Gamma-ray Burst 251001B

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN/BACODINE POSITION NOTICE

    
TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 01 Oct 25 14:31:20 UT
NOTICE_TYPE:     Swift-BAT GRB Position
TRIGGER_NUM:     1400467,   Seg_Num: 0
GRB_RA:           42.665d {+02h 50m 40s} (J2000),
                  42.953d {+02h 51m 49s} (current),
                  42.104d {+02h 48m 25s} (1950)
GRB_DEC:         -23.020d {-23d 01' 09"} (J2000),
                 -22.914d {-22d 54' 51"} (current),
                 -23.225d {-23d 13' 30"} (1950)
GRB_ERROR:       3.00 [arcmin radius, statistical only]
GRB_INTEN:       3145 [cnts]    Image_Peak=178 [image_cnts]
TRIGGER_DUR:     1.024 [sec] 
TRIGGER_INDEX:   146     E_range: 25-100 keV
BKG_INTEN:       18354 [cnts]
BKG_TIME:        51427.57 SOD {14:17:07.57} UT
BKG_DUR:         8 [sec]
GRB_DATE:        20949 TJD;   274 DOY;   25/10/01
GRB_TIME:        51440.56 SOD {14:17:20.56} UT
GRB_PHI:         168.55 [deg]
GRB_THETA:        22.43 [deg]
SOLN_STATUS:     0x20000003
RATE_SIGNIF:     15.39 [sigma]
IMAGE_SIGNIF:    7.73 [sigma]
MERIT_PARAMS:     +1  +0  +0  +0  +2  -2  +0  +0 +73  +0 
SUN_POSTN:       187.96d {+12h 31m 51s}   -3.44d {-03d 26' 08"}
SUN_DIST:        136.88 [deg]   Sun_angle= 9.7 [hr] (West of Sun)
MOON_POSTN:      300.01d {+20h 00m 03s}  -24.74d {-24d 44' 10"}
MOON_DIST:        91.40 [deg]
MOON_ILLUM:      66 [%]
GAL_COORDS:      210.91,-62.76 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       31.62,-37.36 [deg] ecliptic lon,lat of the burst (or transient)
COMMENTS:        SWIFT-BAT GRB Coordinates.  
COMMENTS:        This Notice was delayed by more than 60 sec past the end of the trigger integration interval;  
COMMENTS:        probably due to it occurring during a Malindi downlink session.  
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 = 172.68,16.75 [deg].
COMMENTS:           
COMMENTS:        NOTE: This BAT event is temporally(1.0<100sec) coincident with the FERMI_GBM event (trignum=781021044).

red DSS finding chart ps-file

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 01 Oct 25 14:31:54 UT
NOTICE_TYPE:     Swift-XRT Nack-Position
TRIGGER_NUM:     1400467,   Seg_Num: 0
POINT_RA:         42.664d {+02h 50m 39s} (J2000)
POINT_DEC:       -23.015d {-23d 00' 54"} (J2000)
IMG_START_DATE:  20949 TJD;   274 DOY;   25/10/01
IMG_START_TIME:  51557.09 SOD {14:19:17.09} UT,   116.5 [sec] since BAT Trigger 
Time
COUNTS:          36    Min_needed= 20
STD_DEV:         0.00   Max_StdDev_for_Good=28.44 [arcsec]
PH2_ITER:        4    Max_iter_allowed= 4
ERROR_CODE:      2
COMMENTS:        SWIFT-XRT Nack Position.  
COMMENTS:        Algorithm did not converge; too many iterations.

GCN Circular #42038
The Fermi GBM team reports the detection of a likely LONG GRB
At 14:17:19 UT on 1 Oct 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251001B (trigger 781021044.051374 / 251001595).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 52.0, Dec = -11.2 (J2000 degrees, equivalent to J2000 03h 28m, -11d 11'), with a statistical uncertainty of 5.6 degrees.
The angle from the Fermi LAT boresight is 67.0 degrees.
