X-ray source variability study of the M 31 central field using Chandra HRC-I

TL;DR

This study systematically analyzed long-term X-ray variability in M 31's central field using Chandra HRC-I data from 1999 to 2012, creating a comprehensive catalogue of 318 sources, including X-ray binaries, supernova remnants, and novae.

Contribution

It provides the first extensive catalogue of X-ray sources in M 31 with detailed variability data, revealing new sources and insights into their nature and distribution.

Findings

Identified 318 X-ray sources, 28 of which are new.

Classified 115 X-ray binary candidates with high variability.

Discovered a new super-soft X-ray outburst and nova candidates.

Abstract

[Abridged] The central field of the Andromeda galaxy (M 31) has been monitored, using the Chandra HRC-I detector (about 0.1-10 keV energy range) from 2006 to 2012 with the main aim to detect X-rays from optical novae. We present a systematic analysis of all X-ray sources found in the 41 nova monitoring observations, along with 23 M 31 central field HRC-I observations available from the Chandra data archive starting in December 1999. Based on these observations, we studied the X-ray long-term variability of the source population and especially of X-ray binaries in M 31. We created a catalogue of sources, detected in the 64 available observations, which add up to a total exposure of about 1 Ms. We present a point-source catalogue, containing 318 X-ray sources with detailed long-term variability information, 28 of which are published for the first time. The spatial and temporal resolution of the catalogue allows us to classify 115 X-ray binary candidates showing high X-ray variability or even outbursts in addition to 14 globular cluster X-ray binary candidates showing no significant variability. The analysis may suggest, that outburst sources are less frequent in globular clusters than in the field of M 31. We detected 7 supernova remnants, one of which is a new candidate and in addition resolved the first X-rays from a known radio supernova remnant. Besides 33 known optical nova/X-ray source correlations, we also discovered one previously unknown super-soft X-ray outburst and several new nova candidates. The catalogue contains a large sample of detailed long-term X-ray light curves in the M 31 central field, which helps to understand the X-ray population of our neighbouring spiral galaxy M 31.