TITLE:   GCN CIRCULAR
NUMBER:  44806
SUBJECT: Konus-Wind detection of GRB 260601B/EP260601a
DATE:    26/06/03 16:14:45 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 260601B
(GECAM-B detection: Yu et al., GCN 44767; Wang et al., GCN 44768;
NuSTAR detection: Waratkar et al., GCN 44771;
CALET-GBM detection: Sugita et al., GCN 44773;
Insight-HXMT detection: Wang et al., GCN 44785;
Glowbug detection: Cheung et al., GCN 44801)
triggered Konus-Wind at T0=69173.304 s UT (19:12:53.304).

The burst light curve shows a multipeaked structure
which starts at ~T0-3.2 s and has a total duration of ~230 s.
The emission is seen up to ~8 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB260601_T69173/

As observed by Konus-Wind, the burst
had the total fluence of 3.34(-0.17,+0.17)x10^-4 erg/cm2,
and the 64-ms peak flux, measured from T0+27.552 s,
of 1.66(-0.19,+0.19)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+210.688 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 = -0.79(-0.10,+0.12),
the high energy photon index beta = -1.74(-0.05,+0.04),
the peak energy Ep = 441(-68,+82) keV
(chi2 = 125/97 dof).

The 'peak-flux' spectrum 
(measured from T0+24.832 to T0+31.232 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 = -0.89(-0.07,+0.07),
the high energy photon index beta = -1.82(-0.06,+0.05),
the peak energy Ep = 671(-98,+117) keV
(chi2 = 108/82 dof).

Assuming a standard cosmology with H_0 = 67.3 km/s/Mpc, Omega_M = 0.315,
and Omega_Lambda = 0.685 (Planck Collaboration, 2014), we estimated the burst
isotropic energy release E_iso, the isotropic peak luminosity L_iso, and
the rest-frame peak spectral energy Ep,z of the burst at different redshifts.
With the obtained estimates GRB 260601B is consistent with the 68% prediction band
of the 'Amati' relation for z >~0.2 and with the 68% prediction band of the 'Yonetoku' relation
for z>~1.0 (see http://www.ioffe.ru/LEA/GRBs/GRB260601_T69173/GRB260601B_rest_frame.pdf)

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