  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.
