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Durham University

Department of Physics

Staff profile

Publication details for Prof Tim Roberts

Anderson, G. E., Miller-Jones, J. C. A., Middleton, M. J., Soria, R., Swartz, D. A., Urquhart, R., Hurley-Walker, N., Hancock, P. J., Fender, R. P., Gandhi, P., Markoff, S. & Roberts, T. P. (2019). Discovery of a radio transient in M81. Monthly Notices of the Royal Astronomical Society 489(1): 1181-1196.

Author(s) from Durham

Abstract

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 LR,ν = 1.5 ± 0.1 × 1024 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 1044 . Emin . 1046 erg and jet power of
Pmin ∼ 1039 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
Lx . 1036 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.