No bright optical counterpart to the Galactic plane gamma-ray transient Fermi J1820-1648

ATel #17689; Kirill Sokolovsky (TTU), Oxana Chernyshenko, Alexander Getmansky, Ekaterina Saveleva, Stanislav Korotkiy (Astroverty, Ka-Dar), Elias Aydi (TTU)
on 19 Feb 2026; 17:28 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

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Following the report of the Galactic plane gamma-ray transient Fermi J1820-1648 detected with Fermi-LAT in a week-long integration from 2026-02-09.0 to 2026-02-16.0 (ATel #17688), we inspected wide-field optical images of the region obtained with the New Milky Way (NMW) survey cameras around that time. No new optical sources brighter than magnitude 13 appear within the Fermi error circle during the gamma-ray transient detection time. The dates and approximate limiting magnitudes of the NMW survey images of the Fermi J1820-1648 region are listed below:

 
# Date_(UTC) lim.mag. Camera 
2026-01-29.144 >12.5 ST-8300M 
2026-01-30.142 >13.0 ST-8300M 
2026-02-10.112 >13.3 ST-8300M 
2026-02-10.130 >13.0 STL-11000M 
2026-02-12.107 >13.5 ST-8300M 
2026-02-12.124 >13.1 STL-11000M 
2026-02-16.096 >13.3 ST-8300M 

The images were obtained with two unfiltered CCD cameras, ST-8300M and STL-11000M, attached to identical Canon 135mm f/2.0 telephoto lenses providing 8x6 degrees (8.35"/pix) and 15x10 degrees (13.8"/pix) fields of view, respectively. The cameras are located in a rolling-roof pavilion at the Astroverty astrofarm in Nizhnii Arkhyz, Karachay-Cherkessia, Russia. The magnitude scale zero-point is set using V magnitudes of Tycho-2 stars in the field.

The optical non-detections largely exclude the possibility of Fermi J1820-1648 being associated with a classical nova. The faintest Fermi-detected classical nova in the sample of Craig et al. (arXiv:2508.15900) was V1707 Sco, which peaked at V=11.8, while the other novae in that sample peaked at V magnitudes brighter than 10. An association with a classical nova would require either an unusually optically faint gamma-ray nova or a nova with a very fast decline time (fast enough to fall within the gaps between NMW observations) to have escaped optical detection.

Plate-solved FITS images of the Fermi J1820-1648 region obtained with the NMW wide-field cameras