TITLE: GCN CIRCULAR NUMBER: 42527 SUBJECT: Konus-Wind detection of GRB 251028A DATE: 25/10/31 16:09:13 GMT FROM: Dmitry Svinkin at Ioffe Institute svinkin@mail.ioffe.ru

D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia, A. Tsvetkova, M. Ulanov, and T. Cline on behalf of the Konus-Wind team, report:

The long-duration GRB 251028A (Ferm-GBM detection: The Fermi GBM team, GCN 42497; Veres and Meegan, GCN 42520; BALROG localization: Preis & Greiner, GCN 42499; MAXI-GSC detection: Sugai et al., GCN 42498; Glowbug detection: Cheung et al., GCN 42506; SVOM-GRM detection: Guo et al., GCN 42522; GECAM-B detection: Guo et al., GCN 42523) triggered Konus-Wind at T0=12396.343 s UT (03:26:36.343).

The burst light curve starts with a possible rise of a weak emission at ~T0-67 s, which, at ~T0-5.5 s, evolves into the main multi-peaked emission episode with a duration of ~38 s. The emission is seen up to ~5 MeV.

The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB251028_T12396/

As observed by Konus-Wind, the burst had the total fluence of 3.95(-0.25,+0.27)x10^-5 erg/cm2, and the 64-ms peak flux, measured from T0+19.568 s, of 3.17(-0.62,+0.64)x10^-6 erg/cm2/s (both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the main episode (measured from T0 to T0+33.024 s) is best fit in the 20 keV - 10 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.70(-0.12,+0.13), the high energy photon index beta = -2.81(-0.39,+0.22), the peak energy Ep = 229(-15,+16) keV (chi2 = 92/97 dof).

The spectrum near the maximum count rate (measured from T0+16.640 to T0+24.832 s) is best fit in the 20 keV - 10 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.31(-0.20,+0.22), the high energy photon index beta = -3.22(-1.23,+0.42), the peak energy Ep = 217(-15,+17) keV (chi2 = 77/82 dof).

All the quoted errors are at the 68% confidence level. All the quoted values are preliminary.

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