TITLE:   GCN CIRCULAR
NUMBER:  44597
SUBJECT: Konus-Wind detection of GRB 260511B
DATE:    26/05/13 12:12:06 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 260511B
(Fermi-GBM detection: Fermi GBM team, GCN 44542;
Roberts et al., GCN 44554;
SVOM detection: Godet et al., GCN 44543;
NuSTAR detection: Waratkar & Grefenstette, GCN 44556;
AstroSat-CZTI detection: Arya et al., GCN 44560) 
triggered Konus-Wind at T0=39635.267 s UT (11:00:35.267).

The burst light curve shows two separate multipeaked episodes: the first
from ~T0-1.1 s to ~T0+10 s, and the second, which is brighter and spectrally
harder, from ~T0+26 s to ~T0+42 s. The total duration of the burst is ~43.4 s.
The emission is seen up to ~6 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB260511_T39635/

As observed by Konus-Wind, the burst
had a fluence of 2.48(-0.11,+0.12)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+30.512 s,
of 6.82(-0.83,+0.84)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+41.216 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -1.12(-0.09,+0.10)
and Ep = 259(-20,+24) keV (chi2 = 92/99 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.4
(chi2 = 92/98 dof).

The spectrum near the maximum count rate
(measured from T0+24.832 to T0+33.024 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 = -0.87(-0.23,+0.17),
the high energy photon index beta = -2.26(-0.66,+0.13),
the peak energy Ep = 273(-40,+102) keV
(chi2 = 93/81 dof).

Assuming the redshift z=2.006 (de Ugarte Postigo et al., GCN 44565; Wang et al., GCN 44571)
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 2.53(-0.11,+0.12)x10^53 erg,
the peak luminosity L_iso is 2.09(-0.25,+0.26)x10^53 erg/s,
the rest-frame peak energy of the time-averaged spectrum,
Ep,i,z is 778(-60,+72) keV and the spectrum near the maximum count rate 
Ep,p,z is 821(-120,+307) keV.

With the obtained estimates, GRB 260511B is inside 68% prediction bands for both 
the 'Amati' and the '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/GRB260511_T39635/GRB260511B_rest_frame.pdf

All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.