The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251001595/quicklook/glg_skymap_all_bn251001595.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/bn251001595/quicklook/glg_healpix_all_bn251001595.fit
The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251001595/quicklook/glg_lc_medres34_bn251001595.gif

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 01 Oct 25 14:33:14 UT
NOTICE_TYPE:     Swift-BAT GRB Lightcurve
TRIGGER_NUM:     1400467,   Seg_Num: 0
GRB_RA:           42.665d {+02h 50m 40s} (J2000),
                  42.953d {+02h 51m 49s} (current),
                  42.104d {+02h 48m 25s} (1950)
GRB_DEC:         -23.020d {-23d 01' 09"} (J2000),
                 -22.914d {-22d 54' 51"} (current),
                 -23.225d {-23d 13' 30"} (1950)
GRB_DATE:        20949 TJD;   274 DOY;   25/10/01
GRB_TIME:        51440.56 SOD {14:17:20.56} UT
TRIGGER_INDEX:   146
GRB_PHI:         168.55 [deg]
GRB_THETA:        22.43 [deg]
DELTA_TIME:      -18.00 [sec]
TRIGGER_DUR:     1.024 [sec] 
SOLN_STATUS:     0x3
RATE_SIGNIF:     15.39 [sigma]
IMAGE_SIGNIF:    7.73 [sigma]
LC_URL:          sw01400467000msb.lc
SUN_POSTN:       187.96d {+12h 31m 51s}   -3.44d {-03d 26' 10"}
SUN_DIST:        136.88 [deg]   Sun_angle= 9.7 [hr] (West of Sun)
MOON_POSTN:      300.03d {+20h 00m 07s}  -24.73d {-24d 43' 55"}
MOON_DIST:        91.39 [deg]
MOON_ILLUM:      66 [%]
GAL_COORDS:      210.91,-62.76 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       31.62,-37.36 [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 = 172.68,16.75 [deg].

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 01 Oct 25 14:33:49 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     1400467,   Seg_Num: 0
POINT_RA:         42.664d {+02h 50m 39s} (J2000)
POINT_DEC:       -23.015d {-23d 00' 53"} (J2000)
POINT_ROLL:      123.483d
IMG_START_DATE:  20949 TJD;   274 DOY;   25/10/01
IMG_START_TIME:  51559.35 SOD {14:19:19.35} UT,   118.8 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        2.072
N_STARS:         185
X_OFFSET:        504 [pixels]
Y_OFFSET:        557 [pixels]
X_MAX:           1463 [pixels]
Y_MAX:           1516 [pixels]
DET_THRESH:      12
PHOTO_THRESH:    6
SL_URL:          sw01400467000msufc0118.fits
SUN_POSTN:       187.96d {+12h 31m 51s}   -3.44d {-03d 26' 10"}
SUN_DIST:        136.88 [deg]   Sun_angle= 9.7 [hr] (West of Sun)
MOON_POSTN:      300.04d {+20h 00m 09s}  -24.73d {-24d 43' 50"}
MOON_DIST:        91.38 [deg]
MOON_ILLUM:      66 [%]
GAL_COORDS:      210.90,-62.76 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       31.62,-37.36 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 01 Oct 25 14:34:27 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     1400467,   Seg_Num: 0
POINT_RA:         42.664d {+02h 50m 39s} (J2000)
POINT_DEC:       -23.015d {-23d 00' 53"} (J2000)
POINT_ROLL:      123.483d
IMG_START_DATE:  20949 TJD;   274 DOY;   25/10/01
IMG_START_TIME:  51559.35 SOD {14:19:19.35} UT,   118.8 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        2.072
N_STARS:         185
X_OFFSET:        504 [pixels]
Y_OFFSET:        557 [pixels]
X_MAX:           1463 [pixels]
Y_MAX:           1516 [pixels]
DET_THRESH:      12
PHOTO_THRESH:    6
SL_URL:          sw01400467000msufc0118.fits
SUN_POSTN:       187.96d {+12h 31m 51s}   -3.44d {-03d 26' 11"}
SUN_DIST:        136.88 [deg]   Sun_angle= 9.7 [hr] (West of Sun)
MOON_POSTN:      300.04d {+20h 00m 10s}  -24.73d {-24d 43' 45"}
MOON_DIST:        91.38 [deg]
MOON_ILLUM:      66 [%]
GAL_COORDS:      210.90,-62.76 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       31.62,-37.36 [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:     Wed 01 Oct 25 14:35:28 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     1400467,   Seg_Num: 0
POINT_RA:         42.664d {+02h 50m 39s} (J2000)
POINT_DEC:       -23.015d {-23d 00' 53"} (J2000)
ROLL:            123.483d
IMG_START_DATE:  20949 TJD;   274 DOY;   25/10/01
IMG_START_TIME:  51559.35 SOD {14:19:19.35} UT,   118.8 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     781021195
X_OFFSET:        823 [pixels]
Y_OFFSET:        876 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       983
Y_GRB_POS:       1036
BINNING_INDEX:   1
IM_URL:          sw01400467000msuni0155.fits
SUN_POSTN:       187.97d {+12h 31m 52s}   -3.44d {-03d 26' 12"}
SUN_DIST:        136.88 [deg]   Sun_angle= 9.7 [hr] (West of Sun)
MOON_POSTN:      300.05d {+20h 00m 12s}  -24.73d {-24d 43' 36"}
MOON_DIST:        91.37 [deg]
MOON_ILLUM:      66 [%]
GAL_COORDS:      210.90,-62.76 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       31.62,-37.36 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Image.  
