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

Abstract

We present very long baseline interferometric (VLBI) radio imaging and Fermi/LAT GeV γ-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 γ-rays. VLBI imaging shows the eruption begins asymmetric on day 8 and becomes rather circularly symmetric by day 32. 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 of eruption, VLBI images capture >80percnt of the integrated flux 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 synchrotron halo. The γ-rays appear around optical maximum and remain detectable for 23 days; marginally significant γ-rays reappear around day 60, concurrent with the second radio peak. Modelling indicates radio and γ-ray emission arise in distinct shock regions: γ-rays from CSM in the orbital plane, radio from the 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, εB =few × 10−3.