GCN Circular 45196

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

The long-duration GRB 260721A
(NuSTAR-ACS detection: Grefenstette and Waratkar, GCN 45192
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triggered Konus-Wind at T0=04620.710 s UT (01:17:00.710).

The burst light curve shows a single pulse 
which starts at ~T0-0.6 s and has a total duration of ~33 s (75-1200 keV).
The emission is seen up to ~2 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB260721_T04620/

As observed by Konus-Wind, the burst
had the total fluence of 2.03(-0.14,+0.18)x10^-5 erg/cm2,
and the 64-ms peak flux, measured from T0+1.568 s,
of 2.41(-0.44,+0.45)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+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 = -0.48(-0.11,+0.14),
the high energy photon index beta = -2.69(-0.22,+0.19),
the peak energy Ep = 161(-11,+10) keV
(chi2 = 93/97 dof).

The spectrum near the maximum count rate
(measured from T0+0.256 to T0+8.448 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 = 0.02(-0.15,+0.17),
the high energy photon index beta = -2.76(-0.27,+0.18),
the peak energy Ep = 196(-12,+13) keV
(chi2 = 84/82 dof).

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