An <i>e</i>-MERLIN &amp; EVN radio counterpart to the ultraluminous X-ray source M82 X−1

ABSTRACT Ultra-luminous X-ray sources (ULXs) are X-ray bright (L$_{\rm X-ray} &amp;gt;3\times 10^{39}$ erg s$^{-1}$) extragalactic 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$_{\rm \nu =4.88\, GHz}$ = 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$^{\rm h}55^{\rm m}50\rm{.\!\!^{ {\mathrm{s}}}}$1172, Dec.: +69$^{\circ }40^\prime 46{_{.}^{\prime\prime}}$606 ($\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.