TITLE: GCN CIRCULAR NUMBER: 44931 SUBJECT: GRB 260610B: Fermi GBM Observation DATE: 26/06/13 18:21:44 GMT FROM: Matt Godwin <msg0028@uah.edu> Matt Godwin (UAH) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team: "At 23:46:14.25 UT on 10 June 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260610B (trigger 802827979/260610990). An optical counterpart is identified by GOTO (O'Neill, et al. 2026, GCN 44903). The redshift of the source is measured to be z = 0.473, ALFOSC/NOT (ONeill et al. 2026, GCN 44914). The Fermi GBM on-ground location is consistent with the GOTO position. The angle from the Fermi LAT boresight is 97 degrees. The GBM light curve consists of three peaks with a duration (T90) of about 62 s (50-300 keV). The time-averaged spectrum from T0-2.4 to T0+55.4 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.50 +/- 0.01 and the cutoff energy, parameterized as Epeak, is 46.8 +/- 0.4 keV. The event fluence (10-1000 keV) in this time interval is (1.30 +/- 0.02)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+3.8 s in the 10-1000 keV band is 7.7 +/- 0.3 ph/s/cm^2. A Band function fits the spectrum equally well with Epeak= 45.9 +/- 0.5 keV, alpha = -1.4800 +/- 0.0009 and beta = -3.17 +/- 0.01. 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/" View this GCN Circular online at https://gcn.nasa.gov/circulars/44931. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...