TITLE:   GCN CIRCULAR
NUMBER:  29641
SUBJECT: Konus-Wind detection of GRB 210305A
DATE:    21/03/10 10:04:15 GMT
FROM:    Anna Ridnaia at Ioffe Institute  <ridnaia@mail.ioffe.ru>

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

The long-duration GRB 210305A
(Swift-BAT detection: D'Ai et al., GCN 29595, Krimm et al., GCN 29611;
Fermi-GBM observation: Veres et al., GCN 29628)
triggered Konus-Wind at T0=68578.330 s UT (19:02:58.330).

The burst light curve shows a multipeaked structure
which starts at ~T0-20 s and has the total duration of ~78 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/GRB210305_T68578/

As observed by Konus-Wind, the burst
had a fluence of 1.63(-0.31,+0.43)x10^-5 erg/cm2,
and a 64-ms peak flux, measured from T0+4.224 s,
of 1.23(-0.51,+0.51)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+57.600 s)
is best fit in the 20 keV - 1 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -0.33(-0.65,+1.71),
the high energy photon index beta = -2.45(-0.73,+0.30),
the peak energy Ep = 84(-26,+25) keV
(chi2 = 75/49 dof).

The spectrum near the peak count rate
(measured from T0+0.256 to T0+8.448 s)
is best fit in the 20 keV - 1 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with  alpha = -0.54(-0.32,+0.37)
and Ep = 133(-16,+21) keV (chi2 = 44/50 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -2.3
(chi2 = 42/49 dof).

All the quoted errors are at the 90% confidence level.
All the quoted values are preliminary.