Detection of RS Oph with LST-1 and modelling of its HE/VHE gamma-ray emission

The recurrent nova RS Ophiuchi (RS Oph) underwent a thermonuclear eruption in August 2021. In this event, RS Oph was detected by the High Energy Stereoscopic System ( H.E.S.S. ), the Major Atmospheric Gamma Imaging Cherenkov (MAGIC), and the first Large-Sized Telescope ( LST-1 ) of the future Cherenkov Telescope Array Observatory (CTAO) at very-high gamma-ray energies above 100 This means that novae are a new class of very-high-energy (VHE) gamma-ray emitters. We report the analysis of the RS Oph observations with LST-1 . We constrain the particle population that causes the observed emission in hadronic and leptonic scenarios. Additionally, we study the prospects of detecting further novae using LST-1 and the upcoming LST array of CTAO-North. We conducted target-of-opportunity observations with LST-1 from the first day of this nova event. The data were analysed in the framework of cta-lstchain and Gammapy the official CTAO-LST reconstruction and analysis packages. One-zone hadronic and leptonic models were considered to model the gamma-ray emission of RS Oph using the spectral information from Fermi -LAT and LST-1 together with public data from the MAGIC and H.E.S.S. telescopes. RS Oph was detected at $6.6σ$ with LST-1 in the first 6.35 hours of observations following the eruption. The hadronic scenario is preferred over the leptonic scenario considering a proton energy spectrum with a power-law model with an exponential cutoff whose position increases from $(0.26± 0.08) TeV$ on day 1 up to $(1.6± 0.6) TeV$ on day 4 after the eruption. The deep sensitivity and low energy threshold of the LST-1/LST array will allow us to detect faint novae and increase their discovery rate.