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

¹ INAF Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
e-mail: giroletti@ira.inaf.it
² INAF Astronomical Observatory of Padova, 36012 Asiago (VI), Italy
³ Department of Astrophysics/IMAPP, Radboud University Nijmegen, 6500 GL Nijmegen, The Netherlands
⁴ National Radio Astronomy Observatory, Array Operations Center, 1003 Lopezville Road, Socorro, NM 87801, USA
⁵ Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
⁶ LSST Corproation, 933 North Cherry Avenue, Tucson, AZ 85721, USA
⁷ Space Science Division, Naval Research Laboratory, Washington, DC 20375, USA
⁸ Laboratoire AIM (CEA/IRFU – CNRS/INSU – Université Paris Diderot), CEA DSM/IRFU/SAp, 91191 Gif-sur-Yvette, France
⁹ Station de Radioastronomie de Nançay, Observatoire de Paris, CNRS/INSU, USR 704 – Univ. Orléans, OSUC, 18330 Nançay, France
¹⁰ National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801, USA
¹¹ Department of Physics and Astronomy, Michigan State University, 567 Wilson Rd, East Lansing, MI 48824, USA
¹² Astro Space Center, Lebedev Physical Inst. RAS, Profsoyuznaya 84/32, 117997 Moscow, Russia
¹³ Sternberg Astronomical Institute, Moscow University, Universitetsky 13, 119991 Moscow, Russia
¹⁴ Jodrell Bank Centre for Astrophysics, Alan Turing Building, University of Manchester, Manchester M13 9PL, UK

Received: 10 April 2020
Accepted: 11 May 2020

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.