VLBI detection of internal shocks in nova V959 Mon

TL;DR

This study used VLBI observations to identify internal shocks in nova V959 Mon, linking compact synchrotron emission features to gamma-ray production and revealing asymmetric ejecta influenced by the binary orbit.

Contribution

First direct VLBI detection of internal shocks in a nova, connecting synchrotron emission to gamma-ray production and characterizing ejecta asymmetry.

Findings

Detected compact synchrotron emission features indicating shocks

Observed asymmetric ejecta influenced by binary orbit

Linked VLBI knots to shock interaction regions

Abstract

V959 Mon is a classical nova detected at GeV gamma-ray wavelengths on 2012 June 19. While classical novae are now routinely detected in gamma-rays, the origin of the shocks that produce relativistic particles has remained unknown. We carried out electronic European VLBI Network (e-EVN) observations that revealed a pair of compact synchrotron emission features in V959 Mon on 2012 Sep 18. Since synchrotron emission requires strong shocks as well, we identify these features as the location where the gamma rays were produced. We also detected the extended ejecta in the follow-up EVN observations. They expanded much faster in East-West direction than the compact knots detected in the aforementioned e-EVN measurements. By comparing the VLBI results with lower resolution images obtained using e-MERLIN and the VLA - as reported by Chomiuk et al. (2014) - it appears that 1) influenced by the binary orbit, the nova ejecta was highly asymmetric with a dense and slow outflow in the equatorial plane and low-density and faster ejecta along the poles; and 2) the VLBI knots were related to shocks formed in the interaction region of these outflows.