Gamma-ray Burst 251201B

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN/BACODINE POSITION NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 01 Dec 25 16:26:47 UT
NOTICE_TYPE:     Swift-BAT GRB Position
TRIGGER_NUM:     1418729,   Seg_Num: 0
GRB_RA:            6.409d {+00h 25m 38s} (J2000),
                   6.725d {+00h 26m 54s} (current),
                   5.797d {+00h 23m 11s} (1950)
GRB_DEC:         -44.237d {-44d 14' 12"} (J2000),
                 -44.094d {-44d 05' 36"} (current),
                 -44.514d {-44d 30' 49"} (1950)
GRB_ERROR:       3.00 [arcmin radius, statistical only]
GRB_INTEN:       3195 [cnts]    Image_Peak=298 [image_cnts]
TRIGGER_DUR:     1.024 [sec] 
TRIGGER_INDEX:   146     E_range: 25-100 keV
BKG_INTEN:       15078 [cnts]
BKG_TIME:        59171.21 SOD {16:26:11.21} UT
BKG_DUR:         8 [sec]
GRB_DATE:        21010 TJD;   335 DOY;   25/12/01
GRB_TIME:        59190.35 SOD {16:26:30.35} UT
GRB_PHI:         -164.57 [deg]
GRB_THETA:        30.36 [deg]
SOLN_STATUS:     0x20000003
RATE_SIGNIF:     26.98 [sigma]
IMAGE_SIGNIF:    12.62 [sigma]
MERIT_PARAMS:     +1  +0  +0  +0  +2  +2  +0  +0  +5  +0 
SUN_POSTN:       248.08d {+16h 32m 18s}  -21.91d {-21d 54' 21"}
SUN_DIST:         93.43 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.56d {+01h 22m 14s}  +12.20d {+12d 11' 54"}
MOON_DIST:        57.68 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.13,-72.12 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      344.00,-42.21 [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 = 18.39,20.39 [deg].  
COMMENTS:           
COMMENTS:        NOTE: This BAT event is temporally(6.0<100sec) coincident with the FERMI_GBM event (trignum=786299189).

red DSS finding chart ps-file

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 01 Dec 25 16:28:44 UT
NOTICE_TYPE:     Swift-XRT Processed Image
TRIGGER_NUM:     1418729,   Seg_Num: 0
GRB_RA:            6.3925d {+00h 25m 34.2s} (J2000),
                   6.7086d {+00h 26m 50.0s} (current),
                   5.7809d {+00h 23m 07.4s} (1950)
GRB_DEC:         -44.2312d {-44d 13' 52.3"} (J2000),
                 -44.0879d {-44d 05' 16.2"} (current),
                 -44.5080d {-44d 30' 28.9"} (1950)
GRB_ERROR:       6.8 [arcsec, radius, statistical plus systematic]
GRB_INTEN:       20 [cnts]
IMG_START_DATE:  21010 TJD;   335 DOY;   25/12/01
IMG_START_TIME:  59283.79 SOD {16:28:03.79} UT,   93.4 [sec] since BAT Trigger Time
CENTROID_X:      317.24,  raw= 317  [pixels]
CENTROID_Y:      285.01,  raw= 285  [pixels]
ROLL:            234.88 [deg]
GAIN:            1
MODE:            2, Short Image mode
WAVEFORM:        134
EXPO_TIME:       0.10 [sec]
GRB_POS_XRT_Y:   -32.26
GRB_POS_XRT_Z:   13.29
IMAGE_URL:       sw01418729000msxps_rw.img
SUN_POSTN:       248.08d {+16h 32m 19s}  -21.91d {-21d 54' 21"}
SUN_DIST:         93.42 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.57d {+01h 22m 18s}  +12.21d {+12d 12' 24"}
MOON_DIST:        57.69 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.17,-72.12 [deg] galactic lon,lat of the burst
ECL_COORDS:      343.99,-42.20 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Processed Image.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 01 Dec 25 16:28:30 UT
NOTICE_TYPE:     Swift-XRT Position
TRIGGER_NUM:     1418729,   Seg_Num: 0
GRB_RA:            6.3925d {+00h 25m 34.20s} (J2000),
                   6.7086d {+00h 26m 50.06s} (current),
                   5.7809d {+00h 23m 07.40s} (1950)
GRB_DEC:         -44.2312d {-44d 13' 52.3"} (J2000),
                 -44.0879d {-44d 05' 16.2"} (current),
                 -44.5080d {-44d 30' 28.9"} (1950)
GRB_ERROR:       7.0 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       8.63e-09 [erg/cm2/sec]
GRB_SIGNIF:      4.47 [sigma]
IMG_START_DATE:  21010 TJD;   335 DOY;   25/12/01
IMG_START_TIME:  59283.79 SOD {16:28:03.79} UT,   93.4 [sec] since BAT Trigger Time
TAM[0-3]:        327.71 237.16 261.56 243.80
AMPLIFIER:       2
WAVEFORM:        134
SUN_POSTN:       248.08d {+16h 32m 19s}  -21.91d {-21d 54' 21"}
SUN_DIST:         93.42 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.57d {+01h 22m 17s}  +12.21d {+12d 12' 21"}
MOON_DIST:        57.69 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.17,-72.12 [deg] galactic lon,lat of the burst
ECL_COORDS:      343.99,-42.20 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Coordinates.  
