V1723 Sco and V6598 Sgr: Decoding the Fastest and Brightest Gamma-Ray Eruptions

Classical novae, thermonuclear eruptions on the surface of a white dwarf in a binary system, are known sources of high-energy gamma-rays detected by the Fermi-LAT. This episode explores a multi-wavelength analysis of two recent novae, **V1723 Sco 2024** and **V6598 Sgr 2023**, aiming to constrain the mechanism behind this intense gamma-ray emission.

**V1723 Sco** proved to be a very bright gamma-ray source, with emission lasting 15 days, allowing scientists to constrain the total energy and spectral properties of accelerated protons. Intriguingly, V1723 Sco also showed unexpected gamma-ray and thermal hard X-ray emission more than 40 days after its initial outburst, suggesting that particle acceleration can occur even several weeks post-eruption.

In contrast, **V6598 Sgr** was detected by Fermi-LAT for only two days, marking one of the shortest gamma-ray emission durations ever recorded for a classical nova. Its brief gamma-ray signal coincided with a rapid decline in optical brightness. V6598 Sgr also exhibits peculiar characteristics, including no significant gamma-ray emission below 1 GeV and the possibility that it is an Intermediate Polar (IP) system, which may hint at a different particle acceleration region due to potentially strong magnetic fields.

The detailed analysis, which combined Fermi-LAT data with optical (AAVSO) and X-ray (NuSTAR) observations, strongly supports the hypothesis that the gamma-ray generation in both novae is more consistent with the **hadronic scenario** (involving accelerated protons) than the leptonic scenario. However, the long-standing challenge remains: no non-thermal X-ray emission has been detected simultaneously with the gamma-rays.

**Article Reference:**

Fauverge, P., Jean, P., Sokolovsky, K., et al. (2025). *Fermi-LAT detections of the classical novae V1723 Sco and V6598 Sgr in a multi-wavelength context.* submitted to Astronomy & Astrophysics, arXiv: 2512.14198

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: NASA's Goddard Space Flight Center/S. Wiessinger