TITLE:   GCN CIRCULAR
NUMBER:  44671
SUBJECT: Konus-Wind detection of GRB 260518A
DATE:    26/05/19 22:18:00 GMT
FROM:    Dmitry Svinkin at Ioffe Institute <dmitrysvinkin@gmail.com>

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 260518A
(AstroSat-CZTI detection: Harsha et al., GCN 44668)
triggered Konus-Wind at T0=27873.529 s UT (07:44:33.529).

The burst light curve shows a multipeaked structure
which starts at ~T0-9 s and has a total duration of ~18 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/GRB260518_T27873/

As observed by Konus-Wind, the burst
had the total fluence of 4.59(-0.38,+0.42)x10^-6 erg/cm2,
and the 64-ms peak flux, measured from T0-0.016 s,
of 2.35(-0.47,+0.48)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+8.448 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 = -0.11(-0.36,+0.42)
and Ep = 213(-24,+31) keV (chi2 = 66/85 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.9
(chi2 = 66/84 dof).

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