# Identification of the Hard X-ray Source Dominating the E > 25 keV Emission of the Nearby Galaxy M31

## TL;DR

This study identifies a dominant hard X-ray source in M31 as the counterpart to Swift J0042.6+4112, revealing its spectral properties and likely nature as an X-ray pulsar with an intermediate-mass companion or a symbiotic binary.

## Contribution

First identification of a single X-ray source responsible for the E > 25 keV emission in M31, clarifying its spectral characteristics and possible classification.

## Key findings

- The source exhibits a spectrum with a soft excess and a hard power-law component.
- No pulsations detected, possibly due to limited photon statistics.
- The source is likely an X-ray pulsar with an intermediate-mass companion or a symbiotic binary.

## Abstract

We report the identification of a bright hard X-ray source dominating the M31 bulge above 25 keV from a simultaneous NuSTAR-Swift observation. We find that this source is the counterpart to Swift J0042.6+4112, which was previously detected in the Swift BAT All-sky Hard X-ray Survey. This Swift BAT source had been suggested to be the combined emission from a number of point sources; our new observations have identified a single X-ray source from 0.5 to 50 keV as the counterpart for the first time. In the 0.5-10 keV band, the source had been classified as an X-ray binary candidate in various Chandra and XMM studies; however, since it was not clearly associated with Swift J0042.6+4112, the previous E < 10 keV observations did not generate much attention. This source has a spectrum with a soft X-ray excess (kT~ 0.2 keV) plus a hard spectrum with a power law of Gamma ~ 1 and a cutoff around 15-20 keV, typical of the spectral characteristics of accreting pulsars. Unfortunately, any potential pulsation was undetected in the NuSTAR data, possibly due to insufficient photon statistics. The existing deep HST images exclude high-mass (>3 Msun) donors at the location of this source. The best interpretation for the nature of this source is an X-ray pulsar with an intermediate-mass (<3 Msun) companion or a symbiotic X-ray binary. We discuss other possibilities in more detail.