COMMENTS:        The XRT position is 0.77 arcmin from the BAT position.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 01 Dec 25 16:30:04 UT
NOTICE_TYPE:     Swift-BAT GRB Lightcurve
TRIGGER_NUM:     1418729,   Seg_Num: 0
GRB_RA:            6.409d {+00h 25m 38s} (J2000),
                   6.725d {+00h 26m 54s} (current),
                   5.797d {+00h 23m 11s} (1950)
GRB_DEC:         -44.237d {-44d 14' 12"} (J2000),
                 -44.094d {-44d 05' 36"} (current),
                 -44.514d {-44d 30' 49"} (1950)
GRB_DATE:        21010 TJD;   335 DOY;   25/12/01
GRB_TIME:        59190.35 SOD {16:26:30.35} UT
TRIGGER_INDEX:   146
GRB_PHI:         -164.57 [deg]
GRB_THETA:        30.36 [deg]
DELTA_TIME:      6.00 [sec]
TRIGGER_DUR:     1.024 [sec] 
SOLN_STATUS:     0x3
RATE_SIGNIF:     26.98 [sigma]
IMAGE_SIGNIF:    12.62 [sigma]
LC_URL:          sw01418729000msb.lc
SUN_POSTN:       248.08d {+16h 32m 19s}  -21.91d {-21d 54' 22"}
SUN_DIST:         93.43 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.59d {+01h 22m 21s}  +12.21d {+12d 12' 46"}
MOON_DIST:        57.70 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.13,-72.12 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      344.00,-42.21 [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 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 = 18.39,20.39 [deg].

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 01 Dec 25 16:28:40 UT
NOTICE_TYPE:     Swift-XRT Image
TRIGGER_NUM:     1418729,   Seg_Num: 0
GRB_RA:            6.3925d {+00h 25m 34.2s} (J2000),
                   6.7086d {+00h 26m 50.0s} (current),
                   5.7809d {+00h 23m 07.4s} (1950)
GRB_DEC:         -44.2312d {-44d 13' 52.3"} (J2000),
                 -44.0879d {-44d 05' 16.2"} (current),
                 -44.5080d {-44d 30' 28.9"} (1950)
GRB_ERROR:       6.8 [arcsec, radius, statistical plus systematic]
GRB_INTEN:       20 [cnts]
IMG_START_DATE:  21010 TJD;   335 DOY;   25/12/01
IMG_START_TIME:  59283.79 SOD {16:28:03.79} UT,   93.4 [sec] since BAT Trigger Time
CENTROID_X:      317.24,  raw= 317  [pixels]
CENTROID_Y:      285.01,  raw= 285  [pixels]
ROLL:            234.88 [deg]
GAIN:            1
MODE:            2, Short Image mode
WAVEFORM:        134
EXPO_TIME:       0.10 [sec]
GRB_POS_XRT_Y:   -32.26
GRB_POS_XRT_Z:   13.29
IMAGE_URL:       sw01418729000msxps_rw.img
SUN_POSTN:       248.08d {+16h 32m 19s}  -21.91d {-21d 54' 21"}
SUN_DIST:         93.42 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.57d {+01h 22m 18s}  +12.21d {+12d 12' 23"}
MOON_DIST:        57.69 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.17,-72.12 [deg] galactic lon,lat of the burst
ECL_COORDS:      343.99,-42.20 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Image.

GCN Circular #42923
The Fermi GBM team reports the detection of a likely LONG GRB
At 16:26:24 UT on 1 Dec 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251201B (trigger 786299189.217446 / 251201685).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 13.8, Dec = -40.0 (J2000 degrees, equivalent to J2000 00h 55m, -40d 00'), with a statistical uncertainty of 1.1 degrees.
The angle from the Fermi LAT boresight is 118.0 degrees.
The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251201685/quicklook/glg_skymap_all_bn251201685.png
The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251201685/quicklook/glg_healpix_all_bn251201685.fit
The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251201685/quicklook/glg_lc_medres34_bn251201685.gif
GCN Circular #42924
M. J. Moss (GSFC), S. Dichiara (PSU), R. Gupta (NASA GSFC), J. A. Kennea (PSU), K. L. Page (U Leicester) and T. M. Parsotan (GSFC) report on behalf of the Neil Gehrels Swift Observatory Team:
At 16:26:30 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 251201B (trigger=1418729). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 6.409, -44.237 which is
```
   RA(J2000) = 00h 25m 38s
   Dec(J2000) = -44d 14' 12"
```
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 40 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger.
The XRT began observing the field at 16:28:03.7 UT, 93.4 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 6.3925, -44.2312 which is equivalent to:
```
   RA(J2000)  = 00h 25m 34.20s
   Dec(J2000) = -44d 13' 52.3"
```
with an uncertainty of 7.0 arcseconds (radius, 90% containment). This location is 47 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 0.1 s image was 8.63e-09 erg cm^-2 s^-1 (0.2-10 keV).
