Fermi-LAT detections of novae V1723 Sco and V6598 Sgr

TL;DR

This paper analyzes Fermi-LAT γ-ray data for two classical novae, V1723 Sco 2024 and V6598 Sgr 2023, to characterize emission duration, localization, and spectral properties, and to interpret the particle acceleration mechanisms with multi-wavelength constraints from optical (AAVSO) and X-ray (NuSTAR) data. A dedicated LAT analysis pipeline, including a pre-outburst baseline model, time-window optimization, and spectral-model comparisons, reveals a ~15-day γ-ray outburst for V1723 Sco beginning around the optical discovery, with a spectral fit favoring a hadronic (pion-decay) scenario; V6598 Sgr shows a much shorter, weaker γ-ray signal. Multi-wavelength modeling using NAIMA yields a hard proton spectrum for V1723 Sco (approximately $s_p oughly 1.8$) and a finite total energy in accelerated protons ($W_p oughly 4 imes10^{39}$ erg under fiducial ejecta parameters), with an inferred shock energy budget that implies non-negligible but sub-dominant particle acceleration efficiency and modest γ-ray absorption effects. The results support diffusive shock acceleration at nova shocks as a plausible γ-ray production mechanism, while also demonstrating the importance of absorption, geometry, and delayed/secondary shocks—evident in the late-time NuSTAR detection—highlighting the complexity of particle acceleration in these explosive environments and the need for coordinated multi-wavelength coverage. This work broadens the sample of γ-ray novae and provides quantitative constraints on proton energetics and physical conditions in nova shocks, informing models of shock acceleration in astrophysical transients.

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. 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 4σover 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.

Figures (9)

• Figure 1: $\gamma$-ray and optical light curves of V1723 Sco (top) and V6598 Sgr (bottom) calculated from the Fermi-LAT data (black and gray) and taken from the AAVSO database in the V-band (green) and unfiltered (orange) respectively. Flux estimates with 1$\sigma$ statistical uncertainties only are plotted when TS and $N_{pred}$ > 4 and 95 $\%$ upper limits otherwise. The blue windows indicate the time periods of the NuSTAR observations. $t_{disc}$ corresponds to the optical discovery of both novae (Table \ref{['tab:params']}).
• Figure 2: TS maps of V1723 Sco (left) and V6598 Sgr (right) during the optimal time period (see text). The value in TS is estimated in each pixel by fitting a point source with a power-law model (index fixed to 2). The green point is the optical nova position, the black one is the position found by gta.localize with the 68$\%$, 95$\%$ and 99$\%$ containment radius. The white crosses are known $\gamma$-ray sources from the 4FGL catalog.
• Figure 3: Spectra of V1723 Sco (left) and V6598 Sgr (right) during the optimal time period (see text). Only the best spectral model and its corresponding uncertainty was represented. Flux points with 1$\sigma$ statistical uncertainties (black) and the quadratic sum of statistical and systematic uncertainties (red) are plotted when TS and $N_{pred}$ > 2 and 95 $\%$ upper limits otherwise. Energy bins from the analysis are merged only for the calculation of the flux points.
• Figure 4: Zoom into the Fermi-LAT light curve of V1723 Sco (Figure \ref{['fig:lightcurves']}) recalculated with bins of 6 hours length. See Figure \ref{['fig:lightcurves']} for the legend.
• Figure 5: V1723 Sco and V6598 Sgr in the flux-duration parameter space. We used average fluxes and duration measured by 2018AA...609A.120F, novae_fermi, 2016ApJ...826..142C, V407Lup..Gordon, 2019ApJ...872...86N, lietal2017, 2020ApJ...905..114L, 2020NatAs...4..776A, 2022ApJ...940..141A, 2019ATel13116....1L, 2022MNRAS.514.2239S, 2021ATel14658....1B, Her2021...Sokolovsky and rsoph_fermi for the Fermi-LAT novae points and this work for V1723 Sco and V6598 Sgr. The fastest and brightest nova is V1674 Her, the longest-lasting is V5668 Sgr, and the brightest and long-duration nova is RS Ophiuchi.