Fermi-LAT detections of novae V1723 Sco and V6598 Sgr

Abstract

Context. Numerous classical novae have been observed to emit {\gamma}-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 {\gamma}-rays remains a challenge to this theory. Methods. We performed similar analyses of the Fermi-LAT data for both novae to determine the duration, localization, and spectral properties of the {\gamma}-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 {\gamma}-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 {\gamma}-rays. However, unexpected {\gamma}-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{\sigma}over just two days, one of the shortest {\gamma}-ray emission durations ever recorded, coinciding with a rapid decline in optical brightness. Finally, the high ratio of {\gamma}-ray to optical luminosities and {\gamma}-ray to X-ray luminosities for both novae, as well as the curvature of the {\gamma}-ray spectrum of V1723 Sco below 500 MeV, are all more consistent with the hadronic than the leptonic scenario for {\gamma}-ray generation in novae.