GCN Circular 35701

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

The long GRB 240205B (Fermi GBM observation:
Fermi GBM team, GCN 35682
Not found
 
 
; Fletcher et al., GCN 35693
Not found
 
 
;
Swift detection: Moss et al., GCN 35683
Not found
 
 
;
GECAM detection: Zhang et al., GCN 35689
Not found
 
 
;
CALET detection: Sakamoto et al., GCN 35697
Not found
 
 
)
triggered Konus-Wind (KW) at T0=79986.680 s UT (22:13:06.680).

The burst light curve shows multiple overlapping pulses
in the interval from ~T0-5 s to ~T0+57 s,
with the brightest peak around ~T0+36 s.
The emission is seen up to ~1 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB240205_T79986/

As observed by Konus-Wind, the burst had
a total fluence of (3.22 ± 0.18)x10^-5 erg/cm^2 and
a 64-ms peak energy flux, measured from T0+36.352 s,
of (7.38 ± 0.87)x10^-6 erg/cm^2/s (both in the 20 keV - 10 MeV energy range).

A time-integrated spectrum (measured from T0 to T0+49.408 s)
is best fit in the 20 keV - 1.5 MeV range
by a GRB (Band) function with the following model parameters:
the low-energy photon index alpha = -1.80 (-0.08,+0.09),
the high energy photon index beta = -3.20 (-6.80,+0.35),
the peak energy Ep = 27 (-16,+10) keV,
chi2 = 28/49 dof.

A spectrum near the peak count rate (measured from T0+33.024 to T0+41.216 s)
is best fit in the 20 keV - 15 MeV range by a CPL model
with alpha = -1.70(-0.10,+0.11) and Ep = 49(-10,+8) keV (chi2 = 64/62 dof).
Fitting this spectrum by a Band function yields the same values of alpha and Ep,
and an upper limit on the high energy photon index beta of -3.5 (chi2 = 64/62 dof).

Assuming the redshift z=0.824 (Fausey et al., GCN 35698
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 (6.1 ± 0.3)x10^52 erg,
the isotropic peak luminosity L_iso to (2.6 ± 0.5)x10^52 erg/s,
the rest-frame peak energy of the time-integrated spectrum Ep,i,z to ~49 keV,
and the rest-frame peak energy at the peak of the emission Ep,p,z to ~89 keV.
With the obtained estimates, GRB 240205B is a soft-spectrum outlier in
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/GRB240205_T79986/GRB240205B_rest_frame.pdf


All the quoted errors are estimated at the 90% confidence level.
All the presented results are preliminary.