A subset of ultraluminous X-ray sources (those with luminosities of less than 10 40 erg s -1; ref. 1) are thought to be powered by the accretion of gas onto black holes with masses of ∼5-20, probably by means of an accretion disk. The X-ray and radio emission are coupled in such Galactic sources; the radio emission originates in a relativistic jet thought to be launched from the innermost regions near the black hole, with the most powerful emission occurring when the rate of infalling matter approaches a theoretical maximum (the Eddington limit). Only four such maximal sources are known in the Milky Way, and the absorption of soft X-rays in the interstellar medium hinders the determination of the causal sequence of events that leads to the ejection of the jet. Here we report radio and X-ray observations of a bright new X-ray source in the nearby galaxy M 31, whose peak luminosity exceeded 10 39 erg s -1. The radio luminosity is extremely high and shows variability on a timescale of tens of minutes, arguing that the source is highly compact and powered by accretion close to the Eddington limit onto a black hole of stellar mass. Continued radio and X-ray monitoring of such sources should reveal the causal relationship between the accretion flow and the powerful jet emission.
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Acknowledgements We thank C. Trott and R. Soria for discussions, and C. Gough for making his code available. This work was supported by a Science and Technology Facilities Council (STFC) standard grant (M.J.M.), Netherlands Organization for Scientific Research Vidi Fellowship (S.M.), European Research Council partial funding (R.F.) and grant number BMWI/DLR, FKZ 50 OR 1010 (M. Henze). The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement byAssociated Universities,Inc.We thank thestaffof the Mullard Radio Astronomy Observatory for their assistance in the commissioning and operation of AMI, which is supported by Cambridge University and the STFC. This work is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. This research has also made use of data obtained from NASA’s Swift and Chandra satellites.