UVOT took a finding chart exposure of 150 seconds with the White filter starting 101 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at
```
  RA(J2000)  =	00:25:34.59 =	6.39413
  DEC(J2000) = -44:14:01.1  = -44.23364
```
with a 90%-confidence error radius of about 0.75 arc sec. This position is 9.7 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.28 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.008.
Burst Advocate for this burst is M. J. Moss (mikejmoss3 AT gmail.com). 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:     Mon 01 Dec 25 16:39:26 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     1418729,   Seg_Num: 0
POINT_RA:          6.396d {+00h 25m 35s} (J2000)
POINT_DEC:       -44.241d {-44d 14' 25"} (J2000)
ROLL:            234.883d
IMG_START_DATE:  21010 TJD;   335 DOY;   25/12/01
IMG_START_TIME:  59504.63 SOD {16:31:44.63} UT,   314.3 [sec] since BAT Trigger Time
FILTER:          7, U
EXPOSURE_ID:     786299541
X_OFFSET:        804 [pixels]
Y_OFFSET:        945 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       964
Y_GRB_POS:       1105
BINNING_INDEX:   1
IM_URL:          sw01418729000msuni0351.fits
SUN_POSTN:       248.09d {+16h 32m 21s}  -21.91d {-21d 54' 25"}
SUN_DIST:         93.42 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.68d {+01h 22m 42s}  +12.25d {+12d 15' 14"}
MOON_DIST:        57.77 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.16,-72.12 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      343.99,-42.21 [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:     Mon 01 Dec 25 16:39:34 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     1418729,   Seg_Num: 0
POINT_RA:          6.396d {+00h 25m 35s} (J2000)
POINT_DEC:       -44.241d {-44d 14' 25"} (J2000)
ROLL:            234.883d
IMG_START_DATE:  21010 TJD;   335 DOY;   25/12/01
IMG_START_TIME:  59504.63 SOD {16:31:44.63} UT,   314.3 [sec] since BAT Trigger Time
FILTER:          7, U
EXPOSURE_ID:     786299541
X_OFFSET:        804 [pixels]
Y_OFFSET:        945 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       964
Y_GRB_POS:       1105
BINNING_INDEX:   1
IM_URL:          sw01418729000msuni0351.fits
SUN_POSTN:       248.09d {+16h 32m 21s}  -21.91d {-21d 54' 25"}
SUN_DIST:         93.42 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.68d {+01h 22m 42s}  +12.25d {+12d 15' 16"}
MOON_DIST:        57.77 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.16,-72.12 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      343.99,-42.21 [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:     Mon 01 Dec 25 16:38:16 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     1418729,   Seg_Num: 0
POINT_RA:          6.396d {+00h 25m 35s} (J2000)
POINT_DEC:       -44.241d {-44d 14' 25"} (J2000)
POINT_ROLL:      234.883d
IMG_START_DATE:  21010 TJD;   335 DOY;   25/12/01
IMG_START_TIME:  59504.63 SOD {16:31:44.63} UT,   314.3 [sec] since BAT Trigger Time
FILTER:          7, U
BKG_MEAN:        0.523
N_STARS:         15
X_OFFSET:        484 [pixels]
Y_OFFSET:        625 [pixels]
X_MAX:           1443 [pixels]
Y_MAX:           1584 [pixels]
DET_THRESH:      7
PHOTO_THRESH:    3
SL_URL:          sw01418729000msufc0314.fits
SUN_POSTN:       248.09d {+16h 32m 21s}  -21.91d {-21d 54' 25"}
SUN_DIST:         93.42 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.66d {+01h 22m 39s}  +12.25d {+12d 14' 55"}
MOON_DIST:        57.76 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.16,-72.12 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      343.99,-42.21 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 01 Dec 25 16:52:12 UT
NOTICE_TYPE:     Swift-XRT Position UPDATE
TRIGGER_NUM:     1418729,   Seg_Num: 0
GRB_RA:            6.3952d {+00h 25m 34.84s} (J2000),
                   6.7113d {+00h 26m 50.70s} (current),
                   5.7836d {+00h 23m 08.05s} (1950)
GRB_DEC:         -44.2326d {-44d 13' 57.3"} (J2000),
                 -44.0893d {-44d 05' 21.3"} (current),
                 -44.5094d {-44d 30' 33.9"} (1950)
GRB_ERROR:       3.8 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       1.00e-10 [erg/cm2/sec]
GRB_SIGNIF:      10.00 [sigma]
IMG_START_DATE:  21010 TJD;   335 DOY;   25/12/01
IMG_START_TIME:  59683.00 SOD {16:34:43.00} UT,   492.7 [sec] since BAT Trigger Time
TAM[0-3]:        100.00 100.00 100.00 100.00
AMPLIFIER:       1
WAVEFORM:        31
SUN_POSTN:       248.10d {+16h 32m 23s}  -21.91d {-21d 54' 30"}
SUN_DIST:         93.41 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.80d {+01h 23m 11s}  +12.31d {+12d 18' 36"}
MOON_DIST:        57.84 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.17,-72.12 [deg] galactic lon,lat of the burst
ECL_COORDS:      343.99,-42.20 [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 NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 01 Dec 25 17:00:13 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     1418729,   Seg_Num: 0
POINT_RA:          6.396d {+00h 25m 35s} (J2000)
POINT_DEC:       -44.241d {-44d 14' 25"} (J2000)
POINT_ROLL:      234.883d
IMG_START_DATE:  21010 TJD;   335 DOY;   25/12/01
