The powerful shocks in RS Oph: NuSTAR X-ray data and a complete review
In the 2021 outburst of RS Ophiuchi, the gamma- and the X-ray flux were measured quasi-simultaneously from day 1 after the optical peak, offering the first comprehensive view of shocks in a nova occurring in a symbiotic system. We present a previously unpublished observation done with NuSTAR in the 3-79 keV range, 9 days after maximum, and we review the complex history of the evidence of shocks in the previous outbursts of this nova in the light of the intensive X-ray monitoring of 2021. We find evidence that the shock causing the particle acceleration measured with the Cherenkov telescopes produced also the thermal flux detected in the 0.2-30 keV X-ray range, while the large gamma-ray flux observed with Fermi after about a day, is not consistent with the X-ray observations. We conclude that an initial, strong shock, with particle-particle loss timescale shorter than the timescale of particle acceleration at energy higher than a few GeV, occurred close to the red giant atmosphere,where either the X-rays' emitting volume was reduced by turbulence, or - perhaps less likely - the X-rays were completely absorbed by large column density near the giant and by the accretion wake along the line of sight. We compare RS Oph with other novae in long period systems with evolved companions,discussing how the shocks' phenomenology is a powerful tool to derive other physical parameters. Finally, we discuss predictions that in T CrB, expected to have a new outburst within the next few years, the shocks may not be as energetic as in RS Oph.