Very long baseline interferometry imaging of the advancing ejecta in the first gamma-ray nova V407 Cygni

Context. In 2010 March, the Large Area Telescope on board Fermi revealed a transient gamma-ray source that is positionally coincident with the optical nova in the symbiotic binary, V407 Cyg. This event marked the first discovery of gamma-ray emission from a nova.Aims. We aim to obtain resolved radio imaging of the material involved in the nova event, to determine the ejecta geometry and advance velocity directly in the image plane, and to constrain the physical conditions of the system.Methods. We observed the source with the European VLBI (Very Long Baseline Interferometry) Network in real time mode, at 1.6 and 5 GHz, and the Very Long Baseline Array at 1.6, 5, and 8.4 GHz. In total, we observed the source over 16 epochs, starting 20 days after the optical discovery and continuing for over six months.Results. Milliarcsecond-scale radio emission is detected in 10/16 epochs of observations. The source is initially very dim but it later shows a substantial increase in brightness and a resolved shell-like structure 40-90 days after the optical event. The shell has a projected elliptical shape and is asymmetric in brightness and spectral index, being brighter and characterised by a rising spectrum at the south-eastern edge. We determine a projected expansion velocity of similar to 3500 km s(-1) in the initial phase (for an adopted 2.7 kpc distance), and similar to 2100 km s(-1) between day 20 and 91. We also found an emitting feature about 350 mas (940 AU) to the north-west, advancing at a projected velocity of similar to 700 km s(-1) along the polar axis of the binary. The total flux density in the VLBI images is significantly lower than that previously reported at similar epochs and over much wider angular scales with the VLA.Conclusions. Optical spectra convincingly demonstrated that in 2010 we were viewing V407 Cyg along the equatorial plane and from behind the Mira. Our radio observations image the bipolar flow of the ejecta perpendicular to the orbital plane, where deceleration is much lower than through the equatorial plane probed by the truncated profile of optical emission lines. The separated polar knot at 350 mas and the bipolar flow strictly resemble a similar arrangement seen in Hen 2-104, another symbiotic Mira seen equator-on that went through a large outburst similar to 5700 yrs ago. The observed similar to 700 km s(-1) expansion constrains the launch date of the polar knot around 2004, during the accretion-fed active phase preceding the 2010 n