Astronomy & Astrophysics
Context. 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. Aims. 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. Methods. We created a catalogue of sources, detected in the 64 available observations, which add up to a total exposure of about 1 Ms. To study the variability, we developed a processing pipeline to derive long-term Chandra HRC-I light curves for each source over the 13 years of observations. In the merged images we also searched for extended X-ray sources. Results. 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. Conclusions. 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.
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