TITLE:   GCN CIRCULAR
NUMBER:  37139
SUBJECT: Konus-Wind detection of GRB 240809A
DATE:    24/08/11 17:39:32 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 240809A
(Swift-BAT detection: Evans et al., GCN 37110;
SVOM-GRM detection: Wang et al., GCN 37113)
triggered Konus-Wind at T0=30629.035 s UT (08:30:29.035).

The burst light curve shows a multipeaked structure
which starts at ~T0-4.7 s and has a total duration of ~36.9 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/GRB240809_T30629/

As observed by Konus-Wind, the burst
had a fluence of 1.30(-0.09,+0.09)x10^-4 erg/cm2,
and a 64-ms peak flux, measured from T0+5.488 s,
of 3.14(-0.45,+0.44)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+32.768 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 = -0.84(-0.07,+0.07),
the high energy photon index beta = -2.54(-0.62,+0.28),
the peak energy Ep = 1097(-146,+164) keV
(chi2 = 118/97 dof).

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+6.144 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 = -0.59(-0.07,+0.08),
the high energy photon index beta = -2.50(-0.32,+0.21),
the peak energy Ep = 1240(-146,+160) keV
(chi2 = 111/82 dof).

Assuming the redshift z=1.494 (Schneider et al., GCN 37129)
and a standard cosmology model 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 7.8(-0.5,+0.5)x10^53 erg,
the peak luminosity L_iso is 4.7(-0.7,+0.7)x10^53 erg/s,
the rest-frame peak energy of the time-integrated spectrum Ep,i,z is 2736(-364,+409) keV,
and the rest-frame peak energy at the peak of the emission Ep,p,z is 3093(-364,+399) keV.
With the obtained estimates, GRB 240809A is inside 90% 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/GRB240809_T30629/GRB240809A_rest_frame.pdf

All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.