IMG_START_TIME:  60058.23 SOD {16:40:58.23} UT,   867.9 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        1.615
N_STARS:         63
X_OFFSET:        244 [pixels]
Y_OFFSET:        385 [pixels]
X_MAX:           1683 [pixels]
Y_MAX:           1824 [pixels]
DET_THRESH:      11
PHOTO_THRESH:    5
SL_URL:          sw01418729000msufc0867.fits
SUN_POSTN:       248.10d {+16h 32m 25s}  -21.91d {-21d 54' 33"}
SUN_DIST:         93.40 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.87d {+01h 23m 29s}  +12.35d {+12d 20' 42"}
MOON_DIST:        57.90 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.16,-72.12 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      343.99,-42.21 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 01 Dec 25 17:00:30 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     1418729,   Seg_Num: 0
POINT_RA:          6.396d {+00h 25m 35s} (J2000)
POINT_DEC:       -44.241d {-44d 14' 25"} (J2000)
POINT_ROLL:      234.883d
IMG_START_DATE:  21010 TJD;   335 DOY;   25/12/01
IMG_START_TIME:  60058.23 SOD {16:40:58.23} UT,   867.9 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        1.615
N_STARS:         63
X_OFFSET:        244 [pixels]
Y_OFFSET:        385 [pixels]
X_MAX:           1683 [pixels]
Y_MAX:           1824 [pixels]
DET_THRESH:      11
PHOTO_THRESH:    5
SL_URL:          sw01418729000msufc0867.fits
SUN_POSTN:       248.10d {+16h 32m 25s}  -21.91d {-21d 54' 33"}
SUN_DIST:         93.40 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.87d {+01h 23m 30s}  +12.35d {+12d 20' 47"}
MOON_DIST:        57.90 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.16,-72.12 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      343.99,-42.21 [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:     Mon 01 Dec 25 17:01:35 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     1418729,   Seg_Num: 0
POINT_RA:          6.396d {+00h 25m 35s} (J2000)
POINT_DEC:       -44.241d {-44d 14' 25"} (J2000)
ROLL:            234.883d
IMG_START_DATE:  21010 TJD;   335 DOY;   25/12/01
IMG_START_TIME:  60058.23 SOD {16:40:58.23} UT,   867.9 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     786300095
X_OFFSET:        803 [pixels]
Y_OFFSET:        944 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       963
Y_GRB_POS:       1104
BINNING_INDEX:   1
IM_URL:          sw01418729000msuni0904.fits
SUN_POSTN:       248.10d {+16h 32m 25s}  -21.91d {-21d 54' 34"}
SUN_DIST:         93.40 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.88d {+01h 23m 32s}  +12.35d {+12d 21' 04"}
MOON_DIST:        57.90 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.16,-72.12 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      343.99,-42.21 [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:     Mon 01 Dec 25 17:01:48 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     1418729,   Seg_Num: 0
POINT_RA:          6.396d {+00h 25m 35s} (J2000)
POINT_DEC:       -44.241d {-44d 14' 25"} (J2000)
ROLL:            234.883d
IMG_START_DATE:  21010 TJD;   335 DOY;   25/12/01
IMG_START_TIME:  60058.23 SOD {16:40:58.23} UT,   867.9 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     786300095
X_OFFSET:        803 [pixels]
Y_OFFSET:        944 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       963
Y_GRB_POS:       1104
BINNING_INDEX:   1
IM_URL:          sw01418729000msuni0904.fits
SUN_POSTN:       248.10d {+16h 32m 25s}  -21.91d {-21d 54' 34"}
SUN_DIST:         93.40 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.89d {+01h 23m 32s}  +12.35d {+12d 21' 07"}
MOON_DIST:        57.91 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.16,-72.12 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      343.99,-42.21 [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:        All 4 attachments are included.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 01 Dec 25 17:00:40 UT
NOTICE_TYPE:     Swift-XRT Position UPDATE
TRIGGER_NUM:     1418729,   Seg_Num: 0
GRB_RA:            6.3951d {+00h 25m 34.82s} (J2000),
                   6.7112d {+00h 26m 50.68s} (current),
                   5.7835d {+00h 23m 08.03s} (1950)
GRB_DEC:         -44.2337d {-44d 14' 01.3"} (J2000),
                 -44.0904d {-44d 05' 25.2"} (current),
                 -44.5105d {-44d 30' 37.9"} (1950)
GRB_ERROR:       2.2 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       1.00e-10 [erg/cm2/sec]
GRB_SIGNIF:      10.00 [sigma]
IMG_START_DATE:  21010 TJD;   335 DOY;   25/12/01
IMG_START_TIME:  59683.00 SOD {16:34:43.00} UT,   492.7 [sec] since BAT Trigger Time
TAM[0-3]:        100.00 100.00 100.00 100.00
AMPLIFIER:       1
WAVEFORM:        31
SUN_POSTN:       248.10d {+16h 32m 25s}  -21.91d {-21d 54' 33"}
SUN_DIST:         93.41 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.87d {+01h 23m 30s}  +12.35d {+12d 20' 49"}
MOON_DIST:        57.89 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.17,-72.12 [deg] galactic lon,lat of the burst
ECL_COORDS:      343.99,-42.20 [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:        This position was enhanced using UVOT field astrometry.  
