Discovery of a radio transient in M81

We report the discovery of a radio transient in the spiral galaxy M81. The transient was detected in early 2015 as part of a two-year survey of M81 made up of 12 epochs using the Karl G. Jansky Very Large Array. While undetected on 2014 September 12, the source was first detected on 2015 January 2, from which point it remained visible at an approximately constant luminosity of $L_{R} = 1.5 \pm 0.1 \times 10^{24}$erg s$^{-1}$ Hz$^{-1}$ at the observing frequency of 6 GHz for at least 2 months. Assuming this is a synchrotron event with a rise-time between 2.6 and 112 days, the peak luminosity (at equipartition) corresponds to a minimum energy of $10^{44} \lesssim E_{min} \lesssim 10^{46}$erg and jet power of $P_{min} \sim 10^{39}$erg s$^{-1}$, which are higher than most known X-ray binaries. Given its longevity, lack of short-term radio variability, and the absence of any multi-wavelength counterpart (X-ray luminosity $L_{X} \lesssim 10^{36}$erg s$^{-1}$), it does not behave like known Galactic or extragalactic X-ray binaries. The M81 transient radio properties more closely resemble the unidentified radio transient 43.78+59.3 discovered in M82, which has been suggested to be a radio nebula associated with an accreting source similar to SS 433. One possibility is that both the new M81 transient and the M82 transient may be the birth of a short-lived radio bubble associated with a discrete accretion event similar to those observed from the ULX Holmberg II X-1. However, it is not possible to rule out other identifications including long-term supernova shockwave interactions with the surrounding medium from a faint supernova or a background active galaxy.