High energy emission from recurrent novae

In recent years, several nova explosions - eight classical novae and two symbiotic recurrent novae - have been detected by Fermi/LAT at E$>$100 MeV. In most cases, this emission has been observed early after the explosion, around the optical maximum, and for a short period of time. The high-energy $\gamma$-ray emission is a consequence of $\pi^{0}$ decay and/or Inverse Compton, which are related to particle (p and e$^{-}$) acceleration in the strong shock between the nova ejecta and the circumstellar matter. RS Ophiuchi (2006) was the first nova for which particle acceleration was predicted. This prediction showed that the blast wave decelerated faster than expected as a consequence of the acceleration of particles in the shock and their escape. The aim of our thesis is to study the evolution of the symbiotic recurrent novae the first days after the outburst and the nature of their emission. The present thesis work has been aimed to perform a comprehensive multiwavelength study of the last outburst of RS Oph and V745 Sco.

RS Oph is a recurrent nova in a symbiotic system composed of a white dwarf and a red giant with a recurrence period of $\sim$21 years. In this work, we present a new analysis of XMM-Newton's observations of RS Oph early after its 2006 outburst both with RGS and EPIC-MOS. We compare these results with temperatures, emission measures, absorptions and fluxes obtained for RXTE, Swift and Chandra observations, and previous studies of RGS observations. The evolution of the radio and IR emissions during the first days after the outburst were studied. The multiwavelength studies allow us to get a global picture of the shocked plasma and its relationship with the particle acceleration.

V745 Sco is also a symbiotic recurrent nova with a recurrence period of $\sim$25 years. We present the analysis of the Swift/XRT observations of V745 Sco early after its 2014 outburst simultaneous to Fermi detection. We combine our results with Chandra and NuStar observations to get a global picture of the evolution of the nova ejecta. As in RS Oph we compiled all the radio and IR information about V745 Sco the first days after the explosion. Finally, particle acceleration in V745 Sco can be explained by a diffusive shock model at the blast wave and the subsequent escape of the very high energy particles as in RS Oph.

With the study of RS Oph and V745 Sco, and its comparison, we demonstrate for the first time common features in the early evolution of a nova remnant and their relationship with particle acceleration in the symbiotic recurrent novae. Multiwavelength results provide new insights into the evolution of the shocked plasma and the interaction with the circumstellar material, being a powerful tool to understand the nature of the $\gamma$-ray emission.