COMMENTS:        The probability that this is a serendipitous source in the  
COMMENTS:        SPER window is 0.58% < P(seren) < 1.1%.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 01 Dec 25 16:38:28 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     1418729,   Seg_Num: 0
POINT_RA:          6.396d {+00h 25m 35s} (J2000)
POINT_DEC:       -44.241d {-44d 14' 25"} (J2000)
POINT_ROLL:      234.883d
IMG_START_DATE:  21010 TJD;   335 DOY;   25/12/01
IMG_START_TIME:  59504.63 SOD {16:31:44.63} UT,   314.3 [sec] since BAT Trigger Time
FILTER:          7, U
BKG_MEAN:        0.523
N_STARS:         15
X_OFFSET:        484 [pixels]
Y_OFFSET:        625 [pixels]
X_MAX:           1443 [pixels]
Y_MAX:           1584 [pixels]
DET_THRESH:      7
PHOTO_THRESH:    3
SL_URL:          sw01418729000msufc0314.fits
SUN_POSTN:       248.09d {+16h 32m 21s}  -21.91d {-21d 54' 25"}
SUN_DIST:         93.42 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.67d {+01h 22m 40s}  +12.25d {+12d 14' 58"}
MOON_DIST:        57.76 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.16,-72.12 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      343.99,-42.21 [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:     Mon 01 Dec 25 16:32:19 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     1418729,   Seg_Num: 0
POINT_RA:          6.396d {+00h 25m 35s} (J2000)
POINT_DEC:       -44.242d {-44d 14' 30"} (J2000)
ROLL:            234.883d
IMG_START_DATE:  21010 TJD;   335 DOY;   25/12/01
IMG_START_TIME:  59291.62 SOD {16:28:11.62} UT,   101.3 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     786299328
X_OFFSET:        804 [pixels]
Y_OFFSET:        945 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       964
Y_GRB_POS:       1105
BINNING_INDEX:   1
IM_URL:          sw01418729000msuni0138.fits
SUN_POSTN:       248.08d {+16h 32m 19s}  -21.91d {-21d 54' 23"}
SUN_DIST:         93.42 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.61d {+01h 22m 26s}  +12.22d {+12d 13' 21"}
MOON_DIST:        57.72 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.16,-72.11 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      343.99,-42.21 [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:     Mon 01 Dec 25 16:31:05 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     1418729,   Seg_Num: 0
POINT_RA:          6.396d {+00h 25m 35s} (J2000)
POINT_DEC:       -44.242d {-44d 14' 30"} (J2000)
POINT_ROLL:      234.883d
IMG_START_DATE:  21010 TJD;   335 DOY;   25/12/01
IMG_START_TIME:  59291.62 SOD {16:28:11.62} UT,   101.3 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        1.866
N_STARS:         25
X_OFFSET:        408 [pixels]
Y_OFFSET:        576 [pixels]
X_MAX:           1367 [pixels]
Y_MAX:           1535 [pixels]
DET_THRESH:      11
PHOTO_THRESH:    6
SL_URL:          sw01418729000msufc0101.fits
SUN_POSTN:       248.08d {+16h 32m 19s}  -21.91d {-21d 54' 22"}
SUN_DIST:         93.42 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.60d {+01h 22m 23s}  +12.22d {+12d 13' 02"}
MOON_DIST:        57.72 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.16,-72.11 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      343.99,-42.21 [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:     Mon 01 Dec 25 16:32:33 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     1418729,   Seg_Num: 0
POINT_RA:          6.396d {+00h 25m 35s} (J2000)
POINT_DEC:       -44.242d {-44d 14' 30"} (J2000)
ROLL:            234.883d
IMG_START_DATE:  21010 TJD;   335 DOY;   25/12/01
IMG_START_TIME:  59291.62 SOD {16:28:11.62} UT,   101.3 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     786299328
X_OFFSET:        804 [pixels]
Y_OFFSET:        945 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       964
Y_GRB_POS:       1105
BINNING_INDEX:   1
IM_URL:          sw01418729000msuni0138.fits
SUN_POSTN:       248.08d {+16h 32m 20s}  -21.91d {-21d 54' 23"}
SUN_DIST:         93.42 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.61d {+01h 22m 27s}  +12.22d {+12d 13' 25"}
MOON_DIST:        57.73 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.16,-72.11 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      343.99,-42.21 [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:     Mon 01 Dec 25 16:33:16 UT
NOTICE_TYPE:     Swift-UVOT Position
TRIGGER_NUM:     1418729,   Seg_Num: 0
GRB_RA:            6.3941d {+00h 25m 34.58s} (J2000),
                   6.7102d {+00h 26m 50.44s} (current),
                   5.7825d {+00h 23m 07.79s} (1950)
GRB_DEC:         -44.2336d {-44d 14' 00.9"} (J2000),
                 -44.0903d {-44d 05' 24.9"} (current),
                 -44.5104d {-44d 30' 37.5"} (1950)
GRB_ERROR:       0.8 [arcsec radius, statistical only]
GRB_MAG:         17.28 +/- 0.14 [mag]
FILTER:          10, White
IMG_START_DATE:  21010 TJD;   335 DOY;   25/12/01
IMG_START_TIME:  59288.00 SOD {16:28:08.00} UT,   97.7 [sec] since BAT Trigger Time
SUN_POSTN:       248.08d {+16h 32m 20s}  -21.91d {-21d 54' 23"}
SUN_DIST:         93.42 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.62d {+01h 22m 28s}  +12.23d {+12d 13' 36"}
MOON_DIST:        57.72 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.17,-72.12 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      343.99,-42.20 [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 9.4 arcsec from the XRT position.  
