Radio light curves and imaging of the helium nova V445 Puppis reveal seven years of synchrotron emission

Abstract

V445 Puppis is the only helium nova observed to date; its eruption in late 2000 showed high velocities up to 8500 km/s and a remarkable bipolar morphology cinched by an equatorial dust disc. Here we present multi-frequency radio observations of V445 Pup obtained with the Very Large Array (VLA) spanning 1.5 to 43.3 GHz, and between 2001 January and 2008 March (days 89 to 2700 after eruption). The radio light curve is dominated by synchrotron emission over these seven years, and shows four distinct radio flares. Resolved radio images obtained in the VLA A configuration show that the synchrotron emission hugs the equatorial disc, and comparisons to near-IR images of the nova clearly demonstrate that it is the densest ejecta, not the fastest ejecta, that are the sites of the synchrotron emission in V445 Pup. The data are consistent with a model where the synchrotron emission is produced by a wind from the white dwarf impacting the dense equatorial disc, resulting in shocks and particle acceleration. The individual synchrotron flares may be associated with density enhancements in the equatorial disc and/ or velocity variations in the wind from the white dwarf. This overall scenario is similar to a common picture of shock production in hydrogen-rich classical novae, but V445 Pup is remarkable in that these shocks persist for almost a decade, much longer than the weeks or months for which shocks are typically observed in classical novae.