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The Wikipedia page on the "Cigar Galaxy" (M82) has the following summary about an "unknown object":

In April 2010, radio astronomers working at the Jodrell Bank Observatory of the University of Manchester in the UK reported an object in M82 that had started sending out radio waves, and whose emission did not look like anything seen anywhere in the universe before.

A subsequent article in New Scientist dated 13 December 2010 offers the following update:

The object is still a puzzle, says co-discoverer Tom Muxlow. “It was still there the last time we looked, so its lifetime is now well over a year,” he says. “We are continuing to monitor this object.”

I can find a number of other references to this unknown object, but nothing more recent than 2010. The articles suggest it could perhaps be a new type of microquasar, since it shares some similarities; however, it doesn't have the high X-ray output that known microquasars exhibit.

Has the mystery been solved, or is this still an area of active research?

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The SIMBAD page for the source in question: EQ J095552.5+694045.4

From the list of references, the most relevant ones to this question appear to be as follows:

  • Joseph et al. (2011) suggest that the object could be a microquasar similar to SS 433, which has a high radio to X-ray luminosity ratio.
  • Gendre et al. (2013) argue against a supernova interpretation for the source:

    Muxlow et al. (2010) suggested that this object might either be due to accretion around a massive collapsed object or be a faint and unusual supernova (Joseph, Maccarone & Fender 2010). In the latter case, we would expect the flux density to steadily decrease over time, as the source would be too young to have reached the remnant phase. However, in the observations presented here, it shows a flux density at 5 GHz of ∼0.90 mJy, which does not seem to vary significantly over the period 2009 May–2010. The supernova hypothesis is thus unlikely.

  • Mattila et al. (2013) state that the nature of the source is still unclear but they favour the microquasar interpretation:

    The nature of the 43.78+59.3 transient remains elusive, and on the basis of the limited data available, we regard an extremely bright extragalactic microquasar as the most plausible scenario, perhaps from a high-mass X-ray binary such as LS 5039 (Clark et al. 2001), but with a high ratio of radio to X-ray flux. A bright extragalactic microquasar was proposed as the most likely explanation by Muxlow et al. (2010) and Joseph et al. (2011). We concur with that conclusion although it would mean that the NIR luminosity is a factor of about 30 higher than Galactic microquasars.

  • Varenius et al. (2015) note that the source was not detected with LOFAR.

So from these, the nature of the source hasn't yet been conclusively shown, though the most plausible scenario is that it is a somewhat extreme example of a microquasar.

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    $\begingroup$ I guess it could be some kind of maser process, as described in Cyclotron Maser Emission from Blazar Jets?, which says "Less massive relativistic jet sources, such as microquasars, are even better candidates for producing cyclotron maser emission, primarily in the infrared and optical bands." $\endgroup$ – PM 2Ring Jun 10 '20 at 4:59

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