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Starting on 21 Dec., UT 04:02, we observed the optical
afterglow of GRB 051221A (Parsons et al. GCN 4363) with
the MDM 1.3 m telescope in three 600 s exposures (beginning
2.1, 2.4, and 2.6 hr after the trigger). We detect a faint
source consistent with the locations of the IR (Bloom,
GCN 4367 and 4368), optical (Berger, GCN 4369, 4371;
Berger and Soderberg, GCN 4375; Berger et al. GCN 4383),
and X-ray (Burrows et al., GCN 4366; Grupe et al. GCN 4389)
afterglows. Photometry, calibrated on the R-band magnitudes
of 6 comparison stars in our field of view as given by
Aladin, from the UCD data base (UCD: PHOT_PHG_R), yields
the following magnitude estimates (t = time after GRB trigger):
t | R
-------------------
2.1 hr | 19.88 +/- 0.40
2.4 hr | 19.93 +/- 0.39
2.6 hr | 19.98 +/- 0.34
Consequently, there is no evidence for variability between
the three frames. Seeing was rather poor (~ 2 arc sec.), so
the optical afterglow is not resolved from the host galaxy
(Soderberg and Berger, GCN 4375). Given the RAPTOR upper
limit on the fading counterpart of R > 20.2 at 1.3 hr
after the trigger (Wren et al., GCN 4380), our measurement
might in fact be dominated by the host galaxy contribution.
==
The bright and short burst, GRB 051221A (Parsons et al., GCN4363),
triggered by Swift/BAT was also detected with the Suzaku Wideband
All-sky Monitor (WAM) which covers an energy band of 50 keV - 5 MeV
at 01:51:16 (UT).
The observed light curve exhibits two intense short spikes
with a total duration (T90) of 0.22 sec.
The fluence in 100 - 2000 keV was (2.4 +/- 0.4)X10^-6 erg/cm2.
The 1-sec peak flux was 4.7 +/- 0.8 photons/cm2/s in the
same energy range.
Preliminary result shows that the time-averaged spectrum
is well fitted by a single power law with a photon index
of 1.95 +/- 0.18.
All the quoted errors are at statistical 90% confidence level.
The WAM onboard calibration is still under way, and
systematic errors, such as the flux calibration uncertainties of
about 20%, are not included in the errors.
==
The GRB 051221A (Swift-BAT trigger #173780;
Parsons et al., GCN 4363; Cummings et al., GCN 4365;
Norris et al., GCN 4388)
triggered Konus-Wind at T0=6672.976 s UT (01:51:12.976).
The Konus-Wind light curve consists of a soft weak precursor
and the main episode with five ~15-ms peaks.
The first peak at T0-16 ms to T0+10 is
substantially softer than the others
(there is no emission in the 380-1160 keV energy range).
After T0+0.250 sec a weak soft emission is marginally seen only
in the G1 (18-70 keV) range up to ~1 sec .
As observed by Konus-Wind the burst had
a fluence of 3.2(-1.7, +0.1)x10^-6 erg/cm2 and
peak flux on 4-ms time scale 4.6(-2.5, +0.2)x10^-5 erg/cm2/sec
(both in the 20 keV - 2 MeV energy range).
The time-integrated spectrum of the GRB (from T0 to T0+0.256 sec)
is well fitted (in the 20 keV - 2 MeV range)
by a power law with exponential cutoff model:
dN/dE ~ E^(-alpha) * exp(-E/E0)
with alpha = 1.08(-0.14, +0.13)
and E0 = 436(-116, +165) keV (chi2 = 65/69dof).
The peak energy Ep = 402(-72, +93) keV.
The fitting by a single power law gives an unacceptable
result: chi2 = 152/70dof (null hypothesis probability = 5.423E-08).
All the quoted errors are at the 90% confidence level.
Assuming z = 0.5465 (Berger and Soderberg, GCN 4384)
and a standard cosmology model with H_0 = 70 km/s/Mpc,
Omega_M = 0.3, Omega_\Lambda = 0.7,
the isotropic energy release is E_iso ~2.5x10^51 erg,
the maximum luminosity is (L_iso)_max ~5.5x10^52 erg/sec.
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