Optical Follow-up of New SMC Wing Be/X-ray Binaries

TL;DR

This study identifies and characterizes optical counterparts of four Be/X-ray binaries in the SMC Wing, confirming their nature and variability, and provides insights into their spectral types and circumstellar disk behavior.

Contribution

It presents the discovery and optical classification of two previously unknown Be/X-ray binaries and analyzes their spectral and emission variability.

Findings

All counterparts are Be stars, with three matching known spectral distributions.

SXP101 is the latest spectral type identified among such binaries.

Most sources show highly variable circumstellar disks, linked to orbital periods.

Abstract

We investigate the optical counterparts of recently discovered Be/X-ray binaries in the Small Magellanic Cloud. In total four sources, SXP101, SXP700, SXP348 and SXP65.8 were detected during the Chandra Survey of the Wing of the SMC. SXP700 and SXP65.8 were previously unknown. Many optical ground based telescopes have been utilised in the optical follow-up, providing coverage in both the red and blue bands. This has led to the classification of all of the counterparts as Be stars and confirms that three lie within the Galactic spectral distribution of known Be/X-ray binaries. SXP101 lies outside this distribution becoming the latest spectral type known. Monitoring of the Halpha emission line suggests that all the sources bar SXP700 have highly variable circumstellar disks, possibly a result of their comparatively short orbital periods. Phase resolved X-ray spectroscopy has also been performed on SXP65.8, revealing that the emission is indeed harder during the passage of the X-ray beam through the line of sight.