Shocks in the Symbiotic Recurrent Nova V3890 Sgr: VLBI Radio Imaging and Fermi GeV Gamma-Rays
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, $\epsilon_B = 3 \times 10^{-4} - 2 \times 10^{-3}$.