GCN Circular 35758

D. Svinkin, D. Frederiks, A.Lysenko, A. Ridnaia,
A. Tsvetkova,  M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:

The long-duration GRB 240218A
(Swift-BAT detection: Page et al., GCN 35742
Not found
 
 
;
Fermi-GBM detection: Veres and Meegan, GCN 35755
Not found
 
 
)
was detected by Konus-Wind (KW) in the waiting mode.

A Bayesian block analysis of the KW waiting mode data
in the 20-400 keV band reveals a >10 sigma count rate increase
in the interval from T0-12.646 s to T0+25.626 s,
where T0 = T0(BAT) = 02:00:00 UT.
The burst light curve shows a multi-peaked structure
with the brightest peak around ~T0(BAT)+20 s.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB240218_T07200/

As observed by Konus-Wind, the burst had
a total fluence of 7.62(-2.22,+1.72)x10^-6 erg/cm^2 and
a 2.944-s peak energy flux, measured from T0+19.738 s,
of 5.29(-1.49,+2.21)x10^-7 erg/cm^2/s
(both in the 20 keV - 10 MeV energy range).

Modelling the 3-channel time-integrated spectrum of the burst
(measured from T0-12.646 s to T0+25.626 s)
by a power law with exponential cutoff (CPL) model
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
yields alpha > -0.56 and Ep = 180(-74,+77) keV.

Modelling the 3-channel spectrum near the peak count rate
(measured from T0+19.738 to T0+25.626 s) by the CPL model
yields alpha = -1.33(-0.29,+0.53) and Ep = 293(-120,+207) keV.

Assuming the redshift z=6.782 (Saccardi et al., GCN 35756
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 5.4(-1.6,+1.2)x10^53 erg,
the isotropic peak luminosity L_iso to 2.9(-0.8,+1.2)x10^53 erg/s,
the rest-frame peak energy of the time-integrated spectrum
Ep,i,z to 1401(-576,+599) keV, and the rest-frame peak energy at
the peak of the emission Ep,p,z to 2280(-934,+1611) keV.

With the obtained estimates, GRB 240218A is inside 68% prediction band
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/GRB240218_T07200/GRB240218A_rest_frame.pdf

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