SOAR spectroscopic classification of the Galactic plane gamma-ray transient Fermi J1820-1648

ATel #17700; P. Craig, I. Molina, J. Strader, L. Chomiuk (Michigan State University), E. Aydi, K. Sokolovsky, D. Perla, V. Antoniou, T. Maccaroone, E. Jarvela, and M. Fausnaugh (Texas Tech University)
on 25 Feb 2026; 19:40 UT
Credential Certification: Elias Aydi (eaydi@ttu.edu)

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We report optical spectroscopic follow-up observations of the candidate optical counterpart (ATel #17698) to the Galactic-plane gamma-ray transient Fermi J1820-1648, discovered by Fermi-LAT on 2026-02-18.7 (ATels #17688, #17699).

On 2026-02-25.4 we obtained a low-resolution spectrum of the source using the Goodman spectrograph (Clemens et al. 2004, SPIE, 5492, 331) on the 4.1-m SOAR telescope. The spectrum shows broad, double-peaked emission lines of Halpha, O I, and H I. The full width at zero intensity (FWZI) of Halpha is approximately 4500 km/s. The near-infrared lines are significantly stronger relative to Halpha, consistent with substantial reddening. This is also emphasized by the lack of continuum and emission lines at wavelengths shorter than 6000 Angstrom.

The overall spectral characteristics are consistent with those of a highly reddened classical nova observed after optical maximum. This classification strongly supports the association between the optical source and the gamma-ray transient, as classical novae are now well-established Galactic gamma-ray sources (e.g., Craig et al. 2026, MNRAS 546, 2270).

Determining a reliable distance to this nova will be crucial. Highly reddened and distant novae are not commonly detected by Fermi-LAT, and if this system lies at a large distance, it may represent an unusually gamma-ray luminous Galactic nova.

This ATel is based on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministerio da Ciencia, Tecnologia, Inovacoes e Comunicacoes do Brasil (MCTIC/LNA), the U.S. National Science Foundation's National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU).