TITLE:   GCN CIRCULAR
NUMBER:  37071
SUBJECT: Konus-Wind detection of GRB 240804B (a counterpart of EP240804a)
DATE:    24/08/06 13:53:55 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:

A long-duration GRB 240804B was detected by Konus-Wind (KW)
in the waiting mode at T0=T0(KW)~21:36:48 UT.
A Bayesian block analysis of the KW data in the 100-400 keV
band reveals a >4 sigma count rate increase in the interval
from ~T0-5 s to ~T0+33 s.

Corrected for the propagation time, the burst started ~5 s
before the start time of the fast X-ray transient EP240804a
(T0(EP)=21:36:53 UT; Wang et al., GCN 37034).
The burst source is located in the southern ecliptic hemisphere,
which is consistent with the EP240703a localization.
The positional and temporal coincidence of GRB 240804B with
the EP transient supports the conclusion that EP240804a
is the GRB counterpart.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB240804B/

The time-integrated spectrum of the burst, measured from T0-5 s to T0+33 s,
cat be described by a power law with exponential cutoff (CPL) model
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = (0.16 ± 1.25) and Ep = (310 ± 164) keV.

The total burst fluence is (2.56 ± 1.11)x10^-6 erg/cm^2,
and the 2.944 s peak energy flux, measured from T0-1.96 s,
is (2.1 ± 0.9)x10^-7 erg/cm^2.
(both in the 20 keV - 10 MeV energy range).

Assuming the redshift z=3.662 (Bochenek et al., GCN 37039)
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 (7.2 ± 3.1)x10^52 erg,
the isotropic peak luminosity L_iso to (3.7 ± 1.6)x10^52 erg/s, and
the rest-frame peak spectral energy Ep,z to (1450 ± 770) keV.
With the obtained estimates, GRB 240804B lies at the upper edge, but is consistent,
within errors, with 'Amati' and 'Yonetoku' relations, 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/GRB240804B/GRB240804B_rest_frame.pdf

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