Shocks in the Symbiotic Recurrent Nova V3890 Sgr: VLBI Radio Imaging and Fermi GeV Gamma-Rays

TL;DR

This study combines VLBI radio imaging and Fermi gamma-ray observations to analyze the 2019 eruption of the symbiotic recurrent nova V3890 Sgr, revealing morphological evolution, shock interactions, and emission mechanisms.

Contribution

It provides the first detailed VLBI imaging of the nova's morphological evolution and links radio and gamma-ray emissions to distinct shock regions, advancing understanding of nova eruptions.

Findings

Eruption starts asymmetric, becomes circular, then brighter along north-south axis.

Radio emission dominated by synchrotron, with >80% flux captured early on.

Gamma-ray emission coincides with optical maximum and reappears with the second radio peak.

Abstract

We present very long baseline interferometric (VLBI) radio imaging and Fermi/LAT GeV $\gamma$-ray observations of the 2019 eruption of the symbiotic recurrent nova V3890 Sgr.The VLBI imaging spans 8 -- 51 days after eruption, synchronous with the detected $\gamma$-rays. VLBI imaging shows the eruption starts out asymmetric on day 8 with an eastern component brighter than a western component. By day 32 the blast is rather circularly symmetric, and on day 49, the nova shell is brighter along the north--south axis. This morphological evolution is explained by interaction with circumstellar material (CSM) comprised of a spherical wind plus an over-density in the orbital plane. Comparing radio images to optical line widths gives an expansion parallax distance of 6.8 kpc. In the first 32 days or eruption, VLBI images capture $>$80 per cent of the integrated flux (as measured by the VLA), implying that synchrotron emission dominates. A second peak in the VLA light curve is explained by an image on day 48 that reveals the nova shell surrounded by a diffuse halo, powered by synchrotron emission from particles that have diffused upstream of the shock. The $\gamma$-rays appear around optical maximum and remain detectable for 23 days; marginally significant $\gamma$-rays reappear around day 60, concurrent with the second radio peak. Modelling indicates radio and $\gamma$-ray emission arise in distinct shock regions: $\gamma$-rays from dense CSM in the orbital plane, radio from the more spherical CSM component. X-ray observations constrain the spherical CSM density, which is higher than in other symbiotic recurrent novae. Assuming equipartition, we estimate the fraction of the post-shock pressure in magnetic fields, $\epsilon_B = 3 \times 10^{-4} - 2 \times 10^{-3}$.