COMMENTS:        The GRB Position came from the Window Position in the Mode Command.  
COMMENTS:        The image has 2x2 binning (compression).

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 01 Oct 25 14:35:38 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     1400467,   Seg_Num: 0
POINT_RA:         42.664d {+02h 50m 39s} (J2000)
POINT_DEC:       -23.015d {-23d 00' 53"} (J2000)
ROLL:            123.483d
IMG_START_DATE:  20949 TJD;   274 DOY;   25/10/01
IMG_START_TIME:  51559.35 SOD {14:19:19.35} UT,   118.8 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     781021195
X_OFFSET:        823 [pixels]
Y_OFFSET:        876 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       983
Y_GRB_POS:       1036
BINNING_INDEX:   1
IM_URL:          sw01400467000msuni0155.fits
SUN_POSTN:       187.97d {+12h 31m 52s}   -3.44d {-03d 26' 12"}
SUN_DIST:        136.88 [deg]   Sun_angle= 9.7 [hr] (West of Sun)
MOON_POSTN:      300.05d {+20h 00m 13s}  -24.73d {-24d 43' 35"}
MOON_DIST:        91.37 [deg]
MOON_ILLUM:      66 [%]
GAL_COORDS:      210.90,-62.76 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       31.62,-37.36 [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:     Wed 01 Oct 25 14:37:08 UT
NOTICE_TYPE:     Swift-XRT Position
TRIGGER_NUM:     1400467,   Seg_Num: 0
GRB_RA:           42.6510d {+02h 50m 36.24s} (J2000),
                  42.9397d {+02h 51m 45.52s} (current),
                  42.0906d {+02h 48m 21.74s} (1950)
GRB_DEC:         -23.0190d {-23d 01' 08.3"} (J2000),
                 -22.9138d {-22d 54' 49.7"} (current),
                 -23.2247d {-23d 13' 28.8"} (1950)
GRB_ERROR:       3.5 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       1.00e-10 [erg/cm2/sec]
GRB_SIGNIF:      10.00 [sigma]
IMG_START_DATE:  20949 TJD;   274 DOY;   25/10/01
IMG_START_TIME:  51570.00 SOD {14:19:30.00} UT,   129.4 [sec] since BAT Trigger Time
TAM[0-3]:        100.00 100.00 100.00 100.00
AMPLIFIER:       1
WAVEFORM:        31
SUN_POSTN:       187.97d {+12h 31m 52s}   -3.44d {-03d 26' 13"}
SUN_DIST:        136.89 [deg]   Sun_angle= 9.7 [hr] (West of Sun)
MOON_POSTN:      300.07d {+20h 00m 16s}  -24.72d {-24d 43' 23"}
MOON_DIST:        91.35 [deg]
MOON_ILLUM:      66 [%]
GAL_COORDS:      210.90,-62.77 [deg] galactic lon,lat of the burst
ECL_COORDS:       31.61,-37.36 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Coordinates.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
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 http://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:     Wed 01 Oct 25 14:41:20 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     1400467,   Seg_Num: 0
POINT_RA:         42.663d {+02h 50m 39s} (J2000)
POINT_DEC:       -23.016d {-23d 00' 57"} (J2000)
POINT_ROLL:      123.484d
IMG_START_DATE:  20949 TJD;   274 DOY;   25/10/01
IMG_START_TIME:  52301.09 SOD {14:31:41.09} UT,   860.5 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        4.392
N_STARS:         169
X_OFFSET:        264 [pixels]
Y_OFFSET:        317 [pixels]
X_MAX:           1703 [pixels]
Y_MAX:           1756 [pixels]
DET_THRESH:      17
PHOTO_THRESH:    10
SL_URL:          sw01400467000msufc0860.fits
SUN_POSTN:       187.97d {+12h 31m 52s}   -3.44d {-03d 26' 17"}
SUN_DIST:        136.88 [deg]   Sun_angle= 9.7 [hr] (West of Sun)
MOON_POSTN:      300.11d {+20h 00m 26s}  -24.71d {-24d 42' 49"}
MOON_DIST:        91.33 [deg]
MOON_ILLUM:      66 [%]
GAL_COORDS:      210.90,-62.76 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       31.62,-37.36 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Source List.  
