Gamma-ray Burst 051221

(All information courtesy of the instrument teams.)

Previous IAU Circulars

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Results of Observations

GCN/BACODINE POSITION NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 21 Dec 05 20:06:48 UT
NOTICE_TYPE:     Swift-BAT GRB Position
TRIGGER_NUM:     173904,   Seg_Num: 0
GRB_RA:          312.395d {+20h 49m 35s} (J2000),
                 312.439d {+20h 49m 45s} (current),
                 312.028d {+20h 48m 07s} (1950)
GRB_DEC:         +53.054d {+53d 03' 14"} (J2000),
                 +53.076d {+53d 04' 34"} (current),
                 +52.867d {+52d 52' 00"} (1950)
GRB_ERROR:       3.00 [arcmin radius, statistical only]
GRB_INTEN:       0 [cnts]    Image_Peak=2392 [image_cnts]
TRIGGER_DUR:     192.000 [sec]   (=3.2 [min])
TRIGGER_INDEX:   20000     E_range: 15-50 keV
BKG_INTEN:       0 [cnts]
BKG_TIME:        0.00 SOD {00:00:00.00} UT
BKG_DUR:         0 [sec]
GRB_DATE:        13725 TJD;   355 DOY;   05/12/21
GRB_TIME:        72200.09 SOD {20:03:20.09} UT
GRB_PHI:          17.40 [deg]
GRB_THETA:        38.33 [deg]
SOLN_STATUS:     0x13
RATE_SIGNIF:     0.00 [sigma]
IMAGE_SIGNIF:    7.45 [sigma]
MERIT_PARAMS:     +1  +0  +0  +8  +1  -2  +0  +1  -8  +1 
SUN_POSTN:       270.07d {+18h 00m 17s}  -23.44d {-23d 26' 16"}
SUN_DIST:         84.88 [deg]
MOON_POSTN:      161.20d {+10h 44m 48s}  +10.73d {+10d 44' 02"}
MOON_DIST:       111.63 [deg]
MOON_ILLUM:      68 [%]
GAL_COORDS:       91.22,  5.78 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      347.57, 65.48 [deg] ecliptic lon,lat of the burst (or transient)
COMMENTS:        SWIFT-BAT GRB Coordinates.  
COMMENTS:        This Notice was delayed by more than 90 sec; probably due to it occurring during a Malindi downlink session.  
COMMENTS:        This is an image trigger.  (The RATE_SIGNIF & BKG_{INTEN, TIME, DUR} are undefined.)  
COMMENTS:        A point_source was found.  
COMMENTS:        This does not match any source in the on-board catalog.  
COMMENTS:        This does not match any source in the ground catalog.  
COMMENTS:        This is a GRB.  
COMMENTS:        This trigger occurred at longitude,latitude = 355.22,20.38 [deg].

red DSS finding chart ps-file

GCN/BACODINE POSITION NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 21 Dec 05 20:08:04 UT
NOTICE_TYPE:     Swift-XRT Nack-Position
TRIGGER_NUM:     173904,   Seg_Num: 0
POINT_RA:        312.403d {+20h 49m 37s} (J2000)
POINT_DEC:       +53.058d {+53d 03' 29"} (J2000)
IMG_START_DATE:  13725 TJD;   355 DOY;   05/12/21
IMG_START_TIME:  72475.81 SOD {20:07:55.81} UT,   275.7 [sec] since BAT Trigger
Time
COUNTS:          18    Min_needed= 20
STD_DEV:         0.00   Max_StdDev_for_Good=28.44 [arcsec]
PH2_ITER:        1    Max_iter_allowed= 4
ERROR_CODE:      1
COMMENTS:        SWIFT-XRT Nack Position.
