Be/X-ray binary SXP6.85 undergoes large Type II outburst in the Small Magellanic Cloud

TL;DR

This study analyzes a significant Type II outburst of the Be/X-ray binary SXP6.85 in the Small Magellanic Cloud, correlating X-ray, optical, and IR data to understand disk dynamics and accretion processes.

Contribution

It provides detailed timing and spectroscopic analysis of the outburst, linking optical maximum with disk growth and revealing complex accretion geometry changes.

Findings

Outburst coincides with optical maximum, indicating disk expansion.

IR photometry and H-alpha spectroscopy support disk size increase.

Evidence of inhomogeneous accretion regions on the neutron star.

Abstract

The Small Magellanic Cloud (SMC) Be/X-ray binary pulsar SXP6.85 = XTE J0103-728 underwent a large Type II outburst beginning on 2008 August 10. The source was consistently seen for the following 20 weeks (MJD = 54688 - 54830). We present X-ray timing and spectroscopic analysis of the source as part of our ongoing Rossi X-ray Timing Explorer (RXTE) monitoring campaign and INTEGRAL key programme monitoring the SMC and 47 Tuc. A comparison with the Optical Gravitational Lensing Experiment (OGLE) III light curve of the Be counterpart shows the X-ray outbursts from this source coincide with times of optical maximum. We attribute this to the circumstellar disk increasing in size, causing mass accretion onto the neutron star. Ground based IR photometry and H-alpha spectroscopy obtained during the outburst are used as a measure of the size of the circumstellar disk and lend support to this picture. In addition, folded RXTE light curves seem to indicate complex changes in the geometry of the accretion regions on the surface of the neutron star, which may be indicative of an inhomogeneous density distribution in the circumstellar material causing a variable accretion rate onto the neutron star. Finally, the assumed inclination of the system and H-alpha equivalent width measurements are used to make a simplistic estimate of the size of the circumstellar disk.