An e-MERLIN & EVN radio counterpart to the ultraluminous X-ray source M82 X-1

Abstract

Ultra-luminous X-ray sources (ULXs) are X-ray bright (L${}_{\mathrm{X}-\mathrm{ray}}>$3$\times$10${}^{39}$erg s${}^{-1}$) extra-galactic objects that are powered by either neutron stars, or stellar or intermediate-mass black holes (IMBHs) but few have been detected in the radio waveband. In the nearby galaxy M82, the brightest ULX - M82 X$-$1, is thought to be associated with an IMBH but to date does not have a radio counterpart. We present deep wide-band reprocessed e-MERLIN images observed in 2015 May with an r.m.s. sensitivity of 7$\mu$Jy beam${}^{-1}$ and report the discovery of a new radio source with an integrated flux of S${}_{\nu =4.88\mathrm{GHz}}$ = 174$\pm$15$\mu$Jy, which is spatially co-incident with the Chandra X-ray position of M82 X$-$1. This source is not detected in archival MERLIN/e-MERLIN observations in the last three decades. A search for intra-observation variability in the 2015 e-MERLIN data was inconclusive, but a comparison with 1.5 GHz e-MERLIN observations taken a week prior yielded no detection. We also detect the source at the same position with milliarcsecond angular resolution in EVN+e-MERLIN data from 2021 March at 53$\pm$10$\mu$Jy. The radio source position is ICRF J2000 RA: 09${}^h$55${}^m$50.1172${}^s$, Dec: +69${}^{\circ }$40'46.606" ($\pm$1.5 mas). These radio fluxes are consistent with other radio-detected ULXs on the radio:X-ray plane and points towards a stellar/intermediate-mass black hole. The black hole mass inferred by the `fundamental plane of black hole activity' is 2650 M${}_{\odot }$, but this value remains highly uncertain.

8 pages, 4 figures, accepted to MNRAS