In January of 2012, a new X-ray source flared and rapidly brightened in the Andromeda galaxy (M31), l2.5 million light-years from earth. The event was classified as an ultraluminous X-ray source (ULX), the second ever seen in M31 and efforts with X-ray telescopes and radio observatories resulted in the first detection of radio-emitting jets from a stellar-mass black hole outside our own galaxy.

A ULX is thought to be a binary system containing a black hole that is rapidly accreting gas from its stellar companion. To account for the high-energy output, gas must be flowing into the black hole at a rate very near a theoretical maximum – which astronomers do not yet fully understand.

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