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We report the Fermi-LAT gamma-ray detection of the 2021 outburst of thesymbiotic recurrent nova RS Ophiuchi. In this system, unlike classical novaefrom cataclysmic binaries, the ejecta from the white dwarf form shocks wheninteracting with the dense circumstellar wind environment of the red giantcompanion. We find the LAT spectra from 50 MeV to ~20-23 GeV, thehighest-energy photons detected in some sub-intervals, are consistent with$\pi^{\rm 0}$-decay emission from shocks in the ejecta as proposed byTatischeff & Hernanz (2007) for its previous 2006 outburst. The LAT light-curvedisplayed a fast rise to its peak >0.1 GeV flux of $\simeq$6x10^-6 ph cm^-2s^-1 beginning on day 0.745 after its optically-constrained eruption epoch of2021 August 8.50. The peak lasted for ~1 day, and exhibited a power-law declineup to the final LAT detection on day 45. We analyze the data on shortertimescales at early times and found evidence of an approximate doubling ofemission over ~200 minutes at day 2.2, possibly indicating a localizedshock-acceleration event. Comparing the data collected by the AAVSO, wemeasured a constant ratio of ~2.8x10^-3 between the gamma-ray and opticalluminosities except for a ~5x smaller ratio within the first day of theeruption likely indicating attenuation of gamma rays by ejecta material andlower high-energy proton fluxes at the earliest stages of the shockdevelopment. The hard X-ray emission due to bremsstrahlung from shock-heatedgas traced by the Swift-XRT 2-10 keV light-curve peaked at day ~6, later thanat GeV and optical energies. Using X-ray derived temperatures to constrain thevelocity profile, we find the hadronic model reproduces the observed >0.1 GeVlight-curve.

Fermi-LAT Gamma-ray Detection of the Recurrent Nova RS Ophiuchi during its 2021 Outburst