The Be/neutron star system Swift J004929.5-733107 in the Small Magellanic Cloud -- X-ray characteristics and optical counterpart candidates

TL;DR

This paper identifies and characterizes Swift J004929.5-733107 as a Be star-neutron star X-ray binary in the SMC, revealing its optical counterpart, spectral features, and long binary period through multi-wavelength observations.

Contribution

It presents the first confirmed optical counterpart and detailed characterization of Swift J004929.5-733107 as a BeXRB with a long binary period, using combined X-ray and optical data.

Findings

Optical counterpart is a Be star with strong Hα emission.

Detected 413-day periodic modulation in optical light curve.

X-ray spectrum fitted with photon index ~0.93, luminosity ~10^35 erg/s.

Abstract

Swift J004929.5-733107 is an X-ray source in the Small Magellanic Cloud (SMC) that has been reported several times, but the optical counterpart has been unclear due to source confusion in a crowded region of the SMC. Previous works proposed [MA93] 302 as the counterpart, however we show here, using data obtained from the S-CUBED project, that the X-ray positio is inconsistent with that object. Instead we propose a previously unclassified object which has all the indications of being a newly identified Be star exhibiting strong HU emission. Evidence for the presence of significant I-band variability strongly suggest that this is, in fact, a Be type star with a large circumstellar disk. Over 18 years worth of optical monitoring by the OGLE project reveal a periodic modulation at a period of 413d, probably the binary period of the system. A SALT optical spectrum shows strong Balmer emission and supports a proposed spectral classification of B1-3 III-IVe. The X-ray data obtained from the S-CUBED project reveal a time-averaged spectrum well fitted by a photon index = 0.93 pm 0.16. Assuming the known distance to the SMC the flux corresponds to a luminosity 10E35 erg/s. All of these observational facts suggest that this is confirmed as a Be star-neutron star X-ray binary (BeXRB) in the SMC, albeit one with an unusually long binary period at the limits of the Corbet Diagram.