A Deep Multiwavelength View of Binaries in Omega Centauri

TL;DR

This study uses Chandra and HST to identify and classify X-ray binary stars in Omega Centauri, revealing diverse binary populations including accreting systems and active stars, with implications for stellar evolution in dense clusters.

Contribution

It provides the first comprehensive multiwavelength identification and classification of X-ray binaries in Omega Centauri, highlighting the variety of binary systems present in this massive globular cluster.

Findings

Detected 180 X-ray sources, with 45-70 likely associated with the cluster.

Identified 40 optical counterparts, including accreting binaries and active stars.

Discovered a group of redward of the red giant branch, possibly related to anomalous RGB or sub-subgiants.

Abstract

We summarize results of a search for X-ray-emitting binary stars in the massive globular cluster Omega Centauri (NGC 5139) using Chandra and HST. ACIS-I imaging reveals 180 X-ray sources, of which we estimate that 45-70 are associated with the cluster. We present 40 identifications, most of which we have obtained using ACS/WFC imaging with HST that covers the central 10'x10' of the cluster. Roughly half of the optical IDs are accreting binary stars, including 9 very faint blue stars that we suggest are cataclysmic variables near the period limit. Another quarter comprise a variety of different systems all likely to contain coronally active stars. The remaining 9 X-ray-bright stars are an intriguing group that appears redward of the red giant branch, with several lying along the anomalous RGB. Future spectroscopic observations should reveal whether these stars are in fact related to the anomalous RGB, or whether they instead represent a large group of "sub-subgiants" such as have been seen in smaller numbers in other globular and open clusters.