Discovery of a new transient radio source in the central region of M82

ATel #2073; T. W. B. Muxlow, R. J. Beswick, A. Pedlar (JBCA, Manchester), D. Fenech (UCL), M. K. Argo (Curtin University), M. J. Ward (Durham), A. Zezas (Harvard-Smithsonian CfA)
on 9 Jun 2009; 16:46 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Rob Beswick (Robert.Beswick@manchester.ac.uk)

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We report the discovery of a new compact transient radio source in the central region of the starburst galaxy M82 with MERLIN at 4994MHz. The position measured for the transient source is (J2000) 09 55 52.5083, +69 40 45.420, with an astrometric error of 5mas in each coordinate. The source was discovered in continuous monitoring observations made between 1st and 5th May 2009 with a flux density of 720+/-50microJy. This source was not detected one week earlier (from monitoring observations between 24th-27th April 2009) to a 3 sigma limit of <180microJy/beam. Subsequent MERLIN monitoring observations on 26th-27th May at 4994MHz, 27th-28th May at 6668.4MHz, and 5th-6th June 2009 at 1658MHz confirm the detection of the transient source with measured flux densities of 500+/-80microJy, 620+/-90microJy, and 980+/-80microJy respectively. A comparison with archival Chandra X-ray data shows a weak X-ray source within 0.5 arcseconds of the transient radio source position. The position offset is within the quoted Chandra astrometric position errors and is therefore a potential X-ray counterpart to the radio transient. Within the central kpc of M82 there are of order 50 compact (<1 arcsec diameter) radio sources. Around 2/3 of these objects are known to be associated with radio supernova remnants, and the remainder are identified as compact HII regions. Over the last 15 to 20 years M82 has been subject to regular monitoring at radio wavelengths with typical sampling intervals of around 6 months to 1 year. During this period only 3 transient radio sources have been identified, the most recent being the bright radio supernova SN2008iz (Brunthaler et al., 2009, A&A, 499, 17; ATEL#2020; Beswick et al., ATEL#2060). The two remaining weak transient sources were only detected at a single epoch (Kronberg & Sramek 1985, Science 227, 28; Muxlow et al., 1994, MNRAS, 266, 455), and their nature remains unknown. This new weak transient is the first example to be detected in multiple closely-spaced epochs and thus its age and temporal flux evolution can be accurately constrained. At present, it is unclear as to whether this object is the prompt radio emission from a new supernova, the beamed outburst from an X-ray binary system, or possibly the outburst from a soft gamma-ray repeater. Continued radio and X-ray monitoring of this object is planned.