection of a phase-dependent soft excess below 3 keV

Observation of a broad cyclotron feature at ~50 keV

Abstract

We present a timing and broad-band pulse phase resolved spectral analysis of the transient Be X-ray binary pulsar 1A 1118-61 observed during its outburst in January 2009 using Suzaku observations. Suzaku observations were made twice, once at the peak of the outburst, and once 13 days later at its declining phase.Pulse profiles from both observations exhibit strong energy dependence with several peaks at low energies and a single peak above ~10 keV. A weak, narrow peak is detected at the main dip of the pulse profiles from both observations in the energy bands below 3 keV, indicating the presence of a phase dependent soft excess in the source continuum. The broad-band energy spectrum of the pulsar could be fitted well with a partial covering cutoff power-law model and a narrow iron fluorescence line. We also detect a broad cyclotron feature at ~50 keV from both observations which is a feature common for accretion powered pulsars with high magnetic field strengths. Pulse phase-resolved spectral analysis shows an increase in the absorption column density of the partial covering component, as well as variation in the covering fraction at the dips of the pulse profiles, that naturally explains the energy dependence of the same. The cyclotron line parameters also show significant variation with pulse phase with a ~ 10 keV variation in the cyclotron line energy and a variation in depth by a factor of three. This can be explained either as the effect of different viewing angles of the dipole field at different pulse phases, or a more complex underlying magnetic field geometry