TITLE: GCN CIRCULAR NUMBER: 38887 SUBJECT: GRB 250109A: Fermi GBM Observation DATE: 25/01/10 06:40:17 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 06:17:08.60 UT on 09 January 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250109A (trigger 758096233/250109262). which was also detected by EP-WXT and EP-FXT (Li et al. 2025, GCN 38864). The Fermi GBM on-ground location is consistent with the EP-FXT position. A possible optical counterpart was also detected by SVOM/VT (Qiu et al. 2025, GCN 38872).

The angle from the Fermi LAT boresight is 72 degrees.

The GBM light curve consists of multiple weak spikes from a single emission episode with a duration (T90) of about 15 s (50-300 keV). The time-averaged spectrum from T0-8.2 to T0+9.2 s is best fit by a power law function. The power law index is -1.57 +/- 0.05.

The event fluence (10-1000 keV) in this time interval is (1.9 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+2.9 s in the 10-1000 keV band is 1.3 +/- 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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