AstroSat UVIT counterparts of XMM-Newton X-ray sources in M31

¹ Jülich Supercomputing Centre, Forschungszentrum Jülich, Wilhelm-Johnen-Straße, 52428 Jülich, Germany
² Institute of Astronomy, National Tsing Hua University, No. 101 Sect. 2 Kuang-Fu Road, 30013 Hsinchu, Taiwan

^★ Corresponding authors: h.stiele@fz-juelich.de; akong@gapp.nthu.edu.tw

Received: 25 April 2025
Accepted: 14 October 2025

Abstract

Context. The X-ray source population of M 31 has been intensively studied in numerous investigations using X-ray telescopes. A notable example is the Deep XMM-Newton survey of M 31, which covered the entire D₂₅ ellipse down to a limiting luminosity of ~10³⁵ erg s⁻¹ in the 0.2–4.5 keV band.

Aims. We aimed to identify UV counterparts of the X-ray sources detected with XMM-Newton using the M 31 UVIT catalogue, which is based on AstroSat observations.

Methods. We studied the UV colour-colour diagram of the counterparts and fitted the UV spectra with simple phenomenological models that provide formally acceptable fits for about 35 per cent of the sources. We also analysed the X-ray spectral properties and their temporal evolution, based on the X-ray spectral fits of the XMM2Athena project. We updated the classification of XMM-Newton sources with UVIT counterparts, where necessary, by verifying cross-correlations with more recent studies.

Results. In the UV colour-colour diagram, we find one source that is classified as ‘hard’ in a region where, apart from this source, only foreground stars are located. Apart from foreground stars (23) and hard sources (13), sources located in globular clusters (6) are most numerous among those with UV counterparts that yield statistically acceptable fits with our simple models. We used UVIT data together with XMM-Newton optical monitor data to investigate the spectral energy distributions of the sources. This enabled us to suggest classifications for sources previously classified as hard sources in earlier studies. We provide an updated classification for 24 of the 291 XMM-Newton sources with UVIT counterparts. The list thus contains (including candidates of each class): 105 foreground stars, 22 globular clusters, 11 supernova remnants, nine X-ray binaries, of which six are suggested as high-mass X-ray binary candidates, eight galaxies, eight AGNs, seven supersoft source candidates, two sources that are either an X-ray binary or a galaxy, and another one that is either a galaxy or a globular cluster. A total of 107 sources can still only be classified as hard, while nine remain unclassified.