Radio synchrotron emission from V2672 Oph

ATel #2195; M. I. Krauss Hartman, M. P. Rupen, A. J. Mioduszewski (NRAO)
on 11 Sep 2009; 22:37 UT
Credential Certification: Miriam I. Krauss (miriam@space.mit.edu)

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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

R.A. 17:38:19.7162 +/- 0.0044, Decl. -26:44:13.5630 +/- 0.13

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:

1. The radio brightness temperature of thermal gas is at most equal to the electron temperature of that gas, which for the extended emission from novae is around 10,000 K. Such a source would have to be ~20 milliarcseconds across to produce the observed 8.46 GHz emission. The high extinction (IAUC 9064; ATel #2173) and the Galactic coordinates (l= 1.0, b=2.5) suggest the source is at or beyond the Galactic Center (i.e., D>= 8.5 kpc). For a 17-day-old spherical source this gives an expansion velocity >=17000 km/s, far larger than observed (ATel #2173); the mass required to achieve significant optical depth would also be remarkable. The distance would have to be 3 kpc or less for the 8.46 GHz emission to be entirely thermal.


2. The early detection of hard X-ray emission (ATel #2173) suggests the presence of strong shocks, which can also produce the relativistic electrons and strong magnetic fields needed to generate synchrotron radiation.

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.