TITLE: GCN CIRCULAR NUMBER: 39964 SUBJECT: GRB 250330A: Fermi GBM Observation DATE: 25/03/31 06:48:11 GMT FROM: Utkarsh Pathak at IIT Bombay utkarshpathak.07@gmail.com

U. Pathak (IITB), R. Sonawane (IISER, TVM) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 06:07:38.42 UT on 30 March 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250330A (trigger 765007663/250330255). which was also detected by Swift-BAT (Cenko et al. 2025, GCN 39938). The afterglow has been detected by Swift-XRT (Osborne et al. 2025, GCN 39947). The Fermi GBM Final Localization (GCN 39944) is consistent with the Swift-BAT and Swift-XRT positions (Page et al. 2025, GCN 39940).

The angle from the Fermi LAT boresight is 46 degrees.

The GBM light curve consists of a single emission episode with a duration (T90) of about 4.8 s (50-300 keV). The time-averaged spectrum from T0-0.6 to T0+6.53 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.9 +/- 0.1 and the cutoff energy, parameterized as Epeak, is 95 +/- 8 keV.

The event fluence (10-1000 keV) in this time interval is (1.50 +/- 0.08)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+0.26 s in the 10-1000 keV band is 6.5 +/- 0.3 ph/s/cm^2.

A Band function fits the spectrum equally well with Epeak= 86 +/- 10 keV, alpha = -0.8 +/- 0.2 and beta = -2.7 +/- 0.4.

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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