COMMENTS:        All 4 attachments are included.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 01 Oct 25 14:40:52 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     1400467,   Seg_Num: 0
POINT_RA:         42.663d {+02h 50m 39s} (J2000)
POINT_DEC:       -23.016d {-23d 00' 57"} (J2000)
POINT_ROLL:      123.484d
IMG_START_DATE:  20949 TJD;   274 DOY;   25/10/01
IMG_START_TIME:  52301.09 SOD {14:31:41.09} UT,   860.5 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        4.392
N_STARS:         169
X_OFFSET:        264 [pixels]
Y_OFFSET:        317 [pixels]
X_MAX:           1703 [pixels]
Y_MAX:           1756 [pixels]
DET_THRESH:      17
PHOTO_THRESH:    10
SL_URL:          sw01400467000msufc0860.fits
SUN_POSTN:       187.97d {+12h 31m 52s}   -3.44d {-03d 26' 17"}
SUN_DIST:        136.88 [deg]   Sun_angle= 9.7 [hr] (West of Sun)
MOON_POSTN:      300.10d {+20h 00m 25s}  -24.71d {-24d 42' 52"}
MOON_DIST:        91.34 [deg]
MOON_ILLUM:      66 [%]
GAL_COORDS:      210.90,-62.76 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       31.62,-37.36 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 01 Oct 25 14:42:40 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     1400467,   Seg_Num: 0
POINT_RA:         42.663d {+02h 50m 39s} (J2000)
POINT_DEC:       -23.016d {-23d 00' 57"} (J2000)
ROLL:            123.484d
IMG_START_DATE:  20949 TJD;   274 DOY;   25/10/01
IMG_START_TIME:  52301.09 SOD {14:31:41.09} UT,   860.5 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     781021937
X_OFFSET:        823 [pixels]
Y_OFFSET:        876 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       983
Y_GRB_POS:       1036
BINNING_INDEX:   1
IM_URL:          sw01400467000msuni0897.fits
SUN_POSTN:       187.97d {+12h 31m 53s}   -3.44d {-03d 26' 19"}
SUN_DIST:        136.88 [deg]   Sun_angle= 9.7 [hr] (West of Sun)
MOON_POSTN:      300.12d {+20h 00m 29s}  -24.71d {-24d 42' 38"}
MOON_DIST:        91.32 [deg]
MOON_ILLUM:      66 [%]
GAL_COORDS:      210.90,-62.76 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       31.62,-37.36 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Image.  
COMMENTS:        The GRB Position came from the Window Position in the Mode Command.  
COMMENTS:        The image has 2x2 binning (compression).

