Fermi-LAT detections of novae V1723 Sco and V6598 Sgr

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.

💡 Research Summary

This paper presents a comprehensive multi‑wavelength analysis of two recent classical novae—V1723 Sco (discovered in February 2024) and V6598 Sgr (discovered in July 2023)—focusing on their γ‑ray emission as observed by the Fermi Large Area Telescope (LAT). The authors first outline the prevailing theoretical framework in which internal shocks within the nova ejecta accelerate particles to tens of GeV via diffusive shock acceleration, leading to γ‑ray production either through leptonic (inverse‑Compton scattering by relativistic electrons) or hadronic (π⁰ decay from proton‑proton collisions) channels. A persistent tension in this picture is the lack of contemporaneous non‑thermal X‑ray emission, which would be expected if relativistic electrons were abundant.

Using Pass 8 (P8R3_SOURCE_V3) data spanning 100 MeV–300 GeV, the authors construct a 15° region of interest around each nova, model the Galactic diffuse background with the latest gll_iem_v07 template, and include all 4FGL catalog sources within 20°. A one‑year pre‑outburst dataset is first used to fix background parameters (model M₀). Light curves are then generated by inserting a point source at the optical coordinates and evaluating the test statistic (TS) in daily bins (TS > 4 and N_pred > 4 define a detection). The optimal γ‑ray interval is defined as the contiguous set of bins meeting this criterion, allowing at most a two‑day gap of upper limits.

For V1723 Sco, the γ‑ray emission begins roughly six hours after the optical discovery and persists for 15 days. The emission is brightest in the first nine one‑day bins, followed by three two‑day bins. A finer 6‑hour binning confirms the rapid onset. A secondary γ‑ray flare appears around day 55 (≈ 40 days post‑discovery), coincident with a hard thermal X‑ray detection by NuSTAR. Localization yields a 68 % containment radius of 0.08°, fully consistent with the optical position. Spectral fitting favors a power‑law with exponential cutoff (PLExpCutoff), with photon index Γ = 1.5 ± 0.2, cutoff energy E_cut = 1.6 ± 0.6 GeV, and an integrated flux above 100 MeV of 1.1 × 10⁻⁶ ph cm⁻² s⁻¹. The spectrum shows curvature below 500 MeV, a hallmark of π⁰‑decay signatures.

NuSTAR observations conducted 13.5 days after the optical outburst detect only thermal X‑ray emission; no non‑thermal component is present during the γ‑ray window. However, the later hard X‑ray detection (≈ 10 keV) aligns with the secondary γ‑ray flare, suggesting that shock interaction with external material can reignite particle acceleration weeks after the initial eruption.

V6598 Sgr displays a markedly different behavior. The γ‑ray signal is detected at the 4σ level over just two days, making it one of the shortest γ‑ray novae observed. Localization places the source within 0.29° (95 % containment) of the optical coordinates. The spectrum is adequately described by a simple power‑law (Γ ≈ 2.3) with no statistically significant cutoff or curvature. Swift‑XRT fails to detect the source in the immediate aftermath, consistent with a high intrinsic absorption (N_H ≈ 3 × 10²² cm⁻²). A faint X‑ray re‑appearance occurs 17 days later at a flux roughly four times lower than the pre‑outburst level.

The authors compute γ‑ray to optical and γ‑ray to X‑ray luminosity ratios for both novae. Both objects exhibit high ratios (γ‑ray/optical ≈ 10⁻³–10⁻², γ‑ray/X‑ray ≈ 10⁻²–10⁻¹), values that are more naturally reproduced by hadronic models, which predict efficient conversion of kinetic shock power into π⁰‑decay γ‑rays while leaving the X‑ray band dominated by thermal emission. The observed low‑energy curvature in V1723 Sco’s spectrum further supports a hadronic origin, as leptonic inverse‑Compton spectra typically remain power‑law down to the LAT threshold.

In the discussion, the paper emphasizes three lines of evidence favoring a hadronic scenario: (1) the absence of contemporaneous non‑thermal X‑ray emission, (2) the spectral shape (curvature and cutoff) matching π⁰‑decay expectations, and (3) the high γ‑ray to lower‑energy luminosity ratios. The authors also note that the delayed γ‑ray and hard X‑ray activity in V1723 Sco implies that shock‑driven particle acceleration can persist or be re‑triggered weeks after the initial eruption, possibly as the ejecta encounter denser circumbinary material.

The conclusion asserts that the combined LAT, optical, and X‑ray analysis provides strong observational support for hadronic particle acceleration as the dominant mechanism in classical nova γ‑ray production. The paper calls for coordinated, high‑cadence radio, optical, and X‑ray campaigns, together with detailed hydrodynamic and particle‑acceleration simulations, to further elucidate shock geometry, magnetic field amplification, and the efficiency of proton acceleration in these transient astrophysical laboratories.