TITLE:   GCN CIRCULAR
NUMBER:  43975
SUBJECT: GRB 260310A: Fermi GBM Observation
DATE:    26/03/12 22:19:13 GMT
FROM:    rhamburg@usra.edu

R. Hamburg (USRA) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 04:57:10.81 UT on 10 March 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260310A (trigger 794811435/260310206), which was also detected by AstroSat (Salunke et al. 2026, GCN 43958). This GRB has been reported to have a potential optical counterpart, AT2026fgk, which was discovered by GOTO (O'Neill et al. 2026, TNS Discovery Report 294132) and LAST (Konno et al. 2026, GCN 43974). AT2026fgk has a photometric redshift measurement of z = 0.11 (Hinds et al. 2026, AstroNote 2026-65).

The angle from the Fermi LAT boresight is 124 degrees.

The GBM light curve consists of a single emission episode with a duration (T90) of about 60 s (50-300 keV). The time-averaged spectrum from T0-9.6 to T0+61.6 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.15 +/- 0.01 and the cutoff energy, parameterized as Epeak, is 172 +/- 5 keV.

The event fluence (10-1000 keV) in this time interval is (5.8 +/- 0.4)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+9.8 s in the 10-1000 keV band is 2.3 +/- 0.3 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html

For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"