TITLE: GCN CIRCULAR NUMBER: 40422 SUBJECT: GRB 250509A: Fermi GBM Observation DATE: 25/05/10 16:51:18 GMT FROM: Ava Myers at NASA GSFC <ava.myers@nasa.gov> A. Myers (NPP/GSFC), C. Meegan (UAH) and A. Holzmann Airasca (UniTrento and INFN Bari) report on behalf of the Fermi Gamma-ray Burst Monitor Team: "At 22:33:29.29 UT on 09 May 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250509A (trigger 768522814/250509940). which was also detected by Swift BAT (M. H. Siegel et al. 2025, GCN 40407). The Fermi GBM on-ground location is consistent with the Swift BAT position. The angle from the Fermi LAT boresight is 89 degrees. The GBM light curve consists of main emission episode with a duration (T90) of about 69 s (50-300 keV). The time-averaged spectrum from T0+0.000 to T0+69.120 s is best fit by a power law function. The power law index is -2.19 +/- 0.03. The event fluence (10-1000 keV) in this time interval is (6.18 +/- 0.29)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+6.2 s in the 10-1000 keV band is 3.6 +/- 0.3 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: View this GCN Circular online at https://gcn.nasa.gov/circulars/40422. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...