COMMENTS:        Result based on Genie data.  
COMMENTS:        Notice generated automatically.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 01 Dec 25 16:30:57 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     1418729,   Seg_Num: 0
POINT_RA:          6.396d {+00h 25m 35s} (J2000)
POINT_DEC:       -44.242d {-44d 14' 30"} (J2000)
POINT_ROLL:      234.883d
IMG_START_DATE:  21010 TJD;   335 DOY;   25/12/01
IMG_START_TIME:  59291.62 SOD {16:28:11.62} UT,   101.3 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        1.866
N_STARS:         25
X_OFFSET:        408 [pixels]
Y_OFFSET:        576 [pixels]
X_MAX:           1367 [pixels]
Y_MAX:           1535 [pixels]
DET_THRESH:      11
PHOTO_THRESH:    6
SL_URL:          sw01418729000msufc0101.fits
SUN_POSTN:       248.08d {+16h 32m 19s}  -21.91d {-21d 54' 22"}
SUN_DIST:         93.42 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:       20.60d {+01h 22m 23s}  +12.22d {+12d 12' 60"}
MOON_DIST:        57.72 [deg]
MOON_ILLUM:      85 [%]
GAL_COORDS:      318.16,-72.11 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      343.99,-42.21 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN Circular #42927
J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester) report on behalf of the Swift-XRT team.
Using 1078 s of XRT Photon Counting mode data and 3 UVOT images for GRB 251201B, 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 = 6.39463, -44.23233 which is equivalent to:
```
RA (J2000): 00h 25m 34.71s
Dec (J2000): -44d 13' 56.4"
```
with an uncertainty of 3.8 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 #42928
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-SAAO robotic telescope [1] located in South Africa (South African Astronomical Observatory) was pointed to the Swift GRB 251201B ( M. J. Moss et al., GCN 42924) errorbox 20045 sec after notice time and 20067 sec after trigger time at 2025-12-01 22:00:57 UT, with upper limit up to 17.9 mag. The observations began at zenith distance = 45 deg. The sun altitude is -35.7 deg.
The galactic latitude b = -73 deg., longitude l = 317 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3057197
We obtain a following upper limits.
```
Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

   20158 |         MASTER-SAAO |   C |   180 | 17.9 |        
```
Filter C is a clear (unfiltred) band.
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 Circular #42929
A. Cerón, J. Basurto Merino, P.G. Berdayes, A. Caballero-Almagro, 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), M. Abdul-Masih (IAC and ULL), and I. Pérez-Fournon (IAC and ULL).
Following the detection of GRB 251201B by Fermi (Fermi GBM team, GCN Circ. 42923) and Swift (Moss et al., GCN Circ. 42924; Osborne et al., GCN Circ. 42927), we observed the field with one of the three Las Cumbres Observatory (LCO) 1-m telescopes equipped with Sinistro cameras located at the LCO node at Sutherland Observatory, South Africa. The observation, a single exposure of 300 sec in the SDSS r' filter, started on 2025-12-01 at 22:00:22 UT, about 5.73 hours after the Fermi and Swift trigger. The optical counterpart detected by Swift UVOT (Moss et al., GCN Circ. 42924) is clearly detected in our image with a magnitude of r' = 19.78 +/- 0.11 (AB), calibrated against the Legacy Imaging Surveys DR10 catalog and not corrected for Galactic extinction.
This work makes use of observations from the Las Cumbres Observatory global telescope network (LCO program IAC2025B-010). These observations are part of a course in Astrophysical Techniques of the Master in Astrophysics of the Astrophysics Department of the University of La Laguna in collaboration with the Instituto de Astrofísica de Canarias (La Laguna, Tenerife, Canary Islands, Spain).
This work made use of the Astro-COLIBRI platform (P. Reichherzer et al. 2021, ApJS, 256, 5).
