Time-resolved hadronic particle acceleration in the recurrent nova RS Ophiuchi

Abstract

Recurrent novae are repeating thermonuclear explosions in the outer layers of white dwarfs, due to the accretion of fresh material from a binary companion. The shock generated when ejected material slams into the companion star’s wind can accelerate particles. We report very-high-energy (VHE, ≳ 100 GeV ) gamma rays from the recurrent nova RS Ophiuchi, up to a month after its 2021 outburst, observed using the High Energy Stereoscopic System. The VHE emission has a similar temporal profile to lower-energy GeV emission, indicating a common origin, with a two-day delay in peak flux. These observations constrain models of time-dependent particle energization, favoring a hadronic emission scenario over the leptonic alternative. Shocks in dense winds provide favorable environments for efficient acceleration of cosmic-rays to very high energies.

Evidence for Multiple Shocks from the γ-Ray Emission of RS Ophiuchi

“…The first two items on the above list can be reproduced with hadronic emission arising from a single external shock expanding into a medium of constant density that transitions to a red giant (RG) wind profile with density ρ 0 (r) ∝ r −2 at large radii (3 au). Figure 3 shows an example of shock evolution in such a density profile and the corresponding GeV and TeV light curves (more specifically, the light curves in the 0.06-500 and 250-2500 GeV bands, consistent with the data displayed in Aharonian et al 2022).…”

“…We note that MAGIC (Acciari et al 2022), H.E.S.S. (Aharonian et al 2022), andZheng et al (2022) interpret the combined Fermi and VHE spectra as arising from a single shock. To fit these combined spectra, H.E.S.S.…”

“…Its most recent eruption, beginning on 2021 August 8, qualified RS Ophiuchi as the second embedded nova detected at highsignificance by Fermi-LAT (Cheung et al 2022), and also as the first nova detected at ∼TeV energies, by the atmospheric Cherenkov telescopes H.E.S.S. (Aharonian et al 2022) and MAGIC (Acciari et al 2022). No classical nova has yet been detected at TeV energies, though relatively few upper limits have been reported in the literature (e.g., by MAGIC and HAWC; Ahnen et al 2015;Albert et al 2022) and there are theoretical reasons to believe that the high neutral gas densities surrounding the shock may limit the maximum particle energy (Reville et al 2007;Metzger et al 2016).…”

“…Interestingly, the TeV light-curve peak in RS Ophiuchi is delayed by several days relative to the peak of the GeV light curve, with the Fermi-LAT light curve peaking on 2021 August 9-10 ( Cheung et al 2022) and the H.E.S.S. light curve peaking on 2021 August 12 (Aharonian et al 2022; see their Figure 2). The H.E.S.S.…”

“…Throughout this work, we assume a distance to RS Ophiuchi of 1.4 kpc (Bode et al 2008), consistent with that used in Aharonian et al (2022).…”