AstroSat UVIT Detections of Chandra X-ray Sources in M31

TL;DR

This study combines ultraviolet and X-ray observations of M31 to identify and analyze sources, revealing their nature and associations with globular clusters, stars, and other objects, and providing insights into their emission mechanisms.

Contribution

It presents the first combined UVIT and Chandra catalog of M31 sources, analyzing their properties and associations, and distinguishing between different source types and emission origins.

Findings

67 sources detected in UV and X-ray bands.

UV emission from globular cluster sources is from blue horizontal branch stars.

High UV blackbody temperatures suggest massive stars in M31.

Abstract

An ultraviolet survey of M31 has been carried out during 2017-19 with the UVIT instrument onboard the AstroSat Observatory. Here we match the M31 UVIT source catalog (Leahy et al. 2020) with the Chandra source catalog (Vulic et al. 2016). We find 67 UVIT/Chandra sources detected in a varying number of UV and X-ray bands. The UV and X-ray photometry is analyzed using powerlaw and blackbody models. The X-ray types include 15 LMXBs and 5 AGNs. Cross-matches with catalogs of stars, clusters and other source types yield the following. 20 of the UVIT/Chandra sources match with M31 globular clusters and 9 with foreground stars. 3 more globular clusters and 2 more foreground stars are consistent with the UVIT source positions although outside the Chandra match radius of 1 arcsec. The UV emission of the UVIT/Chandra sources associated with globular clusters is consistent with emission from blue horizontal branch stars rather than from the X-ray source. The LMXBs in globular clusters are among the most luminous globular clusters in M31.Comparison with stellar evolutionary tracks shows that the UVIT/Chandra sources with high UV blackbody temperatures are consistent with massive (10 to 30 M$_{\odot}$) stars in M31.