GCN Circular #42930
D.Buckley (SAAO), V.M. Lipunov (Lomonosov MSU), A.Kuznetsov, I.Panchenko, G.Antipov, P.Balanutsa, K.Zhirkov, V.Senik, N. Tiurina, Ya.Kechin, A.Chasovnikov, D.Vlasenko, E.Gorbovskoy, 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 Juan Uni.,Argentina); 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-SAAO robotic telescope of MASTER Global Robotic Net (Lipunov et al. [1-4], http://observ.pereplet.ru ), started Swift GRB 251201B (Moss et al GCN 42924, Osborne et al. GCN 42927) inspect at 2025-12-01 22:00:57 UT (20067s after trigger time), Lipunov et al. GCN 42929
There is optical counterpart MASTER OT J002534.28-441352.2 at Swift UVOT position (Moss et al. GCN 42924) with unfiltered m_OT~20.4m at summary image at 22:00:57 UT (420s exposition, mlim=21m).
Real time updated cover map and OT position are available here: https://master.sai.msu.ru/site/master2/observ.php?id=3054567 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 #42931
H.L. Li, L.P. Xin, Y. L. Qiu, C. Wu, Z. H. Yao, Y. N. Ma, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. R. Xu, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team.
SVOM/VT performed ToO observation to the field of GRB 251201B (Fermi GBM team, GCN 42923; Moss et al., GCN 42924; Osborne et al., GCN 42927; Goyal et al., GCN 42925). The observation stared at 2025-12-01T17:48:58 UTC, I.e., 1.4 minutes post trigger in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
The optical counterpart (Cerón et al., GCN 42929;Buckley et al., GCN 42930) was detected Within the error box of Swift-UVOT (Moss et al., GCN 42924) and Swift-XRT (Osborne et al., GCN 42927). The position is at R.A., Dec. = 6.39414, -44.23374 degrees, equivalent to:
```
R.A. (J2000) =    0:25:34.59
Dec. (J2000) =  -44:14:01.46
```
with an uncertainty of 0.5 arcsec.
The measurements in AB magnitude without correction for Galactic extinction are given below:
```
Mid_time        Band        Exposure Time       Magnitude (AB)
1.40 hours      VT_B            70 sec         18.56+/-0.03 mag                      
1.40 hours      VT_R            70 sec         18.08+/-0.03 mag 
3.04 hours      VT_B            70 sec         19.46+/-0.04 mag                      
3.04 hours      VT_R            70 sec         18.97+/-0.04 mag 
```
It was fading with a slope of about -1.0 with a single power law.
The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.
GCN Circular #42936
V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti (INAF-OAB), D.N. Burrows (PSU), S. Lanava (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), C. Salvaggio (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 251201B, from 86 s to 23.3 ks after the trigger. The data comprise 362 s in Windowed Timing (WT) mode (the first 6 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.
The late-time light curve (from T0+6.3 ks) can be modelled with a power-law decay with a decay index of alpha=1.6 (+/-0.3).
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.40 (+/-0.04). The best-fitting absorption column is 2.30 (+/-0.12) x 10^21 cm^-2, in excess of the Galactic value of 1.8 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.93 (+0.16, -0.15) and a best-fitting absorption column of 10.0 (+4.5, -4.0) x 10^20 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10^-11 (4.3 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
```
Total column:	     10.0 (+4.5, -4.0) x 10^20 cm^-2
Galactic foreground: 1.8 x 10^20 cm^-2
Excess significance: 3.4 sigma
Photon index:	     1.93 (+0.16, -0.15)
```
If the light curve continues to decay with a power-law decay index of 1.6, the count rate at T+24 hours will be 3.5 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x 10^-13 (1.5 x 10^-13) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at https://www.swift.ac.uk/xrt_products/01418729.
This circular is an official product of the Swift-XRT team.
GCN Circular #42952
Yue Wang, Chen-Wei Wang, Hao-Xuan Guo, Shao-Lin Xiong (IHEP) report on behalf of GECAM team:
GECAM-B was triggered on-ground by GRB 251201B, at 2025-12-01T16:26:32.950 UTC (denoted as T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN #42923).
According to the GECAM-B light curves in about 70-6000 keV, this burst mainly consists of a long pulse with a duration (T90) of 65 +6/-5 s.
The GECAM-B light curve can be found here: https://www.bursthub.cn//admin/static/gecambgrb251201B.png
Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS).
GCN Circular #42954
N.P.M. Kuin (UCL-MSSL) and M. J. Moss (GSFC) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 251201B 101 s after the BAT trigger (Moss et al., GCN Circ. 42924). A source consistent with the XRT position is detected in the initial UVOT exposures.
Preliminary detections and 3-sigma upper limits 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)      Exp(s)           Mag

white              101          251          147         17.31 +/- 0.03
v                  644          663           19         15.97 +/- 0.12
b                  570          589           20         16.71 +/- 0.10
u                  314          564          246         16.48 +/- 0.04
w1                 693         1808          117         18.35 +/- 0.18
m2                 668         1093           39        >18.7
w2                1196         1562           39        >18.8
```
The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.008 in the direction of the burst (Schlegel et al. 1998).