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 01 Oct 25 14:41:33 UT
NOTICE_TYPE:     Swift-XRT Position UPDATE
TRIGGER_NUM:     1400467,   Seg_Num: 0
GRB_RA:           42.6507d {+02h 50m 36.16s} (J2000),
                  42.9394d {+02h 51m 45.44s} (current),
                  42.0903d {+02h 48m 21.67s} (1950)
GRB_DEC:         -23.0200d {-23d 01' 11.9"} (J2000),
                 -22.9148d {-22d 54' 53.3"} (current),
                 -23.2257d {-23d 13' 32.4"} (1950)
GRB_ERROR:       1.9 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       1.00e-10 [erg/cm2/sec]
GRB_SIGNIF:      10.00 [sigma]
IMG_START_DATE:  20949 TJD;   274 DOY;   25/10/01
IMG_START_TIME:  51570.00 SOD {14:19:30.00} UT,   129.4 [sec] since BAT Trigger Time
TAM[0-3]:        100.00 100.00 100.00 100.00
AMPLIFIER:       1
WAVEFORM:        31
SUN_POSTN:       187.97d {+12h 31m 53s}   -3.44d {-03d 26' 18"}
SUN_DIST:        136.89 [deg]   Sun_angle= 9.7 [hr] (West of Sun)
MOON_POSTN:      300.11d {+20h 00m 26s}  -24.71d {-24d 42' 47"}
MOON_DIST:        91.32 [deg]
MOON_ILLUM:      66 [%]
GAL_COORDS:      210.91,-62.77 [deg] galactic lon,lat of the burst
ECL_COORDS:       31.61,-37.36 [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 http://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:     Wed 01 Oct 25 14:43:03 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     1400467,   Seg_Num: 0
POINT_RA:         42.663d {+02h 50m 39s} (J2000)
POINT_DEC:       -23.016d {-23d 00' 57"} (J2000)
ROLL:            123.484d
IMG_START_DATE:  20949 TJD;   274 DOY;   25/10/01
IMG_START_TIME:  52301.09 SOD {14:31:41.09} UT,   860.5 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     781021937
X_OFFSET:        823 [pixels]
Y_OFFSET:        876 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       983
Y_GRB_POS:       1036
BINNING_INDEX:   1
IM_URL:          sw01400467000msuni0897.fits
SUN_POSTN:       187.97d {+12h 31m 53s}   -3.44d {-03d 26' 19"}
SUN_DIST:        136.88 [deg]   Sun_angle= 9.7 [hr] (West of Sun)
MOON_POSTN:      300.12d {+20h 00m 30s}  -24.71d {-24d 42' 35"}
MOON_DIST:        91.32 [deg]
MOON_ILLUM:      66 [%]
GAL_COORDS:      210.90,-62.76 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       31.62,-37.36 [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 Circular #42039
A. P. Beardmore (U Leicester), J. A. Kennea (PSU), M. J. Moss (GSFC) and K. L. Page (U Leicester) report on behalf of the Neil Gehrels Swift Observatory Team:
At 14:17:20 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 251001B (trigger=1400467). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 42.665, -23.020 which is
```
   RA(J2000) = 02h 50m 40s
   Dec(J2000) = -23d 01' 09"
```
with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single-peaked structure with a duration of about 25 sec. The peak count rate was ~2,000 counts/sec (15-350 keV), at ~4 sec after the trigger.
The XRT began observing the field at 14:19:17.0 UT, 116.5 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 42.65098, -23.02014 which is equivalent to:
```
   RA(J2000)  = 02h 50m 36.24s
   Dec(J2000) = -23d 01' 12.5"
```
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This location is 46 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at https://www.swift.ac.uk/sper.
A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 2.65 x 10^20 cm^-2 (Willingale et al. 2013).
UVOT took a finding chart exposure of nominal 150.000 seconds with the White filter starting 860 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. Data from the 2.7'x2.7' sub-image are not available at this time. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.023.
Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.ac.uk). 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 Circular #42040
B. P. Gompertz, D. O'Neill, R. Starling, M. Kennedy, B. Godson, J. Lyman, K. Ackley, M. J. Dyer, K. Ulaczyk, A. Kumar, G. Ramsay, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares report on behalf of the GOTO collaboration:
We report on observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to GRB 251001B (Fermi GBM team, GCN 42038; Beardmore et al. GCN 42039). Targeted observations were performed at 2025-10-01 14:33:37 (starting 16.3 minutes after trigger). Each observation consisted of 4x90s exposures in the GOTO L-band (400-700 nm).
Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations of the same pointings.
At a position consistent with the Swift/XRT localisation, we detect a new optical source, GOTO25icy, with L = 19.33 ± 0.12 AB mag, at coordinates:
RA,DEC (J2000) = 42.651328, -23.02001 | 02:50:36.32, -23:01:12.03
We find no evidence of this source prior to the GRB trigger time in the previous GOTO observation taken at 2025-10-01 14:01:01 UT (16.3 min pre-trigger) down to a 3-sigma depth of L > 20.1 AB mag. We note the source is coincident (~0.4”) with a faint g~25 AB mag source in Legacy Survey DR10, which may be the host galaxy of the GRB.
Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction.
Observations are ongoing.
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 and the Instituto de Astrofisica de Canarias (IAC).
GCN Circular #42041
J. An, X. Liu (NAOC), K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), S.Q. Jiang, L.B. He, Z.P. Zhu, D. Xu (NAOC), S.Y. Fu (HUST) report on behalf of a large collaboration:
We observed the field of GRB 251001B detected by Swift/BAT (Beardmore et al., GCN 42039), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at New South Wales, Australia (SBO). A series of images were obtained in the R band.
