# SXP7.92: A Recently Rediscovered Be/X-ray Binary in the Small Magellanic Cloud, Viewed Edge On

## TL;DR

This study provides a comprehensive optical and X-ray analysis of the 2013 outburst of the Be/X-ray binary SXP7.92 in the Small Magellanic Cloud, revising its position, identifying its optical counterpart, and interpreting its edge-on viewing geometry.

## Contribution

It offers the first detailed optical spectral classification and orbital period measurement, and interprets the system's edge-on geometry and disc precession mechanisms.

## Key findings

- Revised source position with 1.5 arcsec accuracy
- Identified optical counterpart as B1Ve star with shell profile
- Derived orbital period of 40 days and edge-on viewing geometry

## Abstract

We present a detailed optical and X-ray study of the 2013 outburst of the Small Magellanic Cloud Be/X-ray binary SXP7.92, as well as an overview of the last 18 years of observations from OGLE, RXTE, Chandra and XMM-Newton. We revise the position of this source to RA(J2000)=00:57:58.4, Dec(J2000)=-72:22:29.5 with a $1\sigma$ uncertainty of 1.5 arcsec, correcting the previously reported position by Coe et al. (2009) by more than 20 arcminutes. We identify and spectrally classify the correct counterpart as a B1Ve star. The optical spectrum is distinguished by an uncharacteristically deep narrow Balmer series, with the H$\alpha$ line in particular having a distinctive shell profile, i.e. a deep absorption core embedded in an emission line. We interpret this as evidence that we are viewing the system edge on and are seeing self obscuration of the circumstellar disc. We derive an optical period for the system of 40.0$\pm0.3$ days, which we interpret as the orbital period, and present several mechanisms to describe the X-ray/Optical behaviour in the recent outburst, in particular the "flares" and "dips" seen in the optical light curve, including a transient accretion disc and an elongated precessing disc.