CXOU J005047.9-731817: a 292-s X-ray binary pulsar in the Small Magellanic Cloud

TL;DR

This paper reports the discovery and analysis of a 292-second X-ray pulsar in the Small Magellanic Cloud, characterized by a transient outburst, spectral properties, and a likely Be star companion, contributing to understanding of Be X-ray binaries.

Contribution

It presents the first detailed observation of a transient 292-s X-ray pulsar in the SMC, including spectral analysis, optical identification, and classification as a Be X-ray binary outburst.

Findings

Detected a 292-s pulsar with a two-week outburst in the SMC.

Spectral analysis shows an absorbed power law with photon index ~0.6.

Optical spectra suggest a late-Oe or early-Be star companion.

Abstract

We report on the discovery of a transient X-ray pulsars, located in the Small Magellanic Cloud, with a pulse period of 292 s. A series of Chandra pointings fortuitously recorded in 2010 April-May the occurrence of a two-weeks-long outburst, during which the source luminosity increased by a factor of about 100, reaching a peak of ~1E36 erg/s (for a distance of 61 kpc). Complex-shape and energy-dependent pulsations were detected close to the outburst peak and during the very first part of its decay phase. During the outburst, the phase-averaged spectrum of the pulsar was well described by an absorbed power law with photon index ~0.6, but large variations as a function of phase were present. The source was also detected by Chandra several times (during 2002, 2003, 2006, and 2010) at a quiescent level of ~1E34 erg/s. In 2012 we performed an infrared photometric follow-up of the R ~ 15 mag optical counterpart with the ESO/VLT and a spectroscopic observation by means of the CTIO telescope. The optical spectra suggest a late-Oe or early-Be V-III luminosity-class star, though a more evolved companion cannot be ruled out by our data (we can exclude a luminosity class I and a spectral type later than B2). Finally, we show that the outburst main parameters (duration and peak luminosity) can be accounted for by interpreting the source transient activity as a type I outburst in a Be X-ray binary.