GCN Circular #42955
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 251201B detected by Swift/BAT (Moss et al., GCN 42924) and Fermi/GBM (Fermi GBM team, GCN 42923) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with the iTelescope 72 located in Chile and operated by M. Freeberg. Our observations started at TGRB+8.4h and were taken with Rc filter.
In our stacked frames, subtracted from the Legacy Survey DR10 template image, we detect the optical counterpart reported by Swift/UVOT(Moss et al., GCN 42924, Kuin et al., GCN 42954), LCO/1m (Cerón et al., GCN 42929), MASTER (Buckley et al., GCN 42930), SVOM/VT (Li et al., GCN 42931).
We report our follow-up results in the table below:
```
+---------------+-----------+-----------+----------------+-------------+
| Tmid-TGRB (hr)| Exp (s)   | Filter    | Magnitude      | Instrument  |
+===============+===========+===========+================+=============+
| 8.89          | 17 x 180s | Rc (Vega) | 20.33 +/- 0.09 | iT72        | 
+---------------+-----------+-----------+----------------+-------------+
```
All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the Johnson-Cousins filters were calibrated using the GAIA DR3 Synphot 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/).
GCN Circular #42965
Eva Palafox (INAOE) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 16:26:24.22 UT on 01 December 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251201B (trigger 786299189/251201685), which was also detected by Swift BAT (M. J. Moss et al. 2025, GCN 42924), and Swift XRT (J.P. Osborne et al. 2025, GCN 42927). The Fermi GBM on-ground location (GCN 42923) is consistent with the Swift BAT position.
The angle from the Fermi LAT boresight is 119 degrees.
The GBM light curve consists of a single emission episode with a duration (T90) of about 91 s (50-300 keV). The time-averaged spectrum from T0-22.5 to T0+105.5 s is best fit by a simple power law function with index -1.83 +/- 0.04. A power law function with an exponential high-energy cutoff fits equally well with power law index = -1.71 +/- 0.08 and Epeak = 269.8 +/- 166.0.
The event fluence (10-1000 keV) in this time interval is (1.2 +/- 0.1)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+4.7 s in the 10-1000 keV band is 4 +/- 0.3 ph/s/cm^2.
The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog: https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html
For Fermi GBM data and info, please visit the official Fermi GBM Support Page: https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"
GCN Circular #42971
Yue Wang, Chen-Wei Wang, Chao Zheng, and Shao-Lin Xiong (IHEP) report on behalf of the Insight-HXMT team:
At 2025-12-01T16:26:24.000 (T0), Insight-HXMT/HE detected a long-duration GRB 251201B, which is also detected by Fermi/GBM (Fermi GBM team, GCN #42923) and GECAM-B (Yue Wang et. al., GCN #42952).
The Insight-HXMT/HE light curve mainly consists of a long pulse with a T90 of 42 +11/-8 s. The 1s peak rate, measured from T0+6.5 s, is 494 cnts/sec. Insight-HXMT/HE detected a total of 8654 counts from this burst.
The Insight-HXMT /HE light curve can be found here: https://www.bursthub.cn//admin/static/hxmtgrb251201B.png
All measurements above are made with the CsI detectors of Insight-HXMT/HE operating in the regular mode with the energy range of about 60-900 keV (deposited energy). Only gamma-rays with energy greater than about 200 keV can penetrate the spacecraft and leave signals in the CsI detectors installed inside of the telescope.
Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://www.hxmt.org
GCN Circular #42994
D. B. Malesani (DAWN/NBI and Radboud), A. J. Levan (Radboud and Warwick), S. de Wet (DTU Space), A. de Ugarte Postigo (LAM), D. A. H. Buckley (SAAO), A. Martin-Carrillo (UCD), P. M. Vreeswijk (Radboud) report on behalf of a larger collaboration:
We observed the optical afterglow of the Swift and Fermi GRB 251201B (Moss et al., GCN 42924; Palafox & Meegan, GCN 42965), using the Robert Stobie Spectrograph (RSS) installed on the 9.2 m Southern African Large Telescope (SALT) at the Sutherland observatory (South Africa).
In a 10-s acquisition image, taken with the SALTICAM instrument on 2025 Dec 1.871 UT (4.47 hr after trigger), we measure a magnitude i = 19.25 +- 0.09 AB, calibrated against nearby stars from the Pan-STARRS catalog, and not corrected for Galactic extinction.
A 2400-s spectrum was acquired using the PG0700 grating, covering the wavelength range 3600-7500 AA. Faint continuum is detected across the entire spectrum, setting a secure upper limit to the redshift z < 2. Towards the red end, we identify a few weak absorption features, which we match to Mg I 2852, Mg II 2804, 2796, Fe II 2600, 2587, 2383, all at a common redshift of z = 1.599, which we suggest to be the redshift of GRB 251201B.
We acknowledge excellent support from the SALT observing staff, in particular Alexei Kniazev and Veronica Van Dyk.

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Jochen Greiner, last update: 05-Dec-2025 [Disclaimer]