TITLE: GCN CIRCULAR NUMBER: 40243 SUBJECT: Konus-Wind detection of GRB 250424A DATE: 25/04/24 22:12:37 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 GRB 250424A (Swift-BAT detection: Cenko et al., GCN 40224; AstroSat CZTI detection: Harsha et al., GCN 40231) triggered Konus-Wind at T0=24733.431 s UT (06:52:13.431).
The burst light curve shows a single pulse which starts at ~T0-17.9 s and has a total duration of ~40.5 s. The emission is seen up to ~2 MeV.
The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB250424_T24733/
As observed by Konus-Wind, the burst had a fluence of 5.83(-0.16,+0.16)x10^-5 erg/cm2, and a 64-ms peak flux, measured from T0+4.192 s, of 8.88(-0.94,+0.95)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+21.760 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 = -1.30(-0.06,+0.06), the high energy photon index beta = -2.87(-0.15,+0.11), the peak energy Ep = 104(-3,+3) keV (chi2 = 94/82 dof).
The spectrum near the maximum count rate (measured from T0+0.256 to T0+5.376 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 = -1.09(-0.06,+0.07), the high energy photon index beta = -2.91(-0.15,+0.12), the peak energy Ep = 129(-4,+5) keV (chi2 = 72/66 dof).
Assuming the redshift z=0.310 (Saccardi et al., GCN 40228) 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 following rest-frame parameters: the isotropic energy release E_iso is 1.49(-0.04,+0.04)x10^52 erg, the peak luminosity L_iso is 2.97(-0.31,+0.32)x10^51 erg/s, the rest-frame peak energy of the time-averaged spectrum Ep,i,z is 137(-4,+4) keV and the spectrum near the maximum count rate Ep,p,z is 169(-5,+7) keV.
With the obtained estimates, GRB 250424A 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/GRB250424_T24733/GRB250424A_rest_frame.pdf
All the quoted errors are at the 68% confidence level. All the quoted values are preliminary.
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