GCN Circular 43977
K-Ryan Hinds (Caltech), Daniel A. Perley (LJMU), Anna Y. Q. Ho (Cornell), Sam Rose (Caltech), Jillian Rastinejad (UMd), Robert Stein (UMd), Geoffrey Mo (Caltech), Mansi Kasliwal (Caltech), Jesper Sollerman (OKC), Xander J. Hall (CMU), Jacob Wise (LJMU), Rahul Jayaraman (Cornell), Aleksandra Bochenek (LJMU)
We previously reported the optical transient AT 2026fgk (ID 294132) as an afterglow candidate based on its fast evolution, red color, and high implied luminosity (Hinds et al., AstroNote 2026-65). Konno et al. (GCN 43974) reported additional detections of AT 2026fgk using the Large Array Survey Telescope (LAST) whilst tiling the probability region for GRB 260310A (Fermi GBM Team, GCN 43951; Salunke et al., GCN 43958), and noted the temporal and spatial coincidence that identifies AT 2026fgk as the likely optical counterpart of this GRB.
We observed GRB 260310A starting at 2026-03-12 10:23:42 UTC (~2.2 d after the Fermi GBM trigger time) with the Next Generation Palomar Spectrograph (NGPS; AstroNote 2024-340) on the Palomar 5.1m Hale Telescope (P200). The optical spectrum shows a featureless red continuum, although there is a strong, narrow emission line at 7566 Angstroms that we attribute to H-alpha from the candidate host galaxy at z = 0.153. If this is the redshift of the GRB, it would be among the closest long-duration bursts to date.
For GRB 260310A, using a fluence of 5.8e-6 erg/cm^2 (10-1000 keV) reported by Hamburg et al. (GCN 43795) and assuming the redshift above, we find the gamma-ray E_iso is ~3.69e50 erg, characteristic of low-luminosity long-GRBs. The peak observed optical luminosity (from ATLAS forced photometry) is M_o ~ -22.7 mag at 7.2 hours post-trigger, more luminous than the equivalent value seen in most other nearby LLGRBs.
An associated supernova similar to SN 1998bw would peak at an apparent magnitude of r = 19.7 at this redshift in the absence of host extinction. Further observations are encouraged.