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

¹ Univ. Bordeaux, CNRS, LP2i Bordeaux, UMR 5797, F-33170 Gradignan, France
² IRAP, Université de Toulouse, CNRS, CNES, UPS, 9 Avenue Colonel Roche, 31028 Toulouse Cedex 4, France
³ Department of Astronomy, University of Illinois Urbana-Champaign, 1002 W. Green Street, Urbana, IL 61801, USA
⁴ Space Science Division, Naval Research Laboratory, Washington, DC 20375-5352, USA
⁵ Center for Data Intensive and Time Domain Astronomy, Department of Physics and Astronomy, Michigan State University, 567 Wilson Rd, East Lansing, MI 48824, USA
⁶ National Radio Astronomy Observatory, Domenici Science Operations Center, 1003 Lopezville Road, Socorro, NM 87801, USA
⁷ CRESST and X-ray Astrophysics Laboratory, NASA/GSFC, Greenbelt, MD 20771, USA
⁸ Department of Physics and Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 2 July 2025
Accepted: 8 November 2025

Abstract

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 4σ 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.