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 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 $γ$-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 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, $ε_B = 3 \times 10^{-4} - 2 \times 10^{-3}$.

Figures (10)

• Figure 1: VLBI radio images of the 2019 eruption of V3890 Sgr. The top two panels, (a) and (b), show 4.87 and 8.37 GHz VLBA images on day 8.1. The second row---panels (c) and (d)---show 4.87 and 8.37 GHz VLBA images on day 16. The third row are 4.87 GHz VLBA images from day 32.0 (panel (e)) and day 51.0 (panel (f)). The bottom row show the EVN+e-MERLIN image from day 48.7; panel (g) is on the same scale as panels (a--f), while panel (h) zooms out to show a larger field of view. In each panel, the contour levels are set to $-1.5, 1, 3, 5$ and $7 \sigma$ (see Table \ref{['tab:radio']} for $\sigma$ values). The white dot is located at the Gaia position of V3890 Sgr. In the upper left corner is a compass showing the North (up) and East (left) directions. The synthesized beam is plotted in the bottom left corner. The field of view is $80$ mas across for panels (a--g), and $225$ mas across for panel (h). Note that each panel has its own color scale, as denoted in its color bar.
• Figure 2: The H$\alpha$ emission line profiles 470 days before V3890 Sgr's 2019 eruption and 1, 8, 16, 30, and 53 days after the eruption.
• Figure 3: Integrated flux densities from our VLBA observations and the EVN+e-MERLIN observation as a function of time following V3890 Sgr's 2019 eruption, compared to the VLA light curve in similar frequency bands (VLA data published in Nyamai+23).
• Figure 4: Brightness temperature measurements as a function of time following V3890 Sgr's 2019 eruption, measured from our VLBA images in $4.87$ GHz (plotted as blue triangles) and $8.37$ GHz (plotted as orange circles).
• Figure 5: Spectral index map of the September 4 observation, 8.1 days after eruption, using $4.87$ GHz and $8.37$ GHz. The September 4 $8.37$ GHz image contours are overlaid on the SIM.