TITLE: GCN CIRCULAR NUMBER: 39690 SUBJECT: GRB 250226B: Fermi GBM Observation DATE: 25/03/12 20:16:32 GMT FROM: Jacob Smith at Fermi-GBM Team jrs0118@uah.edu

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

At 22:30:23.12 UT on 26 February 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250226B (trigger 762301828/250226938). which was also detected by INTEGRAL SPI-ACS (J. Rodi, et al. 2025, GCN 39519) and Swift/BAT-GUANO (S. Ronchini, et al., 2025, GCN 39673).

The angle from the Fermi LAT boresight is 71 degrees.

The GBM light curve consists of multiple peaks with a duration (T90) of about 26 s (50-300 keV). The time-averaged spectrum from T0+0.003 to T0+27.457 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.09 +/- 0.03 and the cutoff energy, parameterized as Epeak, is 1688 +/- 250 keV.

The event fluence (10-1000 keV) in this time interval is (1.60 +/- 0.03)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+1.7 s in the 10-1000 keV band is 9.7 +/- 0.2 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/

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