The Distance to Nova V959 Mon from VLA Imaging

TL;DR

This study measures the distance to nova V959 Mon using radio imaging and optical spectroscopy, revealing a non-spherical ejecta geometry and a distance of about 1.4 kpc, which impacts understanding of its gamma-ray emission.

Contribution

It combines radio expansion measurements with optical velocities to estimate a nova's distance, providing the first such measurement for a gamma-ray-detected nova.

Findings

Distance to V959 Mon is approximately 1.4 kpc.

V959 Mon has a lower gamma-ray luminosity compared to other gamma-ray novae.

The ejecta geometry is non-spherical.

Abstract

Determining reliable distances to classical novae is a challenging but crucial step in deriving their ejected masses and explosion energetics. Here we combine radio expansion measurements from the Karl G. Jansky Very Large Array with velocities derived from optical spectra to estimate an expansion parallax for nova V959 Mon, the first nova discovered through its gamma-ray emission. We spatially resolve the nova at frequencies of 4.5-36.5 GHz in nine different imaging epochs. The first five epochs cover the expansion of the ejecta from 2012 October to 2013 January, while the final four epochs span 2014 February to 2014 May. These observations correspond to days 126 through 199 and days 615 through 703 after the first detection of the nova. The images clearly show a non-spherical ejecta geometry. Utilizing ejecta velocities derived from 3D modelling of optical spectroscopy, the radio expansion implies a distance between 0.9 +/- 0.2 and 2.2 +/- 0.4 kpc, with a most probable distance of 1.4 +/- 0.4 kpc. This distance implies a gamma-ray luminosity much less than the prototype gamma-ray-detected nova, V407 Cyg, possibly due to the lack of a red giant companion in the V959 Mon system. V959 Mon also has a much lower gamma-ray luminosity than other classical novae detected in gamma-rays to date, indicating a range of at least a factor of 10 in the gamma-ray luminosities for these explosions.