TITLE: GCN CIRCULAR NUMBER: 39530 SUBJECT: GRB 250226A: Fermi GBM Observation DATE: 25/02/28 14:49:24 GMT FROM: Utkarsh Pathak at IIT Bombay utkarshpathak.07@gmail.com
U. Pathak (IITB) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 06:34:57.33 UT on 26 February 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250226A (trigger 762244502/250226274) which was also detected by EP/WXT and EP/FXT (Jiang et al. 2024, GCN 39482). It was also detected by INTEGRAL SPI-ACS and PICsIT (Thakur et al. 2025, GCN 39518), GECAM-B (Zhang et al. 2025, GCN 39492). Optical follow-ups have been done with TRT (An et al. 2025, GCN 39486), COLIBRI/DDRAGO (Magnani et al. 2025, GCN 39488), GSP (Li et al. 2025, GCN 39489), Kinder (Aryan et al. 2025, GCN 39509), 1.6m Mephisto (Zhou et al. 2025, GCN 39511), SVOM/VT (Li et al. 2025, GCN 39514), Xinglong (Jin et al. 2025, GCN 39515). The spectroscopic redshift of the optical counterpart observed by VLT/X-shooter (Zhu et al. 2025, GCN 39487) is 3.315. The Fermi GBM on-ground location is consistent with the EP/FXT position.
The angle from the Fermi LAT boresight is 108 degrees.
The GBM light curve consists of multiple spikes from a single emission episode with a duration (T90) of about 61 s (50-300 keV). The time-averaged spectrum from T0-2.6 to T0+62.0 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.36 +/- 0.03, and the cutoff energy, parameterized as Epeak, is 4408 +/- 1160 keV.
The event fluence (10-1000 keV) in this time interval is (1.96 +/- 0.06)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+3.4 s in the 10-1000 keV band is 4.7 +/- 0.3 ph/s/cm^2.
A Band function fits the spectrum equally well with Epeak= 2864 +/- 2100 keV, alpha = -1.35 +/- 0.04 and beta = -1.8 +/- 0.2.
As the burst occurred near SAA entry, the spectral results of both fits to the data (especially Epeak) are likely to have some contamination and thus may not reflect the burst's true spectral nature.
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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