Fermi-LAT detections of the classical novae V1723 Sco and V6598 Sgr in a multi-wavelength context

Context. Numerous classical novae have been observed to emit γ -rays ( E > 100 MeV) detected by the Fermi-LAT. The prevailing hypothesis attributes this emission to the interaction of accelerated particles within shocks in the nova ejecta. However, the lack of non-thermal X-ray detection coincident with the γ -rays remains a challenge to this theory. Aims. We aim to constrain the γ -ray production mechanism by combining optical and X-ray data with a detailed analysis of the Fermi-LAT observations for two classical novae, V1723 Sco 2024 and V6598 Sgr 2023. Methods. We performed similar analyses of the Fermi-LAT data for both novae to determine the duration, localization, and spectral properties of the γ -ray emission. These results were compared with optical data from the AAVSO database and X-ray observations from NuSTAR, available for V1723 Sco 2024 only, to infer the nature of the accelerated particles. Finally, we used a physical emission model to extract key parameters related to particle acceleration. Results. V1723 Sco 2024 was found to be a very bright γ -ray source with an emission duration of 15 days allowing us to constrain the spectral index and the total energy of accelerated protons. Despite early NuSTAR observations, no non-thermal X-ray emission was detected simultaneously with the γ -rays. However, unexpected γ -ray and thermal hard X-ray emission were observed more than 40 days after the nova outburst, suggesting that particle acceleration can occur even several weeks post-eruption. V6598 Sgr 2023, on the other hand, was detected by the Fermi-LAT at a significance level of 4 σ over just two days, one of the shortest γ -ray emission durations ever recorded, coinciding with a rapid decline in optical brightness. Finally, the high ratio of γ -ray to optical luminosities and γ -ray to X-ray luminosities for both novae, as well as the curvature of the γ -ray spectrum of V1723 Sco below 500 MeV, are all more consistent with the hadronic than the leptonic scenario for γ -ray generation in novae.