A varying optical transient (OT) is detected within the Swift/XRT error circle at coordinates
```
R.A. (J2000) = 2:50:36.33
Dec.(J2000) = -23:01:11.96
```
with an uncertainty of ~ 0.6 arcsec. It has R ~ 18.9 mag at 21.1 min after the Swift/BAT trigger. We thus think the OT is the optical afterglow of the burst.
We also note that there is a galaxy at the OT position from Legacy Survey DR10 with g ~ 25.16, r ~ 24.93, i ~25.06, z ~24.79, and a phot_z ~ 1.14 +/- 0.53, which is presumably the host galaxy of the burst.
GCN Circular #42043
R. Strausbaugh (Eastern Illinois University), A. Cucchiara (NASA) report on behalf of a larger collaboration:
We observed the Swift GRB 251001B field (Beardmore et al., GCN 42039) with the LCOGT 1-meter Sinistro instrument at the Siding Spring Observatory, Australia site, on October 1, from 14:51 to 15:23 UT (corresponding to 0.57 to 1.10 hours after the GRB trigger time) with the SDSS i and r filters.
We performed a series of 3x300s exposures in i-band and r-band. We detect an uncatalogued source consistent with other optical counterpart observations (Gompertz et al., GCN 42040; An et al., GCN 42041) in both bands. We note that our r-band measurement implies a rapidly fading source.
The following magnitudes are calculated using the PanSTARRS catalog as reference:
```
r = 20.26 +/- 0.04
i = 19.81 +/- 0.03
```
These magnitudes are not corrected for galactic extinction.
GCN Circular #42045
A.P. Beardmore, P.A. Evans, M.R. Goad and J.P. Osborne (U. Leicester) report on behalf of the Swift-XRT team.
Using 700 s of XRT Photon Counting mode data and 2 UVOT images for GRB 251001B, 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 = 42.65089, -23.02048 which is equivalent to:
```
RA (J2000): 02h 50m 36.21s
Dec (J2000): -23d 01' 13.7"
```
with an uncertainty of 2.1 arcsec (radius, 90% confidence).
This position may be improved as more data are received. The latest position can be viewed at http://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 #42046
D. O'Neill, B. P. Gompertz, G. Ramsay, R. Starling, M. Kennedy, B. Godson, J. Lyman, K. Ackley, M. J. Dyer, K. Ulaczyk, A. Kumar,, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares report on behalf of the GOTO collaboration:
We report on new observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to GRB 251001B (Fermi GBM team, GCN 42038; Beardmore et al. GCN 42039). Following the detection of the optical counterpart of GRB 251001B (Gompertz et al. GCN 42040, An et al. GCN 42041, Strausbaugh & Cucchiara, GCN 42043) we detect the counterpart again in an additional epoch of GOTO L-band (400-700 nm) imaging taken at 2025-10-01 15:44:25 UT (1.45h post-trigger) consisting of 4x90s exposures. We find the counterpart has faded to L=20.6±0.3 AB mag, consistent with a power law decay with index: alpha = -0.75.
Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are 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 and the Instituto de Astrofisica de Canarias (IAC).
GCN Circular #42047
Eliza Neights (GWU, NASA GSFC) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 14:17:19.05 UT on 01 October 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251001B (trigger 781021044/251001595) which was also detected by Swift BAT (A. P. Beardmore et al. 2025, GCN 42039). The Fermi GBM on-ground location is consistent with the Swift BAT position.
The angle from the Fermi LAT boresight is 53 degrees.
The GBM light curve consists of a single emission episode with a duration (T90) of about 19 s (50-300 keV). The time-averaged spectrum from T0-3.7 to T0+16.7 s is best fit by a Comptonized function with index 0.22 +/- 0.03 and peak energy 107 +/- 5 keV.
The event fluence (10-1000 keV) in this time interval is (1.3 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+0.7 s in the 10-1000 keV band is 1.4 +/- 0.2 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 #42048
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 (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the Swift GRB 251001B ( A. P. Beardmore et al., GCN 42039) errorbox 34585 sec after notice time and 35427 sec after trigger time at 2025-10-02 00:07:47 UT, with upper limit up to 18.4 mag. The observations began at zenith distance = 67 deg. The sun altitude is -32.7 deg.
The galactic latitude b = -62 deg., longitude l = 211 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3004463
We obtain a following upper limits.
