The majority of radio transient studies have focused on blind searches for transients in both dedicated radio surveys and archival data (e.g. Such radio observations allow us to directly probe particle acceleration and shock physics from explosive and outbursting astronomical sources, which provides direct insight into the velocity of the ejecta, the associated magnetic fields, the total energy budget, and the density distribution of the circumstellar and intergalactic media (Fender et al.
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Stars: black holes, supernovae: general, galaxies: individual: M81, radio continuum: general, radio continuum: transients, X-rays: binaries 1 INTRODUCTIONĪs we approach the era of the Square Kilometre Array (SKA), astronomers are investigating how to use the low- and mid-frequency facilities for transient radio astronomy. 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. 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. 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.
Given its longevity, lack of short-term radio variability, and the absence of any multiwavelength counterpart (X-ray luminosity L x ≲ 10 36 erg s −1), it does not behave like known Galactic or extragalactic X-ray binaries. Assuming this is a synchrotron event with a rise-time between 2.6 and 112 d, the peak luminosity (at equipartition) corresponds to a minimum energy of 10 44 ≲ E min ≲ 10 46 erg and jet power of P min ∼ 10 39 erg s −1, which are higher than most known X-ray binaries. 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, ν = 1.5 ± 0.1 × 10 24 erg s −1 Hz −1 at the observing frequency of 6 GHz for at least 2 months. 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. We report the discovery of a radio transient in the spiral galaxy M81.
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