Determining the nature of IC 10 X-2: A comprehensive study of the Optical/Infrared emission from an extragalactic BeHMXB

TL;DR

This study analyzes optical and infrared data of the extragalactic Be high-mass X-ray binary IC 10 X-2 over five years, revealing its orbital period, flaring behavior, and the nature of its stellar companion, enhancing understanding of such systems in low-metallicity galaxies.

Contribution

It provides the first detailed optical/IR characterization of IC 10 X-2, identifying its orbital period, flaring mechanisms, and the role of clumps, advancing knowledge of extragalactic BeHMXBs.

Findings

Detected a ~26.5-day orbital period.

Identified flares caused by accretion onto the neutron star.

Observed color variations due to clumps and orbital effects.

Abstract

We present a comprehensive analysis of the optical and infrared (IR) properties of high-mass X-ray binary (HMXB) IC 10 X-2, classified as a super-giant HMXB and super-fast X-ray transient (SFXT) by previous work. Our analysis of regular (daily and weekly) observations by both the Zwicky Transient Factory and Las Cumbres Observatory over a ~5 year period indicates both periodic flares and variations in the apparent magnitude and color with a $\sim26.5$~d period -- likely the orbital period of this binary system. The periodic flaring suggests the stellar companion is a Be star, with flares resulting from increased accretion onto the neutron star when it enters the stellar decretion disk. The periodic variations in the optical/IR brightness and color likely result from orbital variations in the Hydrogen column density along the line of sight or a transient accretion disk around the neutron star. Lastly, the numerous, short duration, episodes where IC 10 X-2 is significantly ``redder'' or ``bluer'' than normal likely result from from clumps within this system -- which can accrete onto the neutron star (causing IC 10 X-2 to appear bluer), or pass through the line of sight (causing IC 10 X-2 to appear redder). These results substantially increase our understanding of the evolution of this source, a significant source of ionizing photons in its host galaxy IC 10, a low mass, metal-poor starburst galaxy similar in many respect to those thought to be common in the early Universe.