Observations of the new transient radio source in the central region of M82

ATel #2078; A. Brunthaler (MPIfR), K. M. Menten (MPIfR), M. J. Reid (CfA), C. Henkel (MPIfR), G. C. Bower (UC Berkeley), H. Falcke (Uni Nijmegen, ASTRON)
on 11 Jun 2009; 20:01 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Andreas Brunthaler (brunthal@mpifr-bonn.mpg.de)

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We report observations with the Very Large Array (VLA), the Very Long Baseline Array (VLBA), and the European VLBI Network (EVN) in eVLBI made of the new radio transient in M82 that was recently discovered by Muxlow et al. ATEL#2073 in observations from 1st to 5th May 2009 at 5 GHz (0.72+/-0.05 mJy). In our VLA observation from 2009 April 08, no source was found at 22 GHz with a 3 sigma upper limit of 0.6 mJy at the position of the new transient. Further VLA observations were taken on 2009 April 27 and yielded 3 sigma upper limits of 0.9 mJy and 0.7 mJy at 43 and 22 GHz, respectively. Our VLBA observations were taken three days later on 2009 April 30 at 1.6, 4.8, and 8.4 GHz. We detected the source at 1.6 GHz with a flux density of 1.1+/-0.3 mJy within 3 milliarcseconds of the position given by Muxlow et al. ATEL#2073. At 4.8 GHz we tentatively detect a weak source with 0.5+/-0.2 mJy. The source is not resolved with our beam of 2.3 times 1.9 milliarcseconds. At 8.4 GHz, we have an (3 sigma) upper limit of 0.18 mJy. This gives an spectral index of -0.7 between 1.6 and 4.8 GHz, consistent with optical thin synchrotron emission. The eVLBI observations were performed on 2009 May 20 at 1.6 GHz and also detected the source with a flux density of 1.7+/-0.4 mJy. Thus, the source was still rising at 1.6 and 4.8 GHz during the first observations. With its short rise time of a few days and its 4.8 GHz radio luminosity the source fits well into the luminosity vs. time from explosion to peak 4.8 GHz flux density found for type II radio supernovae by Weiler et al. (1998, ApJ 500, 51). Thus, it could be a new radio supernova expanding into a lower density medium compared to SN 2008iz which exploded last year in M82 (Brunthaler et al., 2009, A&A, 499, 17; ATEL#2020; Beswick et al., ATEL#2060). However, these properties can be also explained by beamed emission from a X-ray binary.