Very Long Baseline Interferometry imaging of the advancing ejecta in the
  first gamma-ray nova V407 Cyg
  In 2010/3, the Large Area Telescope on board Fermi revealed a transient
gamma-ray source, positionally coincident with the optical nova in the
symbiotic binary, V407Cyg. This event marked the first discovery of gamma-ray
emission from a nova. We aimed 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; to constrain the physical conditions of
the system. We observed the source with the EVN and the VLBA over 16 epochs,
between 20 days and 6 months after the optical discovery. The source is
initially very dim but it later shows a substantial increase in brightness and
a resolved shell-like structure 40 to 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
S-E edge. We determine a projected velocity of ~3500 km/s in the initial phase,
and ~2100 km/s between day 20 and 91. We also found an emitting feature about
350 mas (940 AU) to the N-W, advancing at a projected velocity of ~700 km/s
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. Optical spectra demonstrated that in
2010 we were viewing V407Cyg 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 the
similar arrangement seen in Hen 2-104. The observed ~700 km/s expansion
constrains the launch-date of the polar knot around 2004. [Abridged]