```
Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

   35517 |   MASTER-Kislovodsk |   C |   180 | 18.4 |        
   35517 |   MASTER-Kislovodsk |   C |   180 | 18.2 |        
```
Filter C is a clear (unfiltred) band.
The observation and reduction will continue. The message may be cited.
GCN Circular #42050
A. de Ugarte Postigo (LAM), D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), B. Schneider (LAM), B. P. Gompertz (Birmingham), A. Martin-Carrillo (UCD) and J. Maiz Apellaniz (IAA-CSIC) report:
We observed the field of GRB 251001B (Fermi GBM team, GCN 42038; Beardmore et al. GCN 42039) using the 2.5 m Nordic Optical Telescope (NOT) equipped with ALFOSC. Observations started on 2 October 2025 at 02:37:23 UT (12.33 hr after the trigger) and consisted of 3x300 s and 5x200 s in the SDSS r and z bands, respectively.
The combined images reveal the presence of the afterglow identified by Gompertz et al. (GCN 42040; An et al. GCN 42041; Strausbaugh & Cucchiara GCN 42043; O'Neill et al. GCN 42046) at a magnitude of r = 21.54 +/- 0.06 (AB) calibrated against field stars of the Pan-STARRS catalogue and not corrected by Galactic extinction.
GCN Circular #42051
Y. N. Ma, L. P. Xin, Z. H. Yao, Y. L. Qiu, C. Wu, H. L. Li, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team.
SVOM/VT performed a Target of Opportunity observation of GRB 251001B detected by Fermi/GBM and Swift/BAT (Fermi GBM team, GCN 42038; Beardmore et al., GCN 42039; Beardmore et al., GCN 42045; Neights & Meegan, GCN 42047). SVOM/VT began observing the field at 2025-10-01T15:03:04 UTC, 0.762 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
With X-band data available, the optical counterpart (Gompertz et al., GCN 42040; An et al., GCN 42041; Strausbaugh & Cucchiara, GCN 42043; O'Neill et al., GCN 42046; Lipunov et al., GCN 42048; de Ugarte Postigo et al., GCN 42050) was clearly detected in both VT_B and VT_R bands. The magnitudes are:
```
mid time (h) | exposure time (s) | band | mag (AB) | mag err
-------------|-------------------|------|----------|--------
    3.790    |      38*70        | VT_B |  22.06   |  0.09 
    3.819    |      35*70        | VT_R |  21.31   |  0.07 
```
Our photometry was not corrected for Galactic extinction.
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 #42052
S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB) and P.A. Evans report on behalf of the Swift-XRT team:
We have analysed 7.8 ks of XRT data for GRB 251001B, from 99 s to 40.1 ks after the trigger. The data comprise 540 s in Windowed Timing (WT) mode (the first 10 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode.
The light curve can be modelled with a power-law decay with a decay index of alpha=1.34 (+/-0.06).
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.92 (+0.08, -0.07). The best-fitting absorption column is 8.8 (+2.0, -1.9) x 10^20 cm^-2, in excess of the Galactic value of 2.6 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.74 (+0.19, -0.18) and a best-fitting absorption column of 8.6 (+5.7, -4.8) x 10^20 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.9 x 10^-11 (4.5 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
```
Total column:	     8.6 (+5.7, -4.8) x 10^20 cm^-2
Galactic foreground: 2.6 x 10^20 cm^-2
Excess significance: 2.1 sigma
Photon index:	     1.74 (+0.19, -0.18)
```
If the light curve continues to decay with a power-law decay index of 1.34, the count rate at T+24 hours will be 9.6 x 10^-4 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.8 x 10^-14 (4.3 x 10^-14) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01400467.
This circular is an official product of the Swift-XRT team.
GCN Circular #42053
R. Sanchez-Ramirez (IAA-CSIC), Y.-D. Hu (GXI), A. J. Castro-Tirado, S. Guziy, M.D. Caballero-Garcia, S.-Y. Wu and I. Perez-Garcia (IAA-CSIC), R. Scarpa (GTC, IAC), D. Gonzalez (GTC), A. Cabrera-Lavers (GTC, IAC), S. B. Pandey (ARIES), M. Gritsevich (Univ. of Helsinki), J. Becerra-Gonzalez (IAC), L. Piro (INAF/IAPS) and B.-B. Zhang (NJU), on behalf of a larger collaboration, report:
Following the detection of GRB 251001B by Fermi (Fermi Team, GCN 42038, Neights et al. GCN 42047) and Swift (Beardmore et al. GCN 42039), we observed the optical afterglow (Gompertz et al. GCN 42040, An et al. GCN 42041, Strausbaugh et al. GCN 42043, O'Neill et al. GCN 42046, de Ugarte Postigo et al., GCN 42050, Ma et al., GCN 42051) with the 10.4m GTC telescope, at the Spanish Roque de los Muchachos Observatory, on the island of La Palma, equipped with the OSIRIS+ instrument.
