"We observed the position of the GRB 051221A afterglow at 8.5 GHz with the Very Large Array on 2005 December 21.99 UT, December 23.02 UT, December 24.83 UT, and December 27.96 UT (GCN 4366; GCN 4367; GCN 4369). On the first epoch (2005 December 21.99 UT) we detect weak radio emission coincident with the X-ray and optical afterglow, with a peak flux density of 88 +/- 26 uJy. The source is...Full Text

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). ...Full Text

The bright and short burst, GRB 051221A (Parsons et al., GCN 4363), 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...Full Text

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, GCN 4371; Berger and Soderberg, GCN 4375; Berger et al....Full Text

We have created a summed image from 2.78 ksec of exposure on GRB 051221A (Parsons et al., GCN 4363) through the "UVW1" filter of the Swift UVOT. From the summed image we detect a source at 4.8-sigma confidence with a position coincident with the one reported by Bloom (GCN 4367) and Berger (GCN 4369). The magnitude of the source is 20.2+/-0.2 as determined by the Swift analysis tool...Full Text

We executed a Chandra ToO observation of GRB 051221A beginning at 2005-12-22 13:55:20 UT (130 ks after the burst) and lasting for 30.2 ks. A fading X-ray source was found within the XRT error circle (Burrows et al., GCN 4366) at ...Full Text

The time profile of GRB 051221A binned to 1 ms resolution reveals that the initial pulse structure (GCN 4363 & GCN 4365) comprises 3 separate pulses of FWHM ~10-15 ms with peak intensities of ~175,000 counts per sec. We note for comparison that GRB 050525A, the brightest long burst so far detected by BAT (in one year) had a peak count rate of 101,000 cts/sec (corrected to match the same partial-coding...Full Text

"We observed the position of the GRB 051221A afterglow at 4.9 GHz with the Westerbork Synthesis Radio Telescope at December 21 13.90 UT to 18.63 UT, i.e. 12.05 - 16.78 hours after the burst (GCN 4363). We do not detect a radio source within the SWIFT/XRT error circle (GCN 4366), in particular at the position of optical (GCN 4369) and infrared (GCN 4368) counterparts. The formal flux measurement...Full Text

"We obtained 2x1200 sec spectra of the afterglow + host galaxy of the short GRB 051221 (GCN 4383) with Gemini/GMOS. We detect several strong emission lines at the position of the afterglow+host which we identify as [OIII]4959,5006, H-beta, and [OII]3727 at a redshift of z=0.5465. Weak absorption at the CaII H&K and G-band wavelengths at the same redshift may also be present, but at a low...Full Text

"We obtained a second epoch of r-band imaging with GMOS on Gemini-north starting on 2005 Dec. 22.195 UT (26.8 hr after the burst and 24 hr after the previous set of observations). A total of 960 sec were obtained in excellent seeing conditions. We find that the optical afterglow candidate (GCN 4375; see also GCN 4367 & 4369) has faded by about 1.1 mag between the two epochs, confirming...Full Text

Starting at 02:57:05 UT (1.1 hours after the burst), the RAPTOR-S telescope began a manually initiated response to the short burst identified by Swift (Parsons et al. GCN 4363). Within the XRT error circle (Burrows et al. 4366) at a position consistent with the location of the candidate J-band infrared (Bloom, GCN 4368) and R-band optical (Berger, GCN 4369) counterparts, a stack of 20...Full Text

"Digital image subtraction performed on the individual 240-s r-band frames obtained with Gemini/GMOS (GCN 4369) reveal a fading source coincident with the position of the bright NIR (GCN 4367) and optical (GCN 4369) object located within the XRT error circle (GCN 4366). The fading source is offset from the center of the bright object, which we identify as the host galaxy, by about 0.2" to...Full Text

We have analyzed the first six orbits of the Swift XRT data of GRB 051221 (Parsons et al., GCN 4363)....Full Text

The Swift/UVOT began the settling exposure of the field of GRB 051221 at 01:52:29 on 2005-12-21, 73s after the BAT trigger (Parsons et al. GCN 4363). The 86s delay reported in GCN 4363 did not include the settling exposure. No source was detected at the XRT position (Burrows et al. GCN 4366) nor the candidate IR position (Bloom, GCN 4368) down to the following 5-sigma upper limits. No correction...Full Text

"Further examination of the Gemini data reveals a clustering of faint galaxies extending about 1.5' eastward from the position of the X-ray afterglow (GCN 4366) and the optical/near-IR source possibly associated with the burst (GCN 4367-9): http://www.ociw.edu/~eberger/grb051221-gemini.tif Additional multi-band photometry and an analysis of spatial clustering are required in order...Full Text

"Further analysis of the Gemini GMOS data reveals that the object located within the XRT error circle (GCN 4367-9) has not changed in brightness between 227 and 271 min after the burst to a limit of about 0.05 mag in comparison to several nearby stars. We note that a typical decay rate of t^-1 would result in a change of about 0.2 mag. The lack of variability suggests that this is either...Full Text

Correction. GCN 4368 incorrectly stated that our discovery of the long-wavelength counterpart to GRB 051221 was the first IR counterpart for a short-hard GRB: the Magellan/PANIC detection of 050724 (Berger et al. Nature, 438, 988-990, 2005) was the first. I apologize for this referencing oversight and thank E. Berger for pointing out this error. ...Full Text

"We observed the BAT error circle of the short GRB 051221 (GCN 4365) with GMOS on the Gemnini-north telescope starting on 2005 Dec. 21.194 UT (2.8 hours after the burst). A total of 40 min were obtained in the r-band under excellent seeing conditions (0.45"). Within the XRT error circle (GCN 4366) we detect a single source at a position coincident with the source reported in GCN 4367...Full Text

We have reduced the second epoch of our PAIRITEL imaging (beginning 2005-12-21 03:02:32 UTC; total integration = 690 sec) and detect the source mentioned in GCN 4367 in a stacked mosaic. The source appears to be best detected in the full stack J-band image at a revised position of J2000: ...Full Text

Following the discovery of the XRT position of GRB 051221 (Burrows GCN 4366), we inspected our PAIRITEL mosaics (GCN 4365) and found a faint, red source to the south of the XRT position by 3.2 arcsec with the coordinates: ...Full Text