TITLE: GCN CIRCULAR NUMBER: 39018 SUBJECT: GRB 250121A: Fermi GBM Observation DATE: 25/01/23 20:33:41 GMT FROM: Matt Godwin msg0028@uah.edu

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

"At 23:36:05.78 UT on 21 January 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250121A (trigger 759195370/250121983). which was also detected by Fermi-LAT (Holzmann Airasca et al. 2025, GCN 39010). The Fermi GBM on-ground location is consistent with the Fermi-LAT position. (Fermi GBM Team et al. 2025, GCN 39006)

The angle from the Fermi LAT boresight is 15 degrees. This burst was also independently detected by INTEGRAL SPI-ACS. The GBM light curve consists of one peak with a duration (T90) of about 5.2 s (50-300 keV). The time-averaged spectrum from T0-0.4 to T0+5.8 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.65 +/- 0.05 and the cutoff energy, parameterized as Epeak, is 230 +/- 10 keV.

The event fluence (10-1000 keV) in this time interval is (5.8 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+0.32 s in the 10-1000 keV band is 14.0 +/- 0.3 ph/s/cm^2.

A Band function fits the spectrum equally well with Epeak= 210 +/- 20 keV, alpha = -0.59 +/- 0.07 and beta = -2.5 +/- 0.3.

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/"

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