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

Abstract

In 2021 August, the Fermi Large Area Telescope, H.E.S.S., and MAGIC detected GeV and TeV γ-ray emission from an outburst of recurrent nova RS Ophiuchi. This detection represents the first very high-energy γ-rays observed from a nova, and it opens a new window to study particle acceleration. Both H.E.S.S. and MAGIC described the observed γ-rays as arising from a single, external shock. In this paper, we perform detailed, multi-zone modeling of RS Ophiuchi’s 2021 outburst, including a self-consistent prescription for particle acceleration and magnetic field amplification. We demonstrate that, contrary to previous work, a single shock cannot simultaneously explain RS Ophiuchi’s GeV and TeV emission, in particular the spectral shape and distinct light-curve peaks. Instead, we put forward a model involving multiple shocks that reproduces the observed γ-ray spectrum and temporal evolution. The simultaneous appearance of multiple distinct velocity components in the nova optical spectrum over the first several days of the outburst supports the presence of distinct shocks, which may arise either from the strong latitudinal dependence of the density of the external circumbinary medium (e.g., in the binary equatorial plane versus the poles) or due to internal collisions within the white dwarf ejecta (which power the γ-ray emission in classical novae).

Hadronic versus Leptonic Origin of Gamma-Ray Emission from Supernova Remnants

“…In addition, the detection of the characteristic "pion bump" in IC443 and W44 confirmed the hadronic nature of the γ-ray emission from these SNRs interacting with molecular clouds (Ackermann et al 2013). The recent measurement of the nova RS Ophiuchi, which was characterized as solidly hadronic, serves as an example of this ongoing exploration (Acciari et al 2022;Aharonian et al 2022;Diesing et al 2023). These findings raise questions about whether hadronic emission from SNRs is common enough for them to be the primary accelerators of Galactic CRs.…”

Lepto-hadronic Interpretation of 2021 RS Ophiuchi Nova Outburst

“…One should note that because the TeV gamma rays produced are within the radiation field of the companion RG star, the pair production process due to gamma-gamma absorption could be an additional mechanism in principle. However, the effect of the gamma-gamma absorption on the model spectrum is minor, as reported by Aharonian et al (2022), Acciari et al (2022), and Diesing et al (2023), and therefore we do not include this process in our current theoretical modeling because its inclusion does not drastically alter the results presented in this work. We discuss this point in detail in Section 4.…”

“…We have neglected the effect of gamma-gamma attenuation in this model following the conclusion drawn by previous studies on RS Oph (Acciari et al 2022;Aharonian et al 2022;Diesing et al 2023). Aharonian et al (2022) have posited that the attenuation of TeV gamma rays occurs due to the interaction with optical-IR photons and X-rays provide the dominant target for GeV photons.…”

“…Acciari et al (2022) have also stated that the effect of gamma-gamma attenuation of the emission in the photosphere radiation field of RS Oph is not significant. Diesing et al (2023) have posited that although close to T-T 0 ;1 day, the gamma-gamma attenuation is expected to be modest (by a factor of ∼2) and this attenuation is negligible at the radius corresponding to days 1 to 4, i.e., during the rise and subsequent peak of the observed TeV luminosity. Consequently, the authors also neglected absorption in their emission estimates.…”

“…Aharonian et al (2022) have explained the HE-VHE gamma-ray emission using a single shock, single particle population model. In contrast, Diesing et al (2023) have explained the HE-VHE gamma-ray emission using a multiple shocks, single particle population scenario. Note that none of these models considered lower energy data points (e.g., radio).…”