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

Abstract

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 ∼3500 km s−1 in the initial phase (for an adopted 2.7 kpc distance), and ∼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 ∼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 ∼5700 yrs ago. The observed ∼700 km s−1 expansion constrains the launch date of the polar knot around 2004, during the accretion-fed active phase preceding the 2010 nova outburst.

The GALAH survey and symbiotic stars – I. Discovery and follow-up of 33 candidate accreting-only systems

“…It quickly reaches the Fermi temperature (a matter of minutes; Starrfield, Iliadis & Hix 2008) and ejects most of the accreted envelope at high velocity (thousands of km s −1 ). Radio interferometry has nicely resolved the expansion of ejecta following, for example, the 2006 outburst of RS Oph (O'Brien et al 2006) and that of 2010 for V407 Cyg (Giroletti et al 2020), the latter being also the first nova event ever detected in GeV γ -rays by the Fermi satellite (Cheung et al 2010). The resulting nova outburst can repeat on time-scales as short as years/decades if the WD mass is close to the Chandrasekhar limit, like in the case for T CrB, RS Oph, V407 Cyg, or V3890 Sgr.…”

Radio Spectral Index Analysis of Southern Hemisphere Symbiotic Stars

“…These are in striking contrast to the high ionization species, e.g. [O VI] and other indicators of high temperature gas such as X-ray or γ ray emission (Giroletti et al 2020). The X-ray spectrum can often distinguish between systems where the hot gas is in the boundary between the accretion disk and the white dwarf, on the surface of the white dwarf, or in a region where the winds from the two stars collide (Luna et al 2013, δ, α, and β types, respectively), or the white dwarf fast wind collides with circumstellar material from an earlier, slower wind (Lucy et al 2020).…”

“…Direct measurement of the sizes of the emission regions in more SySts would be very helpful. VLBI studies such as the recent combined EVN-VLBA monitoring of V 407 Cyg by Giroletti et al (2020) have surface brightness sensitivity limited to T B > 10 5 K, so they mostly show the distribution of non-thermal emission, although if some of the radio flux comes from the X-ray emission region it might be remotely possible that in a few very high density clumps the brightness temperature could be high enough to detect their thermal emission with VLBI. But thermal emission from regions with sizes on the order of 10 −1 arc sec could be measured at higher frequencies with ALMA or the JVLA, or possibly with the ATCA.…”

“…There is good evidence from VLBI maps of transient radio jets that synchrotron emission is required, e.g. in R Aqr (Dougherty et al 1995), in CH Cyg (Crocker et al 2001), and others (Rupen et al 2008;Sokoloski et al 2008;Giroletti et al 2020), to explain the occasional short-lived jet emission seen after some optical and X-ray outbursts. The jets are typically found after optical outbursts (Sokoloski 2003, and references therein).…”

Fermi-LAT Observations of V549 Vel 2017: a Sub-Luminous Gamma-Ray Nova?

“…However, if the distance to the nova is, e.g., d > 5.6 kpc (to make the radius of the giant as > 10 R ), i = 3.2 mas and = 1.91 mas will refer to > 3800 R and > 2300 R , which are also too large for a typical symbiotic binary (the projected size is smaller than a few hundred R with orbital periods of 200-1000 days; see, e.g., Miko lajewska 2003). In some extreme symbiotic systems, the orbital sizes can reach 10,000 R (e.g., V407 Cyg; Hinkle et al 2013;Giroletti et al 2020), but their orbital periods are too long (i.e., ∼ 100 years) to mimic the parallax effect that is on a yearly time-scale.…”