TITLE: GCN CIRCULAR NUMBER: 37302 SUBJECT: Konus-Wind detection of GRB 240825A (bright/long) DATE: 24/08/26 17:10:30 GMT FROM: Dmitry Frederiks at Ioffe Institute fred@mail.ioffe.ru

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

The bright, long GRB 240825A (Fermi GBM detection: Fermi GBM team, GCN 37273; Sharma & Meegan, GCN 37303; Swift detection: Gupta et al., GCN 37274; Fermi LAT detection: Di Lalla et al., GCN 37288; AstroSat CZTI detection: Joshi al., GCN 37298) triggered Konus-Wind (KW) at T0=57185.048 s UT (15:53:05.048).

The burst light curve shows a bright multi-peaked emission pulse, which starts at ~T0 - 0.1 s, peaks around ~T0 + 0.256 s, and has a total duration of ~2 s. This pulse is followed by a weaker emission, which lasts until ~T0 + 25 s. The emission is seen up to ~10 MeV.

The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB240825_T57185/

As observed by Konus-Wind, the burst had a fluence of (1.66 ± 0.08)x10^-4 erg/cm^2 and a 64-ms peak energy flux, measured from T0 + 0.256 s, of (1.52 ± 0.07)x10^-4 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

A time-averaged spectrum (measured from T0 to T0+28.16 s) is best fit in the 20 keV - 15 MeV range by a GRB (Band) function with the following model parameters: the low-energy photon index alpha = -0.79 (-0.04,+0.05), the high energy photon index beta = -2.10 (-0.8,+0.06), the peak energy Ep = 403 (-37,+40) keV, chi2 = 99/97 dof.

A spectrum near the peak count rate (measured from T0 to T0+0.512 s) is best fit in the 20 keV - 15 MeV range by a GRB (Band) function with the following model parameters: the low-energy photon index alpha = -0.30 (-0.11,+0.12), the high energy photon index beta = -2.30 (-0.09,+0.08), the peak energy Ep = 405 (-36,+41) keV, chi2 = 68/63 dof.

Assuming the redshift z=0.659 (Martin-Carrillo et al., GCN 37253) and a standard cosmology with H_0 = 67.3 km/s/Mpc, Omega_M = 0.315, and Omega_Lambda = 0.685 (Planck Collaboration, 2014), we estimate the burst isotropic energy release E_iso to (2.00 ± 0.96)x10^53 erg, the isotropic peak luminosity L_iso to (3.04 ± 0.14)x10^53 erg/s, the rest-frame peak energy Ep,i,z to (670 ± 60) keV. With the obtained estimates, GRB 240825A is inside 68% prediction bands for both 'Amati' and 'Yonetoku' relations derived for the sample of >300 long KW GRBs with known redshifts (Tsvetkova et al., 2017; Tsvetkova et al., 2021), see http://www.ioffe.ru/LEA/GRBs/GRB240825_T57185/GRB240825A_rest_frame.pdf

All the quoted errors are estimated at the 68% confidence level. All the presented results are preliminary.

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