TITLE: GCN CIRCULAR NUMBER: 45038 SUBJECT: Konus-Wind detection of GRB 260618B DATE: 26/06/24 17:14:43 GMT FROM: Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru> A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin, A. Tsvetkova, M. Ulanov, and T. Cline, on behalf of the Konus-Wind team, report: The long-duration GRB 260618B (Fermi-GBM detection: Fermi GBM team, GCN 44979; Veres & Meegan, GCN 45010; Fermi-LAT detection: Gupta et al., GCN 44983) triggered Konus-Wind at T0=60907.637 s UT (16:55:07.637). The burst light curve shows a multipeaked structure which starts at ~T0-1.6 s and has a total duration of ~61.2 s. The emission is seen up to ~4 MeV. The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB260618_T60907/ As observed by Konus-Wind, the burst had the total fluence of 2.26(-0.34,+0.43)x10^-5 erg/cm2, and the 64-ms peak flux, measured from T0+0.378 s, of 4.90(-0.98,+1.01)x10^-6 erg/cm2/s (both in the 20 keV - 10 MeV energy range). The time-averaged spectrum of the burst (measured from T0 to T0+57.600 s) is best fit in the 20 keV - 10 MeV range by a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -0.83(-0.16,+0.19) and Ep = 1066(-241,+365) keV (chi2 = 94/98 dof). Fitting by a GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -2.7 (chi2 = 94/97 dof). The spectrum near the maximum count rate (measured from T0 to T0+8.448 s) is best fit in the 20 keV - 10 MeV range by a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep) with alpha = -0.39(-0.14,+0.15) and Ep = 817(-97,+119) keV (chi2 = 96/88 dof). Fitting by a GRB (Band) model yields the same alpha and Ep, and an upper limit on the high energy photon index: beta < -3.2 (chi2 = 96/87 dof). All the quoted errors are at the 68% confidence level. All the quoted values are preliminary. View this GCN Circular online at https://gcn.nasa.gov/circulars/45038. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...