Particle acceleration and non-thermal emission during the V407 Cygni nova outburst
UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de
Grenoble (IPAG), UMR 5274, Grenoble, France
Accepted: 3 January 2013
Context. On March 2010, the symbiotic binary V407 Cyg erupted as a result of a nova explosion. The event gave rise to a two-week long burst of ≥ 100 MeV γ-rays detected by Fermi/LAT, a unique observation testifying to particle acceleration in the system.
Aims. The outburst can be considered a scaled-down supernova, with short dynamical time scale, and thus can constitute a test case for theories of the origin of Galactic cosmic rays. We aim at determining the properties of the accelerated particles and identifying the origin of the high-energy radiation.
Methods. We developed a model for diffusive shock acceleration and non-thermal emission in V407 Cyg, complemented by an evaluation of the thermal emission from the shocked plasma. We considered both leptonic and hadronic contributions to the non-thermal processes, and investigated the effect of many binary and nova parameters.
Results. The γ-ray emission is mostly of leptonic origin and arises predominantly from inverse-Compton scattering of the nova light. Matching the light curve requires gas accumulation in the vicinity of the white dwarf, as a consequence of wind accretion, while the spectrum imposes particle scattering close to the Bohm limit in the upstream equipartition magnetic field. The nova accelerated protons (respectively electrons) with energies up to ≃300 GeV (respectively ≃20 GeV), for a total non-thermal energy ≃10 erg after two weeks, representing ≃10% of the initial nova kinetic energy. The electron-to-proton ratio at injection is 6%.
Conclusions. The V407 Cyg eruption can be understood from the same principles that are invoked for particle acceleration in supernova remnants, although without the need for strong magnetic field amplification. The population of novae in symbiotic systems is a negligible source of Galactic cosmic rays, and most likely not a class of TeV-emitters.
Key words: binaries: symbiotic / novae, cataclysmic variables / acceleration of particles / stars: individual: V407 Cyg / gamma rays: stars
© ESO, 2013