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ATEL # 2195; M.I. Krauss Hartman, M.P. Rupen, A.J. Mioduszewski (NRAO)
on 11 Sep 2009; 22:37 UT
Password Certification: Miriam I. Krauss (email@example.com)
Subjects: Radio, Binaries, Cataclysmic Variables, Nova, Transients, Variables, Stars
Very Large Array (VLA) C configuration observations of the fast nova V2672 Oph (IAUC 9064; ATel #2173) at 8.46 GHz on 1 September 2009 (MJD 55075.13; 15.61 days after the optical discovery [IAUC 9064]) detect an unresolved source 0.65 arcsec from the reported optical position, with a flux density of 0.526 +/- 0.033 mJy. The radio position (J2000) is
referenced to the nearby (3.3 degrees) calibrator J1751-2524. Another hour-long observation on 3 September 2009 (MJD 55077.18) gives no detection at 22.46 GHz, with a nominal flux density at this position of 0.2 +/- 0.5 mJy/beam.
The radio emission from most novae is dominated by thermal bremsstrahlung which is optically thick at early times (e.g., Bode & Evans 2008), leading to a spectral index alpha= +2 (flux density goes as nu^alpha). The observed alpha < 1.2 (3 sigma) implies that either the 22.46 GHz emission is already optically thin, or a substantial fraction of the observed flux density is synchrotron. Two lines of evidence support the latter as the correct interpretation:
The detection of radio synchrotron emission tends to support the suggestion that V2672 Oph is a recurrent rather than a classical nova (IAUC 9064). The only recurrent nova with a well-sampled radio light curve, RS Oph, shows strong radio synchrotron emission within days of the outburst (e.g., Padin, Davis, & Bode 1985; Eyres et al. 2009). RS Oph would have been ~3 mJy at 8.5 GHz on day 17 at the distance of the Galactic Center (taking its actual distance to be 2.45 kpc; Rupen et al. 2008). By contrast, radio synchrotron emission from classical novae is occasionally seen at late times (QU Vul [e.g., Taylor et al. 1987] and GK Per [e.g., Reynolds & Chevalier 1984]) but is quite rare, and has never been observed so early in an outburst. While this may be an observational bias -- classical novae are seldom observed at early times, since the thermal radio emission can not be readily detected -- recurrent novae do seem to show a much higher ratio of synchrotron to thermal radio emission.