Three X-ray transients in M31 observed with Swift

TL;DR

This study identifies and classifies three transient X-ray sources in M31 using Swift, XMM-Newton, and Chandra data, revealing diverse source types including a Be/X-ray binary, a black hole binary, and a nova, and estimates their occurrence rate.

Contribution

First detailed multi-instrument analysis of three M31 X-ray transients, clarifying their nature and improving source identification accuracy.

Findings

XMMU J004215.8+411924 is a Be/X-ray binary with a one-month outburst.

SWIFT J004217.3+411532 is likely a black hole low mass X-ray binary.

M31N 2006-11a is a nova detected in X-ray and UV after optical maximum.

Abstract

The purpose of this study is to find transient X-ray sources in M31, and to investigate and classify their nature. Three X-ray transients were observed with Swift. For each of the three X-ray transients we use the Swift X-ray and optical data together with observations from XMM-Newton and Chandra to investigate the lightcurves and the spectra of the outburst, and thereby to identify the source types. The outburst of XMMU J004215.8+411924 lasted for about one month. The source had a hard power-law spectrum with a photon index of 1.6. It was previously identified as a Be/X-ray binary based on the optical identification with a star. However, we show that with improved source coordinates it is clear that the optical source is not the counterpart to the X-ray source. The source SWIFT J004217.3+411532 had a bright outburst, after which it slowly decayed over half a year. The spectrum was soft, corresponding to a thermal accretion disk with innermost temperature of 250-600 eV. The source was not seen in the optical, and the soft spectrum indicates that the source is most likely a black hole low mass X-ray binary. M31N 2006-11a is a nova that was previously observed in the optical. We detected it both in X-rays and UV with Swift half a year after the optical maximum, after which it decayed below the Swift detection threshold within a month. The spectrum of the X-ray transient can be modelled by a black-body with a temperature of 50 eV. We use catalogues of X-ray transients in M31 to estimate their rate, and we find a lower limit of 9/yr.