GCN 44122: GRB 260310A / AT2026fgk: Late-time infrared detection with MIRAGE on the MDM 1.3m telescope

R. B. Effress, B. T. Gebreamlak, B. O. Goldman, T. Goodman, J. Kalish, L. Koo, T. Lack, S. A. Lawsky, S. L. Manyan, M. Nebart, K. N. Oqueli-White, A. Ramadorai, N. A. Rodgers, T. A. Sabin, E. Sharman, C. Wu, L. Zheng, R. Zheng, G. Mo, V. Karambelkar, S. Ibrahim, N. Aftab, D. Schiminovich, K. De (Columbia University)

We observed the field of GRB 260310A detected by Fermi/GBM (Fermi GB team GCN 43951) and AstroSat CZTI (Salunke et al. GCN 43958), during commissioning operations of the MDM InfraRed Astronomy InGaAs Explorer (MIRAGE) instrument on the MDM 1.3m telescope.  Observations were carried out starting at UTC 2026-03-22 08:50, approximately three weeks after the GRB trigger. 

We obtained a series of dithered exposures in J-band and Hs-band amounting to a total exposure time of 600 s each. Images were aligned, stacked and calibrated to the 2MASS catalog. Performing forced photometry at the position of the optical counterpart (AT2026fgk; Konno et al. GCN 43974, Hinds et al. AstroNote 2026-65; O’Neill et al. TNS Discovery Report 294132), we clearly detect a point source with magnitudes
J = 18.40 +/- 0.14 Vega mag
Hs = 17.07 +/- 0.16 Vega mag
The measurements confirm continued infrared fading of the source (Gao et al. GCN 44110). Note that these measurements did not include image subtraction, though the host contamination is expected to be small.

Further observations are planned and analysis is being improved. MIRAGE is a new YJH-band near-infrared imager for the MDM 1.3m telescope. We thank the MDM Observatory staff for supporting the commissioning observations.