GCN Circular 5689

S. Golenetskii, R.Aptekar, E. Mazets, V. Pal'shin, D. Frederiks, and
T. Cline on behalf of the Konus-Wind team report:

The long bright GRB 060928 (Hurley et al., GCN 5684
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triggered Konus-Wind at T0=4622.782 s UT (01:17:02.782).

As observed by Konus-Wind the burst shows
two main episodes of emission.
The first started at ~T0 and had a duration of ~15 s.
The second more intense episode started at T-T0 ~150 s
and had a duration of ~75 s.

Both episodes demonstrate strong spectral evolution.

The time-integrated spectrum of the first episode
(from T0 to T0+18.176 s)
can be fitted (in the 20 keV - 2 MeV range)
by a power law with exponential cutoff model:
dN/dE ~ E^alpha * exp(-(2+alpha)*E/Ep)
with alpha = -1.00(+0.07, -0.06),
and Ep = 381(-34, +41) keV (chi2 = 76/59 dof).
The fluence of this part is 2.83(-0.25, +0.12)x10^-5 erg/cm2
(in the 20 keV - 2 MeV range).

The time-integrated spectrum of the second episode
(from T0+149.248 s to T0+221.184 s)
can be fitted (in the 20 keV - 8 MeV range)
by GRBM (Band) model for which:
the low-energy photon index is alpha = -1.28 +/- 0.02,
the high energy photon index beta = -2.27(-0.21, +0.14),
the peak energy Ep = 705(-68, +74) keV (chi2 = 99/81 dof).
The fluence of this part is 3.54(-0.14, +0.19)x10^-4 erg/cm2
(in the 20 keV - 8 MeV range).

The total burst fluence is 3.82(-0.14, +0.19)x10^-4 erg/cm2,
and the 256-ms peak flux measured from T0+198.912 s
2.93(-1.78, +1.38)x10^-5  erg/cm2/s
(both in the 20 keV - 8 MeV energy range).

We also analyzed several time-resolved spectra of this GRB.
The spectra can be fitted either by
by power law with exponential cutoff model
or GRBM (Band) model.
The preliminary results of this analysis
are summarized in the table below.
The chosen time intervals are shown on the K-W light curve
of this GRB available at http://www.ioffe.rssi.ru/LEA/GRBs/GRB060928_T04622/

------------------------------------------------------------------------
Int  Tstart   dT        alpha          Epeak          beta    chi2/dof
        s       s                        keV
------------------------------------------------------------------------
A     0     1.792  -0.36 +/- 0.09     459(-30,+33)     --          82/57

B    1.792  8.192 -0.85(-0.14,+0.15)  150(-12,+14)     --          82/59

C    9.984  8.192 -1.06(-0.16,+0.19)  517(-123,+215)   --          73/75

D  149.248 24.576 -1.19(-0.12,+0.14)  443(-99,+176)    --          80/59

E  173.824 39.168 -1.25 +/- 0.02 757(-65,+71) -2.22(-0.15,+0.10)  103/79

F  212.992  8.192 -1.35(-0.42,+0.49)   91(-20,+39)     --          54/59

G* 185.088  2.816 -1.04+/-0.03 1260(-168,+160) -2.83(-1.18,+0.35) 103/85
---------------------------------------------------------------------------
* - this interval (G) has the hardest spectrum

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