COMMENTS:        No source found in the image.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 21 Dec 05 20:09:45 UT
NOTICE_TYPE:     Swift-BAT GRB Lightcurve
TRIGGER_NUM:     173904,   Seg_Num: 0
GRB_RA:          312.395d {+20h 49m 35s} (J2000),
                 312.439d {+20h 49m 45s} (current),
                 312.028d {+20h 48m 07s} (1950)
GRB_DEC:         +53.054d {+53d 03' 14"} (J2000),
                 +53.076d {+53d 04' 34"} (current),
                 +52.867d {+52d 52' 00"} (1950)
GRB_DATE:        13725 TJD;   355 DOY;   05/12/21
GRB_TIME:        72200.09 SOD {20:03:20.09} UT
TRIGGER_INDEX:   20000
GRB_PHI:          17.40 [deg]
GRB_THETA:        38.33 [deg]
DELTA_TIME:      0.00 [sec]
TRIGGER_DUR:     192.000 [sec]   (=8.0 [min])
LC_URL:          sw00173904000msb.lc
SUN_POSTN:       270.07d {+18h 00m 18s}  -23.44d {-23d 26' 16"}
SUN_DIST:         84.88 [deg]
MOON_POSTN:      161.22d {+10h 44m 53s}  +10.72d {+10d 43' 23"}
MOON_DIST:       111.63 [deg]
MOON_ILLUM:      68 [%]
GAL_COORDS:       91.22,  5.78 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      347.57, 65.48 [deg] ecliptic lon,lat of the burst (or transient)
COMMENTS:        SWIFT-BAT GRB Lightcurve.  
COMMENTS:           
COMMENTS:        The next comments were copied from the BAT_POS Notice:  
COMMENTS:        This is an image trigger.  
COMMENTS:        A point_source was found.  
COMMENTS:        This does not match any source in the on-board catalog.  
COMMENTS:        This does not match any source in the ground catalog.  
COMMENTS:        This is a GRB.  
COMMENTS:        This trigger occurred at longitude,latitude = 355.22,20.38 [deg].

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 21 Dec 05 20:11:47 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     173904,   Seg_Num: 0
POINT_RA:        312.403d {+20h 49m 37s} (J2000)
POINT_DEC:       +53.058d {+53d 03' 29"} (J2000)
POINT_ROLL:      218.339d
IMG_START_DATE:  13725 TJD;   355 DOY;   05/12/21
IMG_START_TIME:  72474.69 SOD {20:07:54.69} UT,   274.6 [sec] since BAT Trigger Time
FILTER:          3, V
BKG_MEAN:        0.848
N_STARS:         135
X_OFFSET:        476 [pixels]
Y_OFFSET:        555 [pixels]
X_MAX:           1435 [pixels]
Y_MAX:           1514 [pixels]
DET_THRESH:      8
PHOTO_THRESH:    4
SL_URL:          sw00173904000msufc0274.fits
SUN_POSTN:       270.08d {+18h 00m 18s}  -23.44d {-23d 26' 16"}
SUN_DIST:         84.88 [deg]
MOON_POSTN:      161.24d {+10h 44m 57s}  +10.72d {+10d 42' 56"}
MOON_DIST:       111.63 [deg]
MOON_ILLUM:      68 [%]
GAL_COORDS:       91.23,  5.78 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      347.58, 65.48 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 21 Dec 05 20:12:35 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     173904,   Seg_Num: 0
POINT_RA:        312.403d {+20h 49m 37s} (J2000)
POINT_DEC:       +53.058d {+53d 03' 29"} (J2000)
POINT_ROLL:      218.339d
IMG_START_DATE:  13725 TJD;   355 DOY;   05/12/21
IMG_START_TIME:  72474.69 SOD {20:07:54.69} UT,   274.6 [sec] since BAT Trigger Time
FILTER:          3, V
BKG_MEAN:        0.848
N_STARS:         135
X_OFFSET:        476 [pixels]
Y_OFFSET:        555 [pixels]
X_MAX:           1435 [pixels]
Y_MAX:           1514 [pixels]
DET_THRESH:      8
PHOTO_THRESH:    4
SL_URL:          sw00173904000msufc0274.fits
SUN_POSTN:       270.08d {+18h 00m 18s}  -23.44d {-23d 26' 16"}
SUN_DIST:         84.88 [deg]
MOON_POSTN:      161.24d {+10h 44m 59s}  +10.71d {+10d 42' 46"}
MOON_DIST:       111.63 [deg]
MOON_ILLUM:      68 [%]
GAL_COORDS:       91.23,  5.78 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      347.58, 65.48 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Source List.  
