A rare outburst from the stealthy BeXRB system Swift J0549.7-6812

TL;DR

This paper reports the first detailed multi-wavelength observation of a rare, luminous outburst from the Be/X-ray binary Swift J0549.7-6812 in the LMC, revealing its binary period and quiescent behavior.

Contribution

It provides the first optical spectrum and comprehensive data analysis of this system's rare outburst, enhancing understanding of BeXRB variability and population estimates.

Findings

Detected a 6s pulsation during outburst

Estimated a binary period of 46.1 days

Showed the system is mostly in X-ray quiescence

Abstract

Swift J0549.7-6812 is an Be/X-ray binary system (BeXRB) in the Large Magellanic Cloud (LMC) exhibiting a 6s pulse period. Like many such systems the variable X-ray emission is believed to be driven by the underlying behaviour of the mass donor Be star. In this paper we report on X-ray observations of the brightest known outburst from this system which reached a luminosity of 8 x 10^37 erg/s. These observations are supported by contemporaneous optical photometric observations, the first reported optical spectrum, as well as several years of historical data from OGLE and GAIA. The latter strongly suggest a binary period of 46.1d. All the observational data indicate that Swift J0549.7-6812 is a system that spends the vast majority of its time in X-ray quiescence, or even switched off completely. This suggests that occasional observations may easily miss it, and many similar systems, and thereby underestimate the massive star evolution numbers for the LMC.