TITLE: GCN CIRCULAR NUMBER: 35835 SUBJECT: Konus-Wind detection of GRB 240225B (long) DATE: 24/02/29 14:46:36 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:

GRB 240225B (MAXI/GCS detection: Nakajima et al., GCN 35796; AstroSat CZTI detection: Joshi al., GCN 35798) triggered Konus-Wind (KW) at T0=72727.251 s UT (20:12:07.251), i.e., ~3.5 min before the MAXI trigger.

The burst light curve shows a multi-peaked emission pulse, which starts at ~T0 - 2 s, peaks around ~T0 + 6 s, and has a total duration of ~35 s. The emission is seen up to ~3 MeV.

The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB240225_T72727/

As observed by Konus-Wind, the burst had a fluence of (2.16 ± 0.35)x10^-5 erg/cm^2 and a 64-ms peak energy flux, measured from T0 + 6.144 s, of (2.87 ± 0.41)x10^-6 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

A time-averaged spectrum (measured from T0 to T0+41.216 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 = -1.37 (-0.14,+0.15), the high energy photon index beta = -2.74 (-7.26,+0.43), the peak energy Ep = 270 (-43,+64) keV, chi2 = 107/97 dof.

A spectrum near the peak count rate (measured from T0 to T0+8.448 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 = -0.85 (-0.110,+0.10), the high energy photon index beta = -2.43 (-0.55,+0.24), the peak energy Ep = 287 (-47,+59) keV, chi2 = 75/79 dof.

Assuming the redshift z=0.946 (Schneider et al., GCN 35832) 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 (5.38 ± 0.95)x10^52 erg, the isotropic peak luminosity L_iso to (1.87 ± 0.27)x10^52 erg/s, the rest-frame peak energy of the time-integrated spectrum Ep,i,z to ~525 keV, and the rest-frame peak energy at the peak of the emission Ep,p,z to ~558 keV. With the obtained estimates, GRB 240225B 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/GRB240225_T72727/GRB240225B_rest_frame.pdf

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

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