The Powerful Shocks in RS Oph: NuSTAR X-Ray Data and a Complete Review

Abstract

Abstract: In the 2021 outburst of RS Ophiuchi, the gamma-ray 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 We present a previously unpublished observation done with NuSTAR in the 3–79 keV range, 9 days after maximum, and we review the complex history of the evidence of shocks in the previous outbursts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 light of the intensive X-ray monitoring of 2021. We find evidence that the shock causing the particle acceleration measured with the Cherenkov telescopes also produced 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 a 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 nova outburst within the next few years, the shocks may not be as energetic as in RS Oph.