COMMENTS:        All 4 attachments are included.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 21 Dec 05 20:13:11 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     173904,   Seg_Num: 0
POINT_RA:        312.403d {+20h 49m 37s} (J2000)
POINT_DEC:       +53.058d {+53d 03' 29"} (J2000)
ROLL:            218.339d
IMG_START_DATE:  13725 TJD;   355 DOY;   05/12/21
IMG_START_TIME:  72474.69 SOD {20:07:54.69} UT,   274.6 [sec] since BAT Trigger Time
FILTER:          3, V
EXPOSURE_ID:     156888474
X_OFFSET:        795 [pixels]
Y_OFFSET:        874 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       955
Y_GRB_POS:       1034
BINNING_INDEX:   1
IM_URL:          sw00173904000msuni0274.fits
SUN_POSTN:       270.08d {+18h 00m 18s}  -23.44d {-23d 26' 16"}
SUN_DIST:         84.88 [deg]
MOON_POSTN:      161.25d {+10h 44m 60s}  +10.71d {+10d 42' 38"}
MOON_DIST:       111.63 [deg]
MOON_ILLUM:      68 [%]
GAL_COORDS:       91.23,  5.78 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      347.58, 65.48 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Image.  
COMMENTS:        The GRB Position came from the Window Position in the Mode Command.  
COMMENTS:        The image has 2x2 binning (compression).

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 21 Dec 05 20:13:44 UT
NOTICE_TYPE:     Swift-XRT Lightcurve
TRIGGER_NUM:     173904,   Seg_Num: 0
POINT_RA:        312.405d {+20h 49m 37s} (J2000)
POINT_DEC:       +53.056d {+53d 03' 22"} (J2000)
LC_START_DATE:   13725 TJD;   355 DOY;   05/12/21
LC_START_TIME:   72478.49 SOD {20:07:58.49} UT,   278.4 [sec] since BAT Trigger Time
LC_STOP_DATE:    13725 TJD;   355 DOY;   05/12/21
LC_STOP_TIME:    72720.59 SOD {20:12:00.59} UT
LC_LIVE_TIME:    238.38 [sec], 98.5%
DELTA_TIME:      86157.90 [sec]
N_BINS:          100
TERM_COND:       0
LC_URL:          sw00173904000msx.lc
SUN_POSTN:       270.08d {+18h 00m 19s}  -23.44d {-23d 26' 16"}
SUN_DIST:         84.88 [deg]
MOON_POSTN:      161.25d {+10h 45m 01s}  +10.71d {+10d 42' 31"}
MOON_DIST:       111.63 [deg]
MOON_ILLUM:      68 [%]
GAL_COORDS:       91.23,  5.78 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      347.58, 65.48 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-XRT Lightcurve.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 21 Dec 05 20:14:03 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     173904,   Seg_Num: 0
POINT_RA:        312.403d {+20h 49m 37s} (J2000)
POINT_DEC:       +53.058d {+53d 03' 29"} (J2000)
ROLL:            218.339d
IMG_START_DATE:  13725 TJD;   355 DOY;   05/12/21
IMG_START_TIME:  72474.69 SOD {20:07:54.69} UT,   274.6 [sec] since BAT Trigger Time
FILTER:          3, V
EXPOSURE_ID:     156888474
X_OFFSET:        795 [pixels]
Y_OFFSET:        874 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       955
Y_GRB_POS:       1034
BINNING_INDEX:   1
IM_URL:          sw00173904000msuni0274.fits
SUN_POSTN:       270.08d {+18h 00m 19s}  -23.44d {-23d 26' 16"}
SUN_DIST:         84.88 [deg]
MOON_POSTN:      161.25d {+10h 45m 01s}  +10.71d {+10d 42' 27"}
MOON_DIST:       111.63 [deg]
MOON_ILLUM:      68 [%]
GAL_COORDS:       91.23,  5.78 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      347.58, 65.48 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Image.  
COMMENTS:        All 4 attachments are included.

GCN Circular #4376
P. Boyd (GSFC), S. Barthelmy (GSFC), D. Burrows (PSU), J. Cummings (GSFC/NRC), N. Gehrels (GSFC), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), F. Marshall (GSFC), K. Page (U Leicester), D. Palmer (LANL), A. Parsons (GSFC), J. Racusin (PSU), P. Roming (PSU), T. Sakamoto (GSFC) on behalf of the Swift team:
At 20:03:20 UT, Swift-BAT triggered and located a source (trigger=173904). The spacecraft slewed promptly after the end of the image trigger. The BAT on-board calculated location is RA,Dec 312.395d,+53.054d {20h 49m 35s,+53d 03' 14"} (J2000), with an uncertainty of 3 arcmin (radius, 90% containment, stat+sys). All we have at this time is the TDRSS lightcurve. This plus the fact that this is a 192-sec image trigger means that we can not say if this trigger is due to a real GRB, a hard x-ray transient, or a noise event. We note that the galactic latitude is 6 deg.
The XRT began observing the location at 20:07:56 UT, 276 sec after the BAT trigger. The on-board detection algorithm did not centroid on a source due to insufficient counts, so no prompt X-ray position is available. However, both the prompt XRT light-curve and raw spectrum indicate there is a faint X-ray source in the field. More information will be available after the next Malindi pass in a few hours.
UVOT took a finding chart exposure of 200 seconds with the V filter starting 275 seconds after the BAT trigger. No afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 25% of the BAT error circle. The typical 3-sigma upper limit has been about 18th mag. The 8'x8' region for the list of sources generated on-board covers 100% of the BAT error circle. The list of sources is typically complete to about 18.0 mag. No correction has been made for the expected visual extinction of about 4.6 magnitudes.
GCN Circular #4378
K.L. Page, J. Racusin, A.P. Beardmore, D.N. Burrows and N. Gehrels report on behalf of the Swift-XRT team:
We have analysed the first orbit of XRT data for BAT trigger 173904 (GCN 4376, Boyd et al.) and find a fading, uncatalogued source located at
```
RA(J2000)  = 20h 49m 35.1s
Dec(J2000) = 53h 02m 12.2s
```
with an estimated uncertainty of 4.5 arcsec (90% containment). This includes the latest XRT boresight correction and is 62 arcsec from the on-board BAT position given in GCN 4376.
From the current data, it is still not possible to determine whether the source is a GRB or a hard X-ray transient.
GCN Circular #4379
A. de Ugarte Postigo, M. Jelinek, S. Vitek, A.J. Castro-Tirado, J. Gorosabel, S.B. Pandey, S. Guziy (IAA-CSIC, Granada), P. Kubanek and R. Hudec (Astronomical Institute of the Academy of Sciences, Ondrejov), report:
"BOOTES-1 in South Spain (El Arenosillo, INTA, Huelva), responded under non-optimal conditions to GRB 051221 (Swift trigger 173904, Boyd et al. GCNC 4376). A sequence of RVI-band exposures started at 20:07:15 UT (235s after the GRB onset). Preliminary analysis does not reveal any transient optical source in the 3' radius SWIFT/BAT error box, and in particular at the position reported by SWIFT/XRT (Page et al. GCNC 4378). We set the following R-band upper limits:
```
  =============================================
   T-To (s)       Exp. Time    Lim. Mag. R-band
   Mid Exposure      (s)         (3 sigma)
  ---------------------------------------------
     235             5               14.5
     307             10              16.0
     353            6x10             16.5
     665         5x100+12x10         18.5
  =============================================
```
We notice an important Galactic extinction in the GRB line of sight (E(B-V)=1.39, Schlegel et al. 1998)"
GCN Circular #4381
J. P. Halpern & S. Tyagi (Columbia U.) report:
"We observed the position of possible Swift GRB 051221B in the R-band with the MDM 1.3m for 30 minutes starting on Dec. 22 01:31 UT, or 5.5 hours after the BAT trigger (Boyd et al., GCN 4376).
We find an object of R=21.8 located at
```
RA(J2000) = 20h 49m 34.8s
Dec(J2000) = +53d 02' 11.8"
```
which is consistent in position with the fading XRT source (Page et al., GCN 4378). (We use a USNO B1.0 comparison star at 20h 49m 36.08s, +53d 01' 29.65".)
We have not established any optical variability at this time."
GCN Circular #4382
E. Fenimore (LANL), L. Barbier (GSFC), S. Barthelmy (GSFC), J. Cummings (GSFC/NRC), N. Gehrels (GSFC), D. Hullinger (GSFC/UMD), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), F. Marshall (GSFC), D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (GSFC/NRC), G. Sato (ISAS), T. Takahashi (ISAS), J. Tueller (GSFC) on behalf of the Swift-BAT team:
Using the full data set from the recent telemetry downlink covering T-300 to T+300 sec, we report further analysis of Swift-BAT GRB 051221B (trigger #173904) (Boyd, et al., GCN 4376). The ground-analysis position is RA,Dec 312.359,+53.040 {20h 49m 26.1s,+53d 02' 23.4"} (J2000) with an uncertainty of 1.6 arcmin (radius, 90%, stat+sys). The partial coding fraction is 45%. The lightcurve has two, possibly 3, peaks starting at T+49 sec with a total duration of 70 sec. T90 is 61 +- 1 sec. Fitting a simple power law over the full interval from T+49 to T+119 sec, the photon index is 1.48 +/- 0.18 with a fluence of 1.13 +/- 0.13 X 10^-6 erg/cm^2. The peak flux in a 1-sec wide window starting at T+59 sec is 0.54 +/- 0.20 ph/cm^2/sec. All values are in the 15-150 keV band at the 90% confidence level. A final note: we think it is very likely that this is a GRB, but we can not rule out the possibility that it is a hard x-ray transient.
GCN Circular #4386
Klotz, A. (CESR-OMP), Boer M. (OHP), and Atteia, J.L. (LAT-OMP) report:
We imaged the entire field of GRB 051221B (173904) detected by SWIFT (Boyd et al. GCNC 4376) with the TAROT robotic telescope (D=25cm) located at the Calern observatory, France. Observations started 9 seconds after the GCN notice (and 216 sec. after the GRB). The field had an elevation of 35 degrees above horizon at the beginning of the observations and then decreased. The humidity was rather high (90%), and haze appeared after few minutes of observation.
We compared the 1.6 arcmin fielld centred on the XRT position (Fenimore et al. GCNC 4382) with stars of the USNO-B1 catalogue and with the DSS2-R atlas. We did not find any new source. Following us the observation log and the upper limits we derived:
```
   Since-trig    Mag.
start      end
+216s to +276s  R>14
+282s to +312s  R>18.2
+319s to +349s  R>18.2
```
This message may be cited.
GCN Circular #4387
B. E. Schaefer (Louisiana State Univ.) report on behalf of the ROTSE collaboration:
ROTSE-IIId, located at the Turkish National Observatory at Bakirlitepe, Turkey, responded to GRB 051221B (Swift trigger 173904), producing images beginning 10 s after the GCN notice time. An automated response took the first image at 20:06:57.2 UT, 217.1 s after the burst, under excellent conditions. We took 10 5-sec, 138 20-sec eposures. These unfiltered images are calibrated relative to USNO A2.0 (R).
Comparison to the DSS (second epoch) reveals no new sources within the 3-sigma error circle, for both single images and coadding into sets of 10. In particular, we find nothing unusual inside the Swift XRT positional circle (Fenimore et al. GCN4382). Individual images have limiting magnitudes ranging from 15.1-17.1; we set the following specific limits.
```
start UT       end UT      t_exp(s)   mlim   t_start-tGRB(s)  Coadd?
--------------------------------------------------------------------
20:06:57.2   20:07:02.2         5     16.6          217.1       N
20:06:57.2   20:09:02.9       125     17.4          217.1       Y
```
GCN Circular #4396
D. Nogami, T. Kawamura, J. Kuriyama, K. Sugiyasu, K. Kubota (Kyoto Univ.), K. Yanagisawa (Okayama Astrophys. Obs.), S. Kouzuma, H. Yamaoka (Kyushu Univ.), and N. Kawai (Tokyo Inst. of Tech.) report:
We observed the field of GRB051117B (Band et al., GCN 4281) with a 50-cm telescope (MITSuME) at the Okayama Astrophysical Observatory, using a three-color (g', Rc, and Ic) CCD imager. The observations started at 14:16:22 (UT), about 54 min after the trigger time, and ended at 16:36:46 (UT). The exposure time was 60 sec, and we obtained 110 images in each band.
We could not find any afterglow candidate around the error box of GRB051117B (GCN 4281). The 3-sigma upper limits in the stacked images of each band are 19.2 mag, 19.2 mag, and 19.5 mag in the g', Rc, and Ic band, respectively. These magnitudes were calibrated with the catalogs of Tycho (VT), USNO A2.0 (R), and USNO B1.0 for g', Rc, and Ic, respectively.
GCN Circular #4417
Dale A. Frail (NRAO) reports on behalf of a larger collaboration:
"We used the Very Large Array to observe the Swift burst GRB051221B (GCN 4376; GCN 4382) at a frequency of 8.46 GHz on 2005 Dec. 27.92 UT (i.e. 6.9 days after the burst). No radio emission is detected within the XRT error circle (GCN 4378). The rms noise was 31 uJy.
No further observations are planned.
The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc."
GCN Circular #4420
P.A.Milne (Steward Obs), on behalf of the Super-LOTIS GRB team reports:
We observed the field of GRB 051221B (SWIFT-BAT trigger 173904) on Dec. 21, 2005, starting at UT=02:35:53 (06:32:33 after the burst) with the 61" Kuiper telescope at Mt. Bigelow, AZ. We obtained 11 x 300-sec R-band images. Observing conditions were considered good. The field was subsequently observed the next night starting at UT=01:33:50. We obtained 7 x 300 second images in the R-band on the second night.
Inspection of a subtraction image produced from co-added images reveals no fading source at the location of the candidate optical counterpart reported by Halpern & Tyagi (GCN 4381). The limiting magnitude of the co-added image taken on the first night is roughly 23.0 magnitude.
This message may be cited.
GCN Circular #4612
D. Sharapov (MAO, and NOT La Palma), M. Ibrahimov (MAO), A. Pozanenko (IKI), V. Rumyantsev (CrAO) on behalf of larger GRB follow up collaboration report:
We observed error box of GRB051221b (Boyd et al., GCN 4376) with 1.5m telescope of Maidanak Astronomical Observatory on Dec. 22 between (UT) 15:41 and 16:06. A set of 7x180 s images were obtained in R-band. We do not detect the object at the location of the candidate optical counterpart reported by Halpern & Tyagi (GCN 4381). Limiting magnitude of the stacked image is based on USNO-B1.0:
```
Obs. time,           Exposure,  mag.(UL),Seeing
(UT)                 (s)

Dec.22 15:41 - 16:06 1260       22.3     1.2"
```
Combined images can be found at http://grb.rssi.ru/GRB051221b/grb051221b_051222_R_AZT22.jpg
The message may be cited.
GCN Circular #4614
Jules Halpern at Columbia U.
"I revise here the report in GCN 4381 of an optical object at the Swift XRT position of the possible GRB 051221B (Boyd et al. GCN 4376, Page et al. GCN 4378, Fenimore et al. GCN 4382), prompted by the non-detections at this position by Milne (GCN 4420) and Sharapov et al. (GCN 4612). The position and magnitude given in GCN 4381 were both in error as a result of using an incorrect entry in the USNO-B1.0 catalog for the quoted comparison star.
The only object that is consistent in position, within 4.5", of the fading XRT source (Page et al. GCN 4378), is located at
```
RA(J2000)  =  20h 49m 34.86s
Dec(J2000) = +53d 02' 08.3"
```
It is also faintly visible on the digitized R and I-band POSS-II plates, and clearly on the image of Sharapov et al.
Since it was not ruled out that this event was a hard X-ray transient, it is still possible to test that hypothesis by searching for an optical counterpart. However, there is no evidence in MDM images that the above object has varied. Observations of it from from 5.5 to 8 hours after the event using the MDM 1.3m, and again at 30.6 hours using the MDM 2.4m, are all consistent with its appearance on the POSS, and with measurement on the CCD images of R = 20.63+/-0.03, referenced to a USNO B1.0 star at 20h 49m 36.16s, +53d 01' 26.4" having R2 = 17.54. The MDM 2.4m image further sets a limit of R > 23.8 on any other object in the XRT error circle, and is shown here:
http://www.astro.columbia.edu/~jules/grb/051221b
Recall that Galactic extinction in this direction, (l,b) = (91.2,+5.8), is considerable. E(B-V) = 1.37 mag, corresponding to A_R = 3.66 mag (Schlegel et al. 1998).
This message may be cited."
GCN Circular #4796
K.L. Page (U. Leicester), D.N. Burrows (PSU) and E. Rol (U. Leicester) report on behalf of the Swift-XRT team:
It has recently been pointed out (Halpern 2006, GCN 4614) that the nature of GRB 051221B has still not been definitively settled (GRB vs hard X-ray transient). Prompted by this report, we are providing further analysis details of the Swift-XRT observation of this source.
The XRT began observing the field of the possible GRB 051221B (GCN 4376, Boyd et al. 2005) 281 seconds after the trigger. 7 seconds of data in Windowed Timing (WT) mode were obtained, before switching into Photon Counting (PC) mode.
The X-ray source was only detected during the first orbit, with 14 counts in the WT data and 51 counts in 385 seconds of PC data (starting 290 seconds after the trigger). The second orbit, covering 1319 seconds of exposure time (starting 3.6 ks after the trigger) shows only 1 count at the position of the source. These later data give a 3-sigma upper limit of 1.5e-3 ct s^-1.
The start of the light-curve falls extremely steeply (alpha ~ 9, calculated with respect to the trigger time). There is an indication of flattening at 400 - 500 seconds after the trigger, though this is based on only 2 bins of data, each containing 5 counts.
Using Cash statistics because of the low number of counts, the spectrum for the first orbit of PC data (290 - 680 seconds after the trigger) can be modelled with a power-law of Gamma = 1.27 +/- 0.37, with the estimated Galactic column fixed at 5.6e21 cm^-2 (Dickey & Lockman 1990). This spectrum has a mean count rate of 0.15 ct s^-1 and an observed (unabsorbed) flux of 1.43e-11 (1.77e-11) erg cm^-2 s^-1.
The XRT observations of this source were terminated early in order to allow followup of the short GRB 051221A. Therefore, no additional data are available. However, the rapid decline in X-ray flux appears to support a GRB origin.
This circular is an official product of the Swift-XRT Team.

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Jochen Greiner, last update: 20-Feb-2006 [Disclaimer]