GCN Circular 37302

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
Not found
 
 
; Sharma & Meegan, GCN 37303
Not found
 
 
;
Swift detection: Gupta et al., GCN 37274
Not found
 
 
;
Fermi LAT detection: Di Lalla et al., GCN 37288
Not found
 
 
;
AstroSat CZTI detection: Joshi al., GCN 37298
Not found
 
 
)
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
Not found
 
 
)
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.