Possible optical counterpart to the Galactic plane gamma-ray transient Fermi J1820-1648

ATel #17698; Kirill Sokolovsky, Darien Perla, Elias Aydi, Vallia Antoniou, Michael Fausnaugh, Tom Maccarone, Emilia Jarvela (Texas Tech University), Peter Craig, Laura Chomiuk (Michigan State University)
on 24 Feb 2026; 22:02 UT
Credential Certification: Kirill Sokolovsky (kirx@scan.sai.msu.ru)

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We imaged the region of the gamma-ray transient Fermi J1820-1648 (ATel #17688, #17689) on 2026-02-22.5087 UTC with a 0.3-m telescope + QHY268M CMOS camera located at TTU Preston Gott Skyview Observatory (Shallowater, Texas). The instrument field of view is 36'x24' (0.7"/pix at 2x2 binning) covering the 0.11 deg radius Fermi-LAT error circle in one pointing. Visual comparison of Ic-band images with archival Pan-STARRS1 i-band images reveals the presence of a new bright Ic=13.12 +/-0.01 source within the error circle. The limiting magnitude of the Ic image (a stack of 9x128s exposures) is Ic~17.0. The position of the optical transient measured relative to Gaia DR3 stars in the field:

 
18:20:24.66 -16:53:36.7 +/-0.2" (J2000.0) 

The object is below the 17.5 mag detection limit of the single 128s V-band image obtained with the same setup on 2026-02-22.4985 just prior to the Ic filter series, implying heavy reddening (V-Ic>4.4). The V and I magnitude zero-points were set using an ensemble of APASS stars. A single-point detection ZTF26aahkwud is reported 0.2" from our measured position by ZTF on 2026-02-23.5509 at r=17.33+/-0.05.

The total line-of-sight extinction in the direction of the transient is A(V)= 15.25, A(Ic)= 8.37 and at 8 kpc A(V)= 11.76, A(Ic)= 6.46 according to the combined 3D extinction map of Bovy et al. (2016, ApJ,818, 130) version Combined19 implemented in mwdust code. A classical nova with a typical absolute magnitude of M_V= -7 and near-zero colors at a fiducial distance of 8 kpc would peak at Ic= 14. The transient could be either a Galactic nova at a distance of <8 kpc characterized by a reddening larger than previously seen in gamma-ray detected novae, or something more exotic.

The finder chart and plate-solved FITS images of the Fermi J1820-1648 region can be found here: