Gamma-ray Burst 251205A

(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:     Fri 05 Dec 25 23:40:57 UT
NOTICE_TYPE:     Swift-BAT GRB Position
TRIGGER_NUM:     1420873,   Seg_Num: 0
GRB_RA:          200.243d {+13h 20m 58s} (J2000),
                 200.547d {+13h 22m 11s} (current),
                 199.658d {+13h 18m 38s} (1950)
GRB_DEC:         +29.962d {+29d 57' 45"} (J2000),
                 +29.827d {+29d 49' 38"} (current),
                 +30.224d {+30d 13' 27"} (1950)
GRB_ERROR:       3.00 [arcmin radius, statistical only]
GRB_INTEN:       24752 [cnts]    Image_Peak=445 [image_cnts]
TRIGGER_DUR:     12.160 [sec] 
TRIGGER_INDEX:   478     E_range: 25-100 keV
BKG_INTEN:       118637 [cnts]
BKG_TIME:        85091.19 SOD {23:38:11.19} UT
BKG_DUR:         64 [sec]
GRB_DATE:        21014 TJD;   339 DOY;   25/12/05
GRB_TIME:        85187.25 SOD {23:39:47.25} UT
GRB_PHI:          98.93 [deg]
GRB_THETA:        26.05 [deg]
SOLN_STATUS:     0x3
RATE_SIGNIF:     10.00 [sigma]
IMAGE_SIGNIF:    7.81 [sigma]
MERIT_PARAMS:     +1  +0  +0  +4  +2  -2  +0  +0 -29  +0 
SUN_POSTN:       252.75d {+16h 51m 00s}  -22.49d {-22d 29' 15"}
SUN_DIST:         72.48 [deg]   Sun_angle= 3.5 [hr] (West of Sun)
MOON_POSTN:       88.22d {+05h 52m 54s}  +28.24d {+28d 14' 09"}
MOON_DIST:        93.15 [deg]
MOON_ILLUM:      98 [%]
GAL_COORDS:       58.27, 82.92 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      185.37, 35.27 [deg] ecliptic lon,lat of the burst (or transient)
COMMENTS:        SWIFT-BAT GRB Coordinates.  
COMMENTS:        This is a rate 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 = 168.30,8.61 [deg].

red DSS finding chart ps-file

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Fri 05 Dec 25 23:42:47 UT
NOTICE_TYPE:     Swift-XRT Processed Image
TRIGGER_NUM:     1420873,   Seg_Num: 0
GRB_RA:          200.2707d {+13h 21m 04.9s} (J2000),
                 200.5740d {+13h 22m 17.7s} (current),
                 199.6852d {+13h 18m 44.4s} (1950)
GRB_DEC:         +29.9296d {+29d 55' 46.5"} (J2000),
                 +29.7943d {+29d 47' 39.5"} (current),
                 +30.1913d {+30d 11' 28.5"} (1950)
GRB_ERROR:       5.0 [arcsec, radius, statistical plus systematic]
GRB_INTEN:       53 [cnts]
IMG_START_DATE:  21014 TJD;   339 DOY;   25/12/05
IMG_START_TIME:  85327.09 SOD {23:42:07.09} UT,   139.8 [sec] since BAT Trigger Time
CENTROID_X:      247.53,  raw= 248  [pixels]
CENTROID_Y:      277.59,  raw= 278  [pixels]
ROLL:            129.87 [deg]
GAIN:            1
MODE:            3, Long Image mode
WAVEFORM:        134
EXPO_TIME:       2.50 [sec]
GRB_POS_XRT_Y:   -49.74
GRB_POS_XRT_Z:   -151.02
IMAGE_URL:       sw01420873000msxps_rw.img
SUN_POSTN:       252.75d {+16h 51m 00s}  -22.49d {-22d 29' 15"}
SUN_DIST:         72.44 [deg]   Sun_angle= 3.5 [hr] (West of Sun)
MOON_POSTN:       88.25d {+05h 52m 59s}  +28.24d {+28d 14' 08"}
MOON_DIST:        93.17 [deg]
MOON_ILLUM:      98 [%]
GAL_COORDS:       57.95, 82.91 [deg] galactic lon,lat of the burst
ECL_COORDS:      185.41, 35.25 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Processed Image.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Fri 05 Dec 25 23:42:33 UT
NOTICE_TYPE:     Swift-XRT Position
TRIGGER_NUM:     1420873,   Seg_Num: 0
GRB_RA:          200.2707d {+13h 21m 04.96s} (J2000),
                 200.5740d {+13h 22m 17.76s} (current),
                 199.6852d {+13h 18m 44.45s} (1950)
GRB_DEC:         +29.9296d {+29d 55' 46.5"} (J2000),
                 +29.7943d {+29d 47' 39.5"} (current),
                 +30.1913d {+30d 11' 28.5"} (1950)
GRB_ERROR:       5.3 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       1.44e-09 [erg/cm2/sec]
GRB_SIGNIF:      7.28 [sigma]
IMG_START_DATE:  21014 TJD;   339 DOY;   25/12/05
IMG_START_TIME:  85327.09 SOD {23:42:07.09} UT,   139.8 [sec] since BAT Trigger Time
TAM[0-3]:        327.64 237.20 261.48 243.58
AMPLIFIER:       2
WAVEFORM:        134
SUN_POSTN:       252.75d {+16h 51m 00s}  -22.49d {-22d 29' 15"}
SUN_DIST:         72.44 [deg]   Sun_angle= 3.5 [hr] (West of Sun)
MOON_POSTN:       88.24d {+05h 52m 58s}  +28.24d {+28d 14' 08"}
MOON_DIST:        93.18 [deg]
MOON_ILLUM:      98 [%]
GAL_COORDS:       57.95, 82.91 [deg] galactic lon,lat of the burst
ECL_COORDS:      185.41, 35.25 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Coordinates.  
COMMENTS:        The XRT position is 2.43 arcmin from the BAT position.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Fri 05 Dec 25 23:42:43 UT
NOTICE_TYPE:     Swift-XRT Image
TRIGGER_NUM:     1420873,   Seg_Num: 0
GRB_RA:          200.2707d {+13h 21m 04.9s} (J2000),
                 200.5740d {+13h 22m 17.7s} (current),
                 199.6852d {+13h 18m 44.4s} (1950)
GRB_DEC:         +29.9296d {+29d 55' 46.5"} (J2000),
                 +29.7943d {+29d 47' 39.5"} (current),
                 +30.1913d {+30d 11' 28.5"} (1950)
GRB_ERROR:       5.0 [arcsec, radius, statistical plus systematic]
GRB_INTEN:       53 [cnts]
IMG_START_DATE:  21014 TJD;   339 DOY;   25/12/05
IMG_START_TIME:  85327.09 SOD {23:42:07.09} UT,   139.8 [sec] since BAT Trigger Time
CENTROID_X:      247.53,  raw= 248  [pixels]
CENTROID_Y:      277.59,  raw= 278  [pixels]
ROLL:            129.87 [deg]
GAIN:            1
MODE:            3, Long Image mode
WAVEFORM:        134
EXPO_TIME:       2.50 [sec]
GRB_POS_XRT_Y:   -49.74
GRB_POS_XRT_Z:   -151.02
IMAGE_URL:       sw01420873000msxps_rw.img
SUN_POSTN:       252.75d {+16h 51m 00s}  -22.49d {-22d 29' 15"}
SUN_DIST:         72.44 [deg]   Sun_angle= 3.5 [hr] (West of Sun)
MOON_POSTN:       88.25d {+05h 52m 59s}  +28.24d {+28d 14' 08"}
MOON_DIST:        93.18 [deg]
MOON_ILLUM:      98 [%]
GAL_COORDS:       57.95, 82.91 [deg] galactic lon,lat of the burst
ECL_COORDS:      185.41, 35.25 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Image.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Fri 05 Dec 25 23:47:53 UT
NOTICE_TYPE:     Swift-BAT GRB Lightcurve
TRIGGER_NUM:     1969542,   Seg_Num: 14
GRB_RA:          200.243d {+13h 20m 58s} (J2000),
                 200.547d {+13h 22m 11s} (current),
                 199.658d {+13h 18m 38s} (1950)
GRB_DEC:         +29.962d {+29d 57' 45"} (J2000),
                 +29.827d {+29d 49' 38"} (current),
                 +30.224d {+30d 13' 27"} (1950)
GRB_DATE:        21014 TJD;   339 DOY;   25/12/05
GRB_TIME:        85187.25 SOD {23:39:47.25} UT
TRIGGER_INDEX:   478
GRB_PHI:          98.93 [deg]
GRB_THETA:        26.05 [deg]
DELTA_TIME:      2332.00 [sec]
TRIGGER_DUR:     12.160 [sec] 
SOLN_STATUS:     0x3
RATE_SIGNIF:     0.00 [sigma]
IMAGE_SIGNIF:    0.00 [sigma]
LC_URL:          sw01420873000msb.lc
SUN_POSTN:       252.75d {+16h 51m 01s}  -22.49d {-22d 29' 17"}
SUN_DIST:         72.48 [deg]   Sun_angle= 3.5 [hr] (West of Sun)
MOON_POSTN:       88.31d {+05h 53m 14s}  +28.24d {+28d 14' 07"}
MOON_DIST:        93.09 [deg]
MOON_ILLUM:      98 [%]
GAL_COORDS:       58.27, 82.92 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      185.37, 35.27 [deg] ecliptic lon,lat of the burst (or transient)
COMMENTS:        SWIFT-BAT GRB Lightcurve.  
COMMENTS:           
COMMENTS:        WARNING: No BAT_Position TDRSS message was received for this trigger_number!  
COMMENTS:           
COMMENTS:           
COMMENTS:        The next comments were copied from the BAT_POS Notice:  
COMMENTS:        This is a rate 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 = 168.30,8.61 [deg].  
COMMENTS:        The 3rd packet (of 3) was missing in the lightcurve data stream.

GCN Circular #43004
V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko, G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU), O.Gress, N.Budnev (ISU), C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A.Sosnovskij (CrAO), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), D.Buckley (SAAO), R.Rebolo (The Instituto de Astrofisica de Canarias), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)
MASTER-Kislovodsk robotic telescope [1] located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the Swift GRB251205.99 (trigger No 1420873,13h 20m 58.42s , +29d 57m 45.0s, R=0.05) errorbox 19 sec after notice time and 92 sec after trigger time at 2025-12-05 23:41:19 UT, with upper limit up to 17.0 mag. The observations began at zenith distance = 68 deg. The sun altitude is -51.1 deg.
The galactic latitude b = 83 deg., longitude l = 55 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3062088
We obtain a following upper limits.
```
Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

     103 |   MASTER-Kislovodsk |  P- |    20 | 16.6 |        
     103 |   MASTER-Kislovodsk |  P| |    20 | 16.5 |        
     131 |   MASTER-Kislovodsk |  P- |    20 | 16.3 |        
     131 |   MASTER-Kislovodsk |  P| |    20 | 16.5 |        
     164 |   MASTER-Kislovodsk |  P- |    30 | 16.8 |        
     164 |   MASTER-Kislovodsk |  P| |    30 | 16.8 |        
     208 |   MASTER-Kislovodsk |  P- |    40 | 16.8 |        
     208 |   MASTER-Kislovodsk |  P| |    40 | 16.8 |        
     261 |   MASTER-Kislovodsk |  P- |    50 | 17.0 |        
     261 |   MASTER-Kislovodsk |  P| |    50 | 17.0 |        
```
The observation and reduction will continue. The message may be cited.
[1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp. http : // www.pereplet.ru/lipunov/625.html

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Fri 05 Dec 25 23:57:21 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     1420873,   Seg_Num: 0
POINT_RA:        200.244d {+13h 20m 59s} (J2000)
POINT_DEC:       +29.967d {+29d 58' 03"} (J2000)
POINT_ROLL:      129.867d
IMG_START_DATE:  21014 TJD;   339 DOY;   25/12/05
IMG_START_TIME:  85336.19 SOD {23:42:16.19} UT,   148.9 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        4.246
N_STARS:         21
X_OFFSET:        504 [pixels]
Y_OFFSET:        568 [pixels]
X_MAX:           1463 [pixels]
Y_MAX:           1527 [pixels]
DET_THRESH:      17
PHOTO_THRESH:    10
SL_URL:          sw01420873000msufc0148.fits
SUN_POSTN:       252.76d {+16h 51m 02s}  -22.49d {-22d 29' 20"}
SUN_DIST:         72.49 [deg]   Sun_angle= 3.5 [hr] (West of Sun)
MOON_POSTN:       88.42d {+05h 53m 41s}  +28.23d {+28d 14' 03"}
MOON_DIST:        93.01 [deg]
MOON_ILLUM:      98 [%]
GAL_COORDS:       58.31, 82.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      185.37, 35.28 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Fri 05 Dec 25 23:57:34 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     1420873,   Seg_Num: 0
POINT_RA:        200.244d {+13h 20m 59s} (J2000)
POINT_DEC:       +29.967d {+29d 58' 03"} (J2000)
POINT_ROLL:      129.867d
IMG_START_DATE:  21014 TJD;   339 DOY;   25/12/05
IMG_START_TIME:  85336.19 SOD {23:42:16.19} UT,   148.9 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        4.246
N_STARS:         21
X_OFFSET:        504 [pixels]
Y_OFFSET:        568 [pixels]
X_MAX:           1463 [pixels]
Y_MAX:           1527 [pixels]
DET_THRESH:      17
PHOTO_THRESH:    10
SL_URL:          sw01420873000msufc0148.fits
SUN_POSTN:       252.76d {+16h 51m 03s}  -22.49d {-22d 29' 20"}
SUN_DIST:         72.49 [deg]   Sun_angle= 3.5 [hr] (West of Sun)
MOON_POSTN:       88.42d {+05h 53m 41s}  +28.23d {+28d 14' 03"}
MOON_DIST:        93.01 [deg]
MOON_ILLUM:      98 [%]
GAL_COORDS:       58.31, 82.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      185.37, 35.28 [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:     Fri 05 Dec 25 23:59:12 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     1420873,   Seg_Num: 0
POINT_RA:        200.244d {+13h 20m 59s} (J2000)
POINT_DEC:       +29.967d {+29d 58' 03"} (J2000)
ROLL:            129.867d
IMG_START_DATE:  21014 TJD;   339 DOY;   25/12/05
IMG_START_TIME:  85336.19 SOD {23:42:16.19} UT,   148.9 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     786670973
X_OFFSET:        1103 [pixels]
Y_OFFSET:        817 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       1263
Y_GRB_POS:       977
BINNING_INDEX:   1
IM_URL:          sw01420873000msuni0185.fits
SUN_POSTN:       252.76d {+16h 51m 03s}  -22.49d {-22d 29' 20"}
SUN_DIST:         72.49 [deg]   Sun_angle= 3.5 [hr] (West of Sun)
MOON_POSTN:       88.44d {+05h 53m 46s}  +28.23d {+28d 14' 02"}
MOON_DIST:        93.00 [deg]
MOON_ILLUM:      98 [%]
GAL_COORDS:       58.31, 82.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      185.37, 35.28 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Image.  
COMMENTS:        The GRB Position came from the XRT Position Command.  
COMMENTS:        The image has 2x2 binning (compression).

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Fri 05 Dec 25 23:59:57 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     1420873,   Seg_Num: 0
POINT_RA:        200.244d {+13h 20m 59s} (J2000)
POINT_DEC:       +29.966d {+29d 57' 58"} (J2000)
POINT_ROLL:      129.867d
IMG_START_DATE:  21014 TJD;   339 DOY;   25/12/05
IMG_START_TIME:  85494.02 SOD {23:44:54.02} UT,   306.8 [sec] since BAT Trigger Time
FILTER:          7, U
BKG_MEAN:        1.861
N_STARS:         22
X_OFFSET:        783 [pixels]
Y_OFFSET:        497 [pixels]
X_MAX:           1742 [pixels]
Y_MAX:           1456 [pixels]
DET_THRESH:      11
PHOTO_THRESH:    6
SL_URL:          sw01420873000msufc0306.fits
SUN_POSTN:       252.76d {+16h 51m 03s}  -22.49d {-22d 29' 21"}
SUN_DIST:         72.49 [deg]   Sun_angle= 3.5 [hr] (West of Sun)
MOON_POSTN:       88.45d {+05h 53m 48s}  +28.23d {+28d 14' 02"}
MOON_DIST:        92.99 [deg]
MOON_ILLUM:      98 [%]
GAL_COORDS:       58.30, 82.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      185.37, 35.28 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Sat 06 Dec 25 00:00:17 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     1420873,   Seg_Num: 0
POINT_RA:        200.244d {+13h 20m 59s} (J2000)
POINT_DEC:       +29.966d {+29d 57' 58"} (J2000)
POINT_ROLL:      129.867d
IMG_START_DATE:  21014 TJD;   339 DOY;   25/12/05
IMG_START_TIME:  85494.02 SOD {23:44:54.02} UT,   306.8 [sec] since BAT Trigger Time
FILTER:          7, U
BKG_MEAN:        1.861
N_STARS:         22
X_OFFSET:        783 [pixels]
Y_OFFSET:        497 [pixels]
X_MAX:           1742 [pixels]
Y_MAX:           1456 [pixels]
DET_THRESH:      11
PHOTO_THRESH:    6
SL_URL:          sw01420873000msufc0306.fits
SUN_POSTN:       252.76d {+16h 51m 03s}  -22.49d {-22d 29' 21"}
SUN_DIST:         72.49 [deg]   Sun_angle= 3.5 [hr] (West of Sun)
MOON_POSTN:       88.45d {+05h 53m 49s}  +28.23d {+28d 14' 02"}
MOON_DIST:        92.99 [deg]
MOON_ILLUM:      98 [%]
GAL_COORDS:       58.30, 82.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      185.37, 35.28 [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:     Sat 06 Dec 25 00:00:52 UT
NOTICE_TYPE:     Swift-UVOT Position
TRIGGER_NUM:     1420873,   Seg_Num: 0
GRB_RA:          200.2722d {+13h 21m 05.32s} (J2000),
                 200.5755d {+13h 22m 18.12s} (current),
                 199.6867d {+13h 18m 44.81s} (1950)
GRB_DEC:         +29.9288d {+29d 55' 43.6"} (J2000),
                 +29.7935d {+29d 47' 36.6"} (current),
                 +30.1904d {+30d 11' 25.6"} (1950)
GRB_ERROR:       0.8 [arcsec radius, statistical only]
GRB_MAG:         17.54 +/- 0.14 [mag]
FILTER:          10, White
IMG_START_DATE:  21014 TJD;   339 DOY;   25/12/05
IMG_START_TIME:  85333.00 SOD {23:42:13.00} UT,   145.8 [sec] since BAT Trigger Time
SUN_POSTN:       252.76d {+16h 51m 03s}  -22.49d {-22d 29' 21"}
SUN_DIST:         72.45 [deg]   Sun_angle= 3.5 [hr] (West of Sun)
MOON_POSTN:       88.46d {+05h 53m 50s}  +28.23d {+28d 14' 02"}
MOON_DIST:        93.03 [deg]
MOON_ILLUM:      98 [%]
GAL_COORDS:       57.94, 82.91 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      185.41, 35.25 [deg] ecliptic lon,lat of the burst (or transient)
COMMENTS:        SWIFT UVOT Position Notice.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The UVOT position is 5.4 arcsec from the XRT position.  
COMMENTS:        Result based on Genie data.  
COMMENTS:        Notice generated automatically.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Sat 06 Dec 25 00:01:19 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     1420873,   Seg_Num: 0
POINT_RA:        200.244d {+13h 20m 59s} (J2000)
POINT_DEC:       +29.966d {+29d 57' 58"} (J2000)
ROLL:            129.867d
IMG_START_DATE:  21014 TJD;   339 DOY;   25/12/05
IMG_START_TIME:  85494.02 SOD {23:44:54.02} UT,   306.8 [sec] since BAT Trigger Time
FILTER:          7, U
EXPOSURE_ID:     786671130
X_OFFSET:        1103 [pixels]
Y_OFFSET:        817 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       1263
Y_GRB_POS:       977
BINNING_INDEX:   1
IM_URL:          sw01420873000msuni0343.fits
SUN_POSTN:       252.76d {+16h 51m 03s}  -22.49d {-22d 29' 21"}
SUN_DIST:         72.49 [deg]   Sun_angle= 3.5 [hr] (West of Sun)
MOON_POSTN:       88.47d {+05h 53m 52s}  +28.23d {+28d 14' 02"}
MOON_DIST:        92.98 [deg]
MOON_ILLUM:      98 [%]
GAL_COORDS:       58.30, 82.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      185.37, 35.28 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Image.  
COMMENTS:        The GRB Position came from the XRT Position Command.  
COMMENTS:        The image has 2x2 binning (compression).

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Sat 06 Dec 25 00:01:27 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     1420873,   Seg_Num: 0
POINT_RA:        200.244d {+13h 20m 59s} (J2000)
POINT_DEC:       +29.966d {+29d 57' 58"} (J2000)
ROLL:            129.867d
IMG_START_DATE:  21014 TJD;   339 DOY;   25/12/05
IMG_START_TIME:  85494.02 SOD {23:44:54.02} UT,   306.8 [sec] since BAT Trigger Time
FILTER:          7, U
EXPOSURE_ID:     786671130
X_OFFSET:        1103 [pixels]
Y_OFFSET:        817 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       1263
Y_GRB_POS:       977
BINNING_INDEX:   1
IM_URL:          sw01420873000msuni0343.fits
SUN_POSTN:       252.76d {+16h 51m 03s}  -22.49d {-22d 29' 21"}
SUN_DIST:         72.49 [deg]   Sun_angle= 3.5 [hr] (West of Sun)
MOON_POSTN:       88.47d {+05h 53m 52s}  +28.23d {+28d 14' 02"}
MOON_DIST:        92.98 [deg]
MOON_ILLUM:      98 [%]
GAL_COORDS:       58.30, 82.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      185.37, 35.28 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Image.  
COMMENTS:        The GRB Position came from the XRT Position Command.  
COMMENTS:        The image has 2x2 binning (compression).  
COMMENTS:        All 4 attachments are included.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Sat 06 Dec 25 00:03:04 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     1420873,   Seg_Num: 0
POINT_RA:        200.244d {+13h 20m 59s} (J2000)
POINT_DEC:       +29.966d {+29d 57' 58"} (J2000)
POINT_ROLL:      129.867d
IMG_START_DATE:  21014 TJD;   339 DOY;   25/12/05
IMG_START_TIME:  86046.87 SOD {23:54:06.87} UT,   859.6 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        0.532
N_STARS:         8
X_OFFSET:        543 [pixels]
Y_OFFSET:        257 [pixels]
X_MAX:           1982 [pixels]
Y_MAX:           1696 [pixels]
DET_THRESH:      7
PHOTO_THRESH:    3
SL_URL:          sw01420873000msufc0859.fits
SUN_POSTN:       252.76d {+16h 51m 04s}  -22.49d {-22d 29' 21"}
SUN_DIST:         72.49 [deg]   Sun_angle= 3.5 [hr] (West of Sun)
MOON_POSTN:       88.49d {+05h 53m 57s}  +28.23d {+28d 14' 01"}
MOON_DIST:        92.97 [deg]
MOON_ILLUM:      98 [%]
GAL_COORDS:       58.30, 82.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      185.37, 35.28 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN Circular #43005
S. Lanava (PSU), S. Dichiara (PSU), C. Gronwall (PSU), R. Gupta (NASA GSFC), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (U Tampa), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team:
At 23:39:47 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 251205A (trigger=1420873). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 200.243, +29.962 which is
```
   RA(J2000) = 13h 20m 58s
   Dec(J2000) = +29d 57' 45"
```
with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 30 sec. The peak count rate was ~500 counts/sec (15-350 keV), at ~0 sec after the trigger.
The XRT began observing the field at 23:42:07.0 UT, 139.8 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 200.2707, 29.9296 which is equivalent to:
```
   RA(J2000)  = 13h 21m 4.97s
   Dec(J2000) = +29d 55' 46.6"
```
with an uncertainty of 5.3 arcseconds (radius, 90% containment). This location is 145 arcseconds from the BAT onboard position, within the BAT error circle. No event data are yet available to determine the column density using X-ray spectroscopy.
The initial flux in the 2.5 s image was 1.44e-09 erg cm^-2 s^-1 (0.2-10 keV).
UVOT took a finding chart exposure of 150 seconds with the White filter starting 148 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at
```
  RA(J2000)  =	13:21:05.34 = 200.27223
  DEC(J2000) = +29:55:44.0  =  29.92888
```
with a 90%-confidence error radius of about 0.75 arc sec. This position is 5.4 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.54 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.014.
Burst Advocate for this burst is S. Lanava (sml7284 AT psu.edu). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/)

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Sat 06 Dec 25 00:03:18 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     1420873,   Seg_Num: 0
POINT_RA:        200.244d {+13h 20m 59s} (J2000)
POINT_DEC:       +29.966d {+29d 57' 58"} (J2000)
POINT_ROLL:      129.867d
IMG_START_DATE:  21014 TJD;   339 DOY;   25/12/05
IMG_START_TIME:  86046.87 SOD {23:54:06.87} UT,   859.6 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        0.532
N_STARS:         8
X_OFFSET:        543 [pixels]
Y_OFFSET:        257 [pixels]
X_MAX:           1982 [pixels]
Y_MAX:           1696 [pixels]
DET_THRESH:      7
PHOTO_THRESH:    3
SL_URL:          sw01420873000msufc0859.fits
SUN_POSTN:       252.76d {+16h 51m 04s}  -22.49d {-22d 29' 22"}
SUN_DIST:         72.49 [deg]   Sun_angle= 3.5 [hr] (West of Sun)
MOON_POSTN:       88.49d {+05h 53m 57s}  +28.23d {+28d 14' 01"}
MOON_DIST:        92.96 [deg]
MOON_ILLUM:      98 [%]
GAL_COORDS:       58.30, 82.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      185.37, 35.28 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Source List.  
COMMENTS:        If you have elected to receive attachments:  
COMMENTS:        The uvot_catalog_srclist.fits.gz file does not exist; skipping the attachment.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Sat 06 Dec 25 00:04:09 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     1420873,   Seg_Num: 0
POINT_RA:        200.244d {+13h 20m 59s} (J2000)
POINT_DEC:       +29.966d {+29d 57' 58"} (J2000)
ROLL:            129.867d
IMG_START_DATE:  21014 TJD;   339 DOY;   25/12/05
IMG_START_TIME:  86046.87 SOD {23:54:06.87} UT,   859.6 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     786671683
X_OFFSET:        1102 [pixels]
Y_OFFSET:        816 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       1262
Y_GRB_POS:       976
BINNING_INDEX:   1
IM_URL:          sw01420873000msuni0896.fits
SUN_POSTN:       252.77d {+16h 51m 04s}  -22.49d {-22d 29' 22"}
SUN_DIST:         72.50 [deg]   Sun_angle= 3.5 [hr] (West of Sun)
MOON_POSTN:       88.50d {+05h 54m 00s}  +28.23d {+28d 14' 00"}
MOON_DIST:        92.96 [deg]
MOON_ILLUM:      98 [%]
GAL_COORDS:       58.30, 82.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      185.37, 35.28 [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:     Sat 06 Dec 25 00:04:24 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     1420873,   Seg_Num: 0
POINT_RA:        200.244d {+13h 20m 59s} (J2000)
POINT_DEC:       +29.966d {+29d 57' 58"} (J2000)
ROLL:            129.867d
IMG_START_DATE:  21014 TJD;   339 DOY;   25/12/05
IMG_START_TIME:  86046.87 SOD {23:54:06.87} UT,   859.6 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     786671683
X_OFFSET:        1102 [pixels]
Y_OFFSET:        816 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       1262
Y_GRB_POS:       976
BINNING_INDEX:   1
IM_URL:          sw01420873000msuni0896.fits
SUN_POSTN:       252.77d {+16h 51m 04s}  -22.49d {-22d 29' 22"}
SUN_DIST:         72.50 [deg]   Sun_angle= 3.5 [hr] (West of Sun)
MOON_POSTN:       88.50d {+05h 54m 01s}  +28.23d {+28d 14' 00"}
MOON_DIST:        92.95 [deg]
MOON_ILLUM:      98 [%]
GAL_COORDS:       58.30, 82.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      185.37, 35.28 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Image.  
COMMENTS:        The GRB Position came from the Window Position in the Mode Command.  
COMMENTS:        The image has 2x2 binning (compression).  
COMMENTS:        If you have elected to receive attachments:  
COMMENTS:        The uvot_catalog_image.fits.gz file does not exist; skipping the attachment.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Fri 05 Dec 25 23:59:27 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     1420873,   Seg_Num: 0
POINT_RA:        200.244d {+13h 20m 59s} (J2000)
POINT_DEC:       +29.967d {+29d 58' 03"} (J2000)
ROLL:            129.867d
IMG_START_DATE:  21014 TJD;   339 DOY;   25/12/05
IMG_START_TIME:  85336.19 SOD {23:42:16.19} UT,   148.9 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     786670973
X_OFFSET:        1103 [pixels]
Y_OFFSET:        817 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       1263
Y_GRB_POS:       977
BINNING_INDEX:   1
IM_URL:          sw01420873000msuni0185.fits
SUN_POSTN:       252.76d {+16h 51m 03s}  -22.49d {-22d 29' 20"}
SUN_DIST:         72.49 [deg]   Sun_angle= 3.5 [hr] (West of Sun)
MOON_POSTN:       88.44d {+05h 53m 46s}  +28.23d {+28d 14' 02"}
MOON_DIST:        93.00 [deg]
MOON_ILLUM:      98 [%]
GAL_COORDS:       58.31, 82.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      185.37, 35.28 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Image.  
COMMENTS:        The GRB Position came from the XRT Position Command.  
COMMENTS:        The image has 2x2 binning (compression).  
COMMENTS:        All 4 attachments are included.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Sat 06 Dec 25 00:15:22 UT
NOTICE_TYPE:     Swift-XRT Position UPDATE
TRIGGER_NUM:     1420873,   Seg_Num: 0
GRB_RA:          200.2724d {+13h 21m 05.37s} (J2000),
                 200.5757d {+13h 22m 18.17s} (current),
                 199.6869d {+13h 18m 44.86s} (1950)
GRB_DEC:         +29.9299d {+29d 55' 47.6"} (J2000),
                 +29.7946d {+29d 47' 40.6"} (current),
                 +30.1915d {+30d 11' 29.5"} (1950)
GRB_ERROR:       3.9 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       1.00e-10 [erg/cm2/sec]
GRB_SIGNIF:      10.00 [sigma]
IMG_START_DATE:  21014 TJD;   339 DOY;   25/12/05
IMG_START_TIME:  85719.00 SOD {23:48:39.00} UT,   531.8 [sec] since BAT Trigger Time
TAM[0-3]:        100.00 100.00 100.00 100.00
AMPLIFIER:       1
WAVEFORM:        31
SUN_POSTN:       252.77d {+16h 51m 06s}  -22.49d {-22d 29' 25"}
SUN_DIST:         72.46 [deg]   Sun_angle= 3.5 [hr] (West of Sun)
MOON_POSTN:       88.63d {+05h 54m 32s}  +28.23d {+28d 13' 56"}
MOON_DIST:        92.90 [deg]
MOON_ILLUM:      98 [%]
GAL_COORDS:       57.95, 82.91 [deg] galactic lon,lat of the burst
ECL_COORDS:      185.41, 35.25 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Coordinates.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        This is an Update Notice -- the RA,Dec values herein supersede the previous XRT_POS Notice.  
COMMENTS:        TAM values, flux and significance fields are not valid.  
COMMENTS:        This position was automatically generated on the ground using  
COMMENTS:        Photon Counting data telemetered via TDRSS (SPER data).  
COMMENTS:        See https://www.swift.ac.uk/sper/docs.php for details.  
COMMENTS:        The probability that this is a serendipitous source in the  
COMMENTS:        SPER window is 0.58% < P(seren) < 1.1%.

GCN Circular #43006
V.M. Lipunov (Lomonosov MSU), V.Senik, E.Gorbovskoy, A.Kuznetsov, I.Panchenko, G.Antipov, P.Balanutsa, K.Zhirkov, N. Tiurina, Ya.Kechin,A.Chasovnikov, D.Vlasenko, Yu.Tselik(Lomonosov MSU, SAI, Moscow), O.A.Gress, N.M.Budnev, O.Ershova (Irkutsk State University), R. Podesta, C.Francile, F. Podesta, E. Gonzalez (OAFA, San JuanUni.,Argentina); D.Buckley (SAAO), A. Sosnovskij (CrAO RAS), A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory), A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity), L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysic Observatory, Mexico) V.M.Pillet, R.Rebolo Lopez (The Instituto de Astrofisica de Canarias,Spain)
MASTER-Kislovodsk robotic telescope of MASTER Global Robotic Net (Lipunov et al. [1-4], http://observ.pereplet.ru ), started Swift GRB 251205A (Lanava et al GCN 43005) error-box observations in alert mode (PP filters) 19s after notice time at at 2025-12-05 23:41:19 UT, Lipunov et al. GCN 43004
MASTER auto-detection system found optical counterpart MASTER OT J132105.37+295545.8 , first publishied by Swift (GCN 43005) with unfiltered m_OT~18.1m We have also prompt wide-field cameras images, the reduction will be later.
There is host(possible) galaxy in 4.36" from GRB counterpart with z_phot=0.25470 (Cavuoti et al. 2014)
Real time updated cover map and OT position are available here: https://master.sai.msu.ru/site/master2/observ.php?id=3062088 Observation and reduction will continue.
```
[1] Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L
[2] Lipunov et al. 2022, Universe, Vol. 8(5), id.271
[3] Lipunov et a. 2019, ARep, vol.63, 293
[4] Lipunov V., Kornilov V., Gorbovskoy E., Tiurina N., Kuznetsov A. 2023,
Astronomical Robotic Networks and Operative Multichanel Astrophysics,Lomonosov
MSU PRESS, 591pp.
http://www.pereplet.ru/lipunov/625.html#625
```
GCN Circular #43007
A.P. Beardmore, P.A. Evans, M.R. Goad and J.P. Osborne (U. Leicester) report on behalf of the Swift-XRT team.
Using 366 s of XRT Photon Counting mode data and 1 UVOT images for GRB 251205A, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 200.27190, +29.92894 which is equivalent to:
```
RA (J2000): 13h 21m 05.26s
Dec (J2000): +29d 55' 44.2"
```
with an uncertainty of 2.4 arcsec (radius, 90% confidence).
This position may be improved as more data are received. The latest position can be viewed at https://www.swift.ac.uk/xrt_positions. Position enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans et al. (2009, MNRAS, 397, 1177).
This circular was automatically generated, and is an official product of the Swift-XRT team.
GCN Circular #43008
A. de Ugarte Postigo (LAM), S. Geier (GTC), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), C. C. Thoene (AbAO), L. Izzo (INAF/OACN and DARK/NBI), J. F. Agui Fernandez (CAHA), M. A. Aloy (UV), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC), G. Lombardi (GTC), N. A. Rakotondrainibe (LAM), B. Schneider (LAM), N. R. Tanvir (Univ. Leicester), D. Perez Valladares (GTC) report:
We observed the optical counterpart of GRB 251205A (Lanava et al., GCN 43005; Lipunov et al., GCN 43006) using the Gran Telescopio Canarias (GTC) equipped with the OSIRIS+ instrument.
In the 30-s acquisition image (beginning on 2025-12-06 at 05:21:34 UT, that is 5.68 hr after trigger), the optical afterglow is well detected with a magnitude i = 17.78 +- 0.03 AB, calibrated against nearby Pan-STARRS objects, and not corrected for Galactic extinction. We also report the following, improved coordinates (0.3" accuracy):
```
RA(J2000) = 13:21:05.32
Dec(J2000) = +29:55:43.6
```
A total of 4 spectra by 900 s were secured, starting on 2025-12-06 at 05:31:22 UT (5.86 hr after trigger), using grism R1000B. Continuum is detected over the range 3750-7750 AA. A number of absorption features are detected, which we interpret as due to Fe II 2344, 2383, 2586, 2600, Mg II 2796, 2803, and Mg I 2852, all at a common redshift z = 1.100, which we suggest to be the redshift of GRB 251205A.
Given the redshift value measured in absorption, it is thus unlikely that the GRB is associated with the nearby, bright galaxy with photometric redshift z = 0.25 noted by Lipunov et al. (GCN 43006).
GCN Circular #43009
D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), A. K. Aris-Kiss (Helsinki), M. Korp-Lagg (Aalto University), S. Wedemeyer (Oslo), S. Armas Perez (NOT) report on behalf of a larger collaboration:
We observed the optical afterglow of GRB 251205A (Lanava et al., GCN 43005; Lipunov et al., GCN 43006; de Ugarte Postigo et al., GCN 43008) using the Nordic Optical Telescope (NOT) equipped with the StanCam camera. Observations started on 2025-12-06 at 05:25:33 UT (5.75 hr after the trigger). Five 150 s observations were taken using the StanCam “interference i” filter.
The optical afterglow is well detected in single images. At a mean time of 5.90 hr after the trigger, we measure a magnitude
i = 17.93 +- 0.03.
This magnitude is in the AB system and is not corrected for Galactic extinction. We note that, due to the small field-of-view of StanCam, only one star from the Pan-STARRS catalog could be used for photometric calibration. Considering that the StanCam i filter is (slightly) different from the Pan-STARRS one, an extra systematic error of 0.1 mag should be conservatively added to the photometric error budget.
GCN Circular #43010
D. O'Neill, G. Ramsay, A. Kumar, B. Gompertz, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, S. Belkin, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien,, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, D. Pollacco, J. Casares Vel'azquez, T. Killestein, B. Godson on behalf of GOTO collaboration:
We report on optical observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to the GRB 251205A.
Serendipitous observations from the all-sky survey covered the position of the counterpart (Malesani et al., GCN 43009; Lanava et al., GCN 43005; Lipunov et al., GCN 43006; de Ugarte Postigo et al., GCN 43008) at 2025-12-06 06:12:01 UT, (+6.5h post trigger). Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations. The counterpart was detected in the GOTO L-band (400-700nm) mag = 18.16 ± 0.12 (AB). Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and not corrected for Galactic extinction.
GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester, the University of Birmingham and the Instituto de Astrofisica de Canarias (IAC).
GCN Circular #43011
Nikos Mandarakas (LAM), Margarita Pereyra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM):
We imaged the field of the Swift GRB 251205A (S. Lanava et al., GCN Circ. 43005) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-12-06 10:04:31 to 10:47:46 UTC (from 10.41 to 11.13 hours after the trigger) and obtained 32 minutes of simultaneous exposure in the r and z filters.
The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
We detected the optical counterpart S. Lanava et al. (GCN Circ. 43005); V. M. Lipunov et al. (GCN Circ. 43006); A. de Ugarte Postigo et al. (GCN Circ. 43008); D. B. Malesani et al. (GCN Circ. 43009) and D. O’Neill et al. (GCN Circ. 43010), at a preliminary magnitude of:
```
r = 18.26 +/- 0.03
z = 17.97 +/- 0.02
```
Further observations are ongoing.
We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.
COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.
GCN Circular #43012
N. Pankov (HSE,IKI), A. Novichonok (Petrozavodsk State University, KIAM), S. Schmalz (KIAM), A. Volnova (IKI), A. Pozanenko (IKI) report on behalf of IKI GRB-FuN:
We observed the optical afterglow of GRB 251205A (Lanava et al., GCN 43005; Lipunov et al., GCN 43006; de Ugarte Postigo et al., GCN 43008; Malesani et al., GCN 43009; O'Neill et al., GCN 43010) using the Zeiss-1000 telescope at Koshka observatory (INASAN). The observations started/ended on 2025-12-06 (UT) 00:57:28/04:07:07. Preliminary photometry of the initial stacked image is the following
```
Date UT start t-T0 Exp. Filter OT Err. UL(3sigma)
(mid, days) (s)

2025-12-06 00:57:28 0.05747 10*90 R  17.92 0.06 20.5
```
The photometry is based on nearby USNO-B1.0 stars and not corrected for Galaxy extinction.
GCN Circular #43015
A. Bochenek, D. A. Perley (LJMU), report:
We observed the field of GRB 251205A (Lanava et al., GCN 43005) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 4x100s exposures the SDSS i and r filters starting at 2025-12-06 05:37:14 UT, approximately 5.95 hours after trigger.
Some exposures were affected by telescope movement and had to be discarded. We report detections in both filters, at the position first reported by Lanava et al. GCN 43005:
```
MJD (mid)          T_mid-T_0        Filter       Mag. (AB)
61015.23465        5.97 h           i          17.93 ± 0.05
61015.24283        6.16 h           r          18.06 ± 0.03
```
The photometry is consistent with measurements by Lanava et al. GCN43005; Lipunov et al. GCN 43006; de Ugarte Postigo et al. GCN 43008; Malesani et al. GCN 43009; O’Neill et al., GCN 43010; Mandarakas et al. GCN 43011 and Pankov et al., GCN 43012. The photometry was obtained using nearby PanSTARRS secondary standards and was not corrected for extinction.

GCN Circular #43016
N. Klingler (NASA-GSFC / UMBC / CRESST II) and Sophia Lanava (PSU) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 251205A 149 s after the BAT trigger (Lanava et al., GCN Circ. 43005). A source consistent with the XRT position (RA, Dec = 13h 21m 05.26s, +29d 55' 44.2"; Beardmore et al., GCN Circ. 43007) is detected in the initial UVOT exposures.
The preliminary UVOT position is:

    RA  (J2000) =  13:21:05.34 = 200.27223 (deg.)
    Dec (J2000) = +29:55:44.0  =  29.92890 (deg.)

with an estimated uncertainty of 0.43 arc sec. (radius, 90% confidence).
Preliminary detections using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are:

Filter  T_start(s)  T_stop(s)    Mag

white      148.9     298.7     17.61 +/- 0.06      
white      586.8     606.5     16.96 +/- 0.09      
white      759.1     869.1     16.89 +/- 0.08      
white    16513.5   17308.6     17.77 +/- 0.05      

b          562.4     582.2     17.32 +/- 0.17      
b          734.9     754.7     17.29 +/- 0.18      
b        15601.3   16508.4     18.18 +/- 0.05      

u          306.8     556.6     16.65 +/- 0.06      
u          710.3     730.0     16.57 +/- 0.16      
u        22323.1   22838.7     17.51 +/- 0.06      

v          636.4     656.2     17.10 +/- 0.27      
v          808.6     828.4     16.92 +/- 0.24      
v        10660.9   11393.6     18.45 +/- 0.10      

uvw1       685.8     705.5     16.18 +/- 0.18      
uvw1     21416.9   22316.6     17.83 +/- 0.07      

uvw2      9754.6   10654.4     18.44 +/- 0.10

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.014 in the direction of the burst (Schlegel et al. 1998).

GCN Circular #43017
S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB) and P.A. Evans report on behalf of the Swift-XRT team:
We have analysed 6.3 ks of XRT data for GRB 251205A, from 129 s to 94.1 ks after the trigger. The data comprise 352 s in Windowed Timing (WT) mode (the first 9 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.
The late-time light curve (from T0+9.7 ks) can be modelled with a power-law decay with a decay index of alpha=0.77 (+0.12, -0.11).
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.14 (+/-0.06). The best-fitting absorption column is 5.6 (+1.3, -1.2) x 10^20 cm^-2, in excess of the Galactic value of 1.2 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.95 (+0.16, -0.15) and a best-fitting absorption column of 3.8 (+3.5, -2.6) x 10^20 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.4 x 10^-11 (3.7 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
```
Total column:	     3.8 (+3.5, -2.6) x 10^20 cm^-2
Galactic foreground: 1.2 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.95 (+0.16, -0.15)
```
The results of the XRT-team automatic analysis are available at https://www.swift.ac.uk/xrt_products/01420873.
This circular is an official product of the Swift-XRT team.
GCN Circular #43018
H. A. Krimm (NSF), S. D. Barthelmy (GSFC), R. Gupta (GSFC), S. Laha (GSFC/UMBC), S. Lanava (PSU), A. Y. Lien (U Tampa), C. B. Markwardt (GSFC), M. J. Moss (GSFC), D. M. Palmer (LANL), T. Parsotan (GSFC), D. Sadaula (GSFC/UMBC), T. Sakamoto (AGU) (i.e. the Swift-BAT team):
Using the data set from T-239 to T+963 sec from the recent telemetry downlink, we report further analysis of BAT GRB 251205A (trigger #1420873) (Lanava et al., GCN Circ. 43005). The BAT ground-calculated position is RA, Dec = 200.260, 29.906 deg which is
```
   RA(J2000)  =  13h 21m 02.5s
   Dec(J2000) = +29d 54' 21.2"
```
with an uncertainty of 2.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 45%.
The mask-weighted light curve shows a weak pulse that starts at ~T0 and peaks at ~T+9 s. The main pulse ends at ~T+40s, followed by a weak tail emission that lasts till ~T+140 s. T90 (15-350 keV) is 128.00 +- 65.97 sec (estimated error including systematics).
The time-averaged spectrum from T+0.94 to T+144.94 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.67 +- 0.27. The fluence in the 15-150 keV band is 1.1 +- 0.2 x 10^-06 erg/cm2. The 1-sec peak photon flux measured from T+8.44 sec in the 15-150 keV band is 0.5 +- 0.2 ph/cm2/sec. All the quoted errors are at the 90% confidence level.
The results of the batgrbproduct analysis are available at https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1420873
GCN Circular #43019
S. Patil, A.P. Saikia, V. Swain, V. Vijaykumar, T. Mohan, V. Bhalerao (IITB), G.C. Anupama, S. Barway (IIA) and K. Angail (IAO) report on behalf of the GIT team:
We observed the field of Swift GRB 251205A (Lanava et al., GCN Circ. 43005), with 0.7m GROWTH-India Telescope (GIT). We started the observation at 2025-12-05T23:46:13 UT, i.e., 6.43 minutes after the Swift BAT trigger. We obtained multiple exposures in r' filter. We detected the optical afterglow in our stacked image of r' filter at position reported by Swift UVOT (Lanava et al., GCN Circ. 43005). The photometry results are as follow:
```
| MJD (mid)     | Filter | t-t0 (in minutes) |Total Exposure Time (sec) | Magnitude (AB) |
| ------------- | ------ |-----| ------------------ | -------------- |
| 61014.99251157 | r'     | 9.43  | 360  | 16.84 +/- 0.06 |
| 61015.00966435 | r'     | 34.13  | 360  | 17.02 +/- 0.06 |
```
The measurement is calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.
Our magnitude is consistent with other optical observations (Lipunov et al., GCN 43004, de Ugarte Postigo et al., GCN 43008, Malesani et al. GCN 43009, O'Neill et al. 43010, Mandarakas et al. GCN 43011, Pankov et al. 43012, Bochenek et al. 43015, Klingler et al. 43016, ).
The GROWTH India Telescope (GIT; Kumar et al. 2022) is a 70-cm telescope with a 0.7-degree field of view, set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge funding by the IITB alumni batch of 1994, which partially supports the operations of the telescope. Telescope technical details are available at https://sites.google.com/view/growthindia/.
GCN Circular #43020
I. Pérez-Fournon (IAC and ULL), J. Basurto Merino, P.G. Berdayes, A. Caballero-Almagro, A. Cerón, M. Contreras, F. Díaz-Segado, T. Ferrer-Laviña, B. Gandolfi, V. Ghiraldo, J. Hernández Fung, L. Juliá-Maroto, E. Lekaroz-Urriza, M. Manzano García, E. Mejía-Martínez, J. Prieto Polo, M. Pulido-Torres, M. Quintana-Ansaldo, A. Schenone-Zanuzzi, A. Selezneva, T. Tundidor Rodríguez, E. Urquijo-Rodríguez (all ULL) and M. Abdul-Masih (IAC and ULL).
We report on ZTF and ATLAS detections of the optical afterglow of the Swift GRB 251205A (Lanava et al., GCN Circ. 43005; Beardmore et al., GCN Circ. 43007; Dichiara et al., GCN Circ. 43017; and Krimm et al., GCN Circ. 43018) at a spectroscopic redshift of z = 1.100 (de Ugarte Postigo et al., GCN Circ. 43008).
Both ZTF and ATLAS detected the optical afterglow of GRB 251205A during standard observations of these time-domain surveys. ZTF first detected this transient (ZTF25acgbeof) on 2025-12-06 at 12:16:12 UT at a ZTF g-band magnitude of g-ZTF = 19.168 +/- 0.169. It was reported to the Transient Name Server by Rehemtulla et al. (AT 2025afws, TNS Astronomical Transient Report No. 279483).
Using the ATLAS forced-photometry server (Shingles et al. 2021, Transient Name Server AstroNote 2021-7), we find three ATLAS detections, as reported in the following table of ZTF and ATLAS photometry:
MJD mag (AB) error survey filter
61015.511250 19.168 0.169 ZTF g
61015.543854 18.788 0.105 ZTF r
61015.625035 19.062 0.279 ATLAS orange
61015.628805 19.225 0.321 ATLAS orange
61015.646040 18.889 0.229 ATLAS orange
These ZTF and ATLAS detections are consistent with other reported UV and optical detections: Lanava et al. (GCN Circ. 43005), Lipunov et al. (GCN Circ. 43006), de Ugarte Postigo et al. (GCN Circ. 43008), Malesani et al. (GCN Circ. 43009), O'Neill et al. (GCN Circ. 43010), Mandarakas et al. (GCN Circ. 43011), Pankov et al. (GCN Circ. 43012), Bochenek and Perley (GCN Circ. 43015), Klingler et al. (GCN Circ. 43016), and Patil et al. (GCN Circ. 43019).
The ZTF coordinates of ZTF25acgbeof / AT 2025afws are RA (J2000) = 13:21:05.324, Dec (J2000) = 29:55:43.64, consistent within the errors with the optical and X-ray positions of the GRB 251205A afterglow reported by Lanava et al. (GCN Circ. 43005), Lipunov et al. (GCN Circ. 43006), Beardmore et al. (GCN Circ. 43007), de Ugarte Postigo et al. (GCN Circ. 43008), and Klingler et al. (GCN Circ. 43016).
This work made use of the Astro-COLIBRI platform (P. Reichherzer et al. 2021, ApJS, 256, 5).
GCN Circular #43024
A. Bochenek, D. A. Perley (LJMU), report:
We observed the field of GRB 251205A (Lanava et al., GCN 43005) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 4x100s exposures the SDSS i and r filters starting at 2025-12-07 04:57:54 UT, approximately 29.3 hours after trigger.
Some exposures were affected by telescope wobble or poor seeing and had to be discarded before stacking. We report detections in both filters, at the position first reported by Lanava et al., GCN 43005:
```
MJD (mid)          T_mid-T_0        Filter       Mag. (AB)
61016.21035        29.4 h           i          19.57 ± 0.06
61016.21529        29.5 h           r          19.72 ± 0.07
```
The photometry is consistent with measurements by Lanava et al. GCN 43005; Lipunov et al. GCN 43006; de Ugarte Postigo et al. GCN 43008; Malesani et al. GCN 43009; O’Neill et al., GCN 43010; Mandarakas et al. GCN 43011; Pankov et al., GCN 43012; Bochenek and Perley, GCN 43015; Klingler et al., GCN 43016; Patil et al., GCN 43019; and Pérez-Fournon et al., GCN 43020. The photometry was obtained using nearby PanSTARRS secondary standards and was not corrected for extinction.
GCN Circular #43026
Malte Busmann (LMU), Xander J. Hall (CMU), Jule Augustin (LMU), Brendan O’Connor (CMU), Daniel Gruen (LMU), and Antonella Palmese (CMU) report:
We observed the counterpart of GRB 251205A (Lavana et al. GCN 43005) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i, z, J, and Ks bands. We detect the counterpart across all bands.
```
| Start Time          | t - t0 (days) | Filter | Exposures             | Magnitude (AB) | 
|---------------------|---------------|--------|-----------------------|----------------|
| 2025-12-06T03:25:42 | 0.157         | r'     | 5 x 150 s + 5 x 180 s | 18.09 +/- 0.05 |
| 2025-12-06T03:25:42 | 0.157         | i'     | 5 x 150 s             | 18.10 +/- 0.05 |
| 2025-12-06T03:41:46 | 0.168         | z'     | 5 x 180 s             | 17.74 +/- 0.05 |
| 2025-12-06T03:42:00 | 0.168         | J      | 5 x 180 s             | 17.46 +/- 0.05 |
| 2025-12-06T03:25:55 | 0.157         | Ks     | 10 x 60 s             | 17.13 +/- 0.05 |
```
These measurements are consistent with the magnitudes reported by Lavana et al. (GCN 43005), Lipunov et al. (GCN 43006), Malesani et al. (GCN 43009), O’Neill et al. (GCN 43010), Mandarakas et al. (GCN 43011), Bochenek & Perley (GCN 43015), Klinger et al. (GCN 43016), Patil et al. (GCN 43019), Pérez-Fournon et al. (GCN 43020), and Bochenek et al. (GCN 13024).
The r, i, and z-band magnitudes are calibrated against the PS1 catalog, and the J and Ks-band magnitudes are calibrated with the 2MASS Catalog. All magnitudes are provided in the AB system and are not corrected for Galactic extinction.
We thank Michael Schmidt from the Wendelstein Observatory for obtaining these observations.
GCN Circular #43029
H. Hagio, Y. Kubo, I. Takahashi, M. Sasada, H. Seki, A. Ochi, R. Kato, S. Joshima, Y. Yatsu and N. Kawai (Science Tokyo) report on behalf of the MITSuME collaboration:
We observed the field of GRB 251205A detected by Swift (Lanava et al., GCN 43005) with the optical three-color (g', Rc, and Ic) CCD cameras attached to the MITSuME 50-cm telescope Akeno.
The observation started at 2025-12-06 16:56:58 UT (16.3 hr after the trigger). We stacked the images taken under good conditions. We detected a point source in the Rc- and Ic-band images at a position of the reported optical candidate (Lipunov et al., GCN 43004; Lanava et al., GCN 43005; Lipunov et al., GCN 43006; Malesani et al., GCN 43009; O'Neill et al., GCN 43010; Mandarakas et al., GCN 43011; Pankov et al., GCN 43012; Bochenek et al., GCN 43015; Klingler et al., GCN 43016; Patil et al., GCN 43019; Pérez-Fournon et al., GCN 43020; Bochenek et al., GCN 43024; Busmann et al., GCN 43026 ). Our photometric results can be contaminated by the flux from the nearby galaxy. Here we report the preliminary magnitudes of the source as follows.
```
T0+[sec] | MID-UT | T-EXP[sec] | magnitudes
---------------------------------------------------------------------------------------------------------------
68759 | 2025-12-06 18:45:46 | 4860 | Rc=19.1+/-0.2, Ic=18.8+/-0.2
---------------------------------------------------------------------------------------------------------------
T0+ : Elapsed time after the trigger
T-EXP: Total exposure time
```
We used the PS1 catalog for flux calibration. The catalog magnitudes in PS1 g, r and i bands were converted to our g', Rc and Ic band magnitudes following Tonry et al. (2012), Table 6. The magnitudes are expressed in the AB system. The images were processed in real-time through the MITSuME GPU reduction pipeline (Niwano et al. 2021, PASJ; https://github.com/MNiwano/Eclaire).
GCN Circular #43038
A. Bochenek, D. A. Perley (LJMU), report:
We observed the field of GRB 251205A (Lanava et al., GCN 43005) using the IO:O optical camera on the 2m robotic Liverpool Telescope. We obtained 4x100s exposures in the SDSS i and r filters starting at 2025-12-08 05:21:03 UT, approximately 2.24 days after trigger.
We report detections in the stacked images of both filters, at the position first reported by Lanava et al., GCN 43005:
```
MJD (mid)          T_mid-T_0        Filter       Mag. (AB)
61017.22543        53.8 h           i           21.02 ± 0.19
61017.23156        53.9 h           r           21.14 ± 0.26
```
The photometry was obtained using nearby PanSTARRS secondary standards and was not corrected for extinction. The photometry is consistent with late-time optical measurements by Mandarakas et al., GCN 43011; Pérez-Fournon et al., GCN 43020; Bochenek & Perley, GCN 43024; and Hagio et al., GCN 43029.
GCN Circular #43044
Geoffrey Mo (Caltech/Carnegie), Tomas Ahumada (NOIRLab), Benjamin Schneider (LAM), Viraj Karambelkar (Columbia), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of GRB 251205A (Lanava et al., GCN 43005; Beardmore et al., GCN 43007; Dichiara et al., GCN 43017; Krimm et al., GCN 43018) in the near-infrared J band with the Palomar 1-m telescope, equipped with the 1.2-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024).
Observations began at 2025-12-06T10:27:54 UTC in the J band (~10.8 hours after the GRB trigger), consisting of 15 x 120 s exposures.The images were processed using the WINTER data reduction pipeline implemented with mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565).
We detect a source at the optical counterpart location (Lipunov et al., GCN 43004; Lanava et al., GCN 43005; Lipunov et al., GCN 43006; de Ugarte Postigo et al., GCN 43008; Malesani et al., GCN 43009; O’Neill et al., GCN 43010; Mandarakas et al., GCN 43011; Pankov et al., GCN 43012; Bochenek et al., GCN 43015; Klinger et al., GCN 43016; Patil et al., GCN 43019; Pérez-Fournon et al., GCN 43020; Bochenek et al., GCN 43024; Busmann et al., GCN 43026; Hagio et al., GCN 43029; Bochenek et al., GCN 43038), with magnitude J = 17.9 ± 0.1 mag (AB).
WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.
GCN Circular #43059
R. Hamburg and P. Veres report on behalf of the Fermi-GBM Team:
The Swift/BAT detected GRB 251205A on 2025-12-05 at 23:39:47 UTC (Lanava et al 2025, GCN 43005) and a redshift of z = 1.1 has been measured by GTC/OSIRIS (de Ugarte Postigo et al 2025, GCN 43008). There was no Fermi-GBM onboard trigger around this event time. An automated, blind search for gamma-ray bursts below the onboard triggering threshold in Fermi-GBM identified no candidates.
The GBM Targeted Search [1], the most sensitive coherent search for GRB-like signals in GBM, identified a transient starting about 2 seconds after the Swift/BAT trigger time, most significantly on the 8 s timescale with an SNR of 7.1 and a false alarm rate of 8.1e-05 Hz. The Targeted Search event was found with highest significance using a hard spectrum (i.e., Comptonized function with Epeak = 1500 keV, alpha = -0.5) for a GRB. The Targeted Search localization is found to be spatially consistent with the Swift BAT location.
[1] Goldstein et al. 2019, arXiv:1903.12597
GCN Circular #43067
Anshika Gupta, Debalina Kar, Koshvendra Singh, Dhruv Jain, Pankaj Pawar, and Kuntal Misra (ARIES) report:
We observed the field of GRB 251205A detected by Swift (Lanava et al. 2025, GCN 43005) with the 1.3m Devasthal Fast Optical Telescope (DFOT), located at the Devasthal Observatory of the Aryabhatta Research Institute of Observational Sciences (ARIES), India. The observations were started on 2025-12-09 at 23:15:22.00 UT, i.e., ~ 3.98 days after the Swift trigger. We have taken multiple frames with an exposure time of 300s in the R filter. We stacked the images after the alignment. We could not detect the optical emission in our stacked image within the error box of Swift/UVOT (Lanava et al. 2025, GCN 43005). We obtain the following 3-sigma upper limit in the stacked image:
```
Date Start_UT T_start-T0 (Days) Filter  Exp time (s)  Magnitude
===============================================================
2025-12-09  23:15:22.00    ~3.98     R     300s*11     >22.9
```
The non-detection of the burst is consistent with (Lipunov et al. 2025, GCN 43004; Lanava et al. 2025, GCN 43005; Lipunov et al. 2025, GCN 43006; de Ugarte Postigo et al. 2025, GCN 43008; Malesani et al. 2025, GCN 43009; O’Neill et al. 2025, GCN 43010; Mandarakas et al. 2025, GCN 43011; Pankov et al. 2025, GCN 43012; Bochenek et al. 2025, GCN 43015; Klinger et al. 2025, GCN 43016; Patil et al. 2025, GCN 43019; Pérez-Fournon et al. 2025, GCN 43020; Bochenek et al. 2025, GCN 43024; Busmann et al. 2025, GCN 43026; Hagio et al. 2025, GCN 43029; Bochenek et al. 2025, GCN 43038; Mo et al. 2025, GCN 43044).
The magnitude is not corrected for the Galactic extinction in the direction of the burst. Photometric calibration is performed using the standard stars from the USNO-B1.0 catalogue.
GCN Circular #43231
M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), S. Antier (OCA/IJCLab), M. Coughlin (UMN), S. Karpov (FZU), P. Hello (IJCLAB), M. Pillas (IAP) on behalf of the GRANDMA/Kilonova-Catcher collaboration:
We observed the field of GRB 251205A detected by Swift/BAT (Lanava et al., GCN 43005), Fermi/GBM (Fermi GBM team, GCN 43059) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with the TEC160FL telescope operated by M. Freeberg. Our observations started at TGRB+10.6h and were taken with sdss r and i filters.
In our stacked frames, subtracted from the Pan STARRS DR2 template image, we detect the optical counterpart reported by Swift/UVOT (Lanava et al., GCN 43005, Klingler et al., GCN 43016), MASTER (Lipunov et al., GCN 43006), GTC/OSIRIS (de Ugarte Postigo et al., GCN 43008), NOT (Malesani et al., GCN 43009), GOTO (O'Neill et al., GCN 43010), COLIBRÍ (Mandarakas et al., GCN 43011) and Zeiss-1000 Koshka observatory (Pankov et al., GCN 43012), Liverpool telescope (Bochenek et al., GCN 43015, GCN 43024, GCN 43038), GROWTH (Patil et al., GCN 43019), ZTF and ATLAS (Pérez-Fournon et al., GCN 43020), FTW (Busmann et al., GCN 43026), MITSuME (Hagio et al., GCN 43029), WINTER (Mo et al., GCN 43044), 1.3m DFOT (Gupta et al., GCN 43067).
We report our follow-up results in the table below:
```
+---------------+-----------+------------+----------------+--------------+
| Tmid-TGRB (hr)| Exp (s)   | Filter     | Magnitude      | Instrument   |
+===============+===========+============+================+==============+
| 10.86         | 10 x 180s | sdssr (AB) | 18.35 +/- 0.09 | TEC160FL     | 
| 12.51         | 22 x 180s | sdssi (AB) | 18.56 +/- 0.09 | TEC160FL     | 
| 35.63         | 20 x 180s | sdssr (AB) | 19.79 +/- 0.15 | TEC160FL     | 
| 36.72         | 19 x 180s | sdssi (AB) | 19.91 +/- 0.30 | TEC160FL     | 
+---------------+-----------+------------+----------------+--------------+
```
All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the sloan filters were calibrated using the PanSTARRS DR1 catalog.
We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023).
GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).

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