GCN Circular 44953

C. Malacaria (INAF-OAR) reports on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 07:05:16.39 UT on 16 June 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260616B (trigger 803286321/260616295).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 257.49, Dec = -32.62 (J2000 degrees, equivalent to J2000 17h 9m, -32d 37'), with a statistical uncertainty of 1.70 degrees. (radius, 1-sigma containment, statistical only; there is additionally a systematic error which we have characterized as a core-plus-tail model, with 90% of GRBs having a 3.7 deg error and a small tail suffering a larger than 10 deg systematic error [Connaughton et al. 2015, ApJS, 216, 32]).

The angle from the Fermi LAT boresight is 76 degrees.

The GBM light curve consists of two main pulses with a duration (T90) of about 13.8 s (50-300 keV). The time-averaged spectrum from T0+0.002 to T0+14.464 s is best fit by a Band function with Epeak = 240 +/- 10 keV, alpha = -0.63 +/- 0.04, and beta = -2.09 +/- 0.06.

The event fluence (10-1000 keV) in this time interval is (2.03 +/- 0.03)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+10 s in the 10-1000 keV band is 21.3 +/- 0.4 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/"