The spectroscopic observations were performed at high airmass and consisted of 3x900s exposures using grism R1000B, with a spectral coverage between 3,600 and 7,700 A (R~600). The observations started on Oct 2, 02:39 UT (i.e. 12.35 h after the burst trigger).
From a preliminary reduction, we find a strong DLA at ~4000A, as well as several metal absorption lines that we interpret as coming from NV, SiII, SiII*, OI, CII, SiIV, CIV, FeII, FeII*, AlII, AlIII, ZnII, CrII, all at z=2.162. The detection of SiII* and FeII* links this system to the GRB. We also note the presence of Lya in emission at the same redshift.
GCN Circular #42054
Benjamin Schneider (LAM), Camila Angulo (UNAM), Rosa L. Becerra (U Roma), Antonio de Ugarte Postigo (LAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (OCA), Nathaniel R. Butler (ASU) , Damien Dornic (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), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), and Fredd Sánchez Álvarez (UNAM):
We imaged the field of the GRB 251001B (Fermi GBM team, GCN Circ. 42038; Beardmore et al. GCN Circ. 42039) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-10-02T06:13:31 to 08:49:34 UTC (from 15.94 to 18.54 hours after the trigger) and obtained 64 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 previously reported by Gompertz et al. GCN Circ. 42040; An et al. GCN Circ. 42041; Strausbaugh & Cucchiara GCN Circ. 42043; O'Neill et al. GCN Circ. 42046; A. de Ugarte Postigo et al., GCN Circ. 42050; Ma et al., GCN Circ. 42051 and Sanchez-Ramirez et al., GCN 42053 at preliminary magnitudes of:
```
r = 21.56 +/- 0.06
z = 21.28 +/- 0.13
```
Our measured r-band magnitude is consistent with the magnitude reported by the NOT (Ugarte Postigo et al., GCN Circ. 42050), indicating a possible flattening in the optical afterglow.
Further observations and analysis are ongoing in g, r, i, z and y filters.
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 #42055
A.A. Breeveld (UCL-MSSL), S.P.R. Shilling (Lancaster U.) and A.P. Beardmore (U. Leicester) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 251001B 119 s after the BAT trigger (Beardmore et al., GCN Circ. 42039). A source consistent with the enhanced XRT position (Beardmore et al., GCN Circ. 42045) and also with the optical counterpart seen by Gompertz et al. (GCN Circ. 42040), An et al. (GCN Circ. 2041), Strausbaugh and Cucchiara (GCN Circ. 42043) and de Ugarte Postigo et al. (GCN Circ. 42050) is detected in the initial UVOT exposures.
Preliminary detections using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposures and subsequent exposures are:
```
Filter         T_start(s)   T_stop(s)      Exp(s)           Mag

white_FC          119          269          147         18.76 +/- 0.06
u_FC              331          581          246         18.86 +/- 0.15
white             611          630           10         19.0  +/- 0.25
white             861         1010          147         19.6  +/- 0.2
b                 587          779           39         > 19.0
v                 661          681           20         > 17.4
```
The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.023 in the direction of the burst (Schlegel et al. 1998).
GCN Circular #42058
H. A. Krimm (NSF), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), R. Gupta (GSFC), S. Laha (GSFC/UMBC), 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-240 to T+962 sec from the recent telemetry downlink, we report further analysis of BAT GRB 251001B (trigger #1400467) (Beardmore, et al., GCN Circ. 42039). The BAT ground-calculated position is RA, Dec = 42.668, -22.990 deg which is
```
   RA(J2000)  =  02h 50m 40.2s 
   Dec(J2000) = -22d 59' 23.9" 
```
with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 91%.
The mask-weighted light curve shows a single FRED pulse from ~T0-5 s to ~T0+20 s. Statistically significant emission is found to occur out to ~T0+90 s. The T90 (15-350 keV) is 75.33 +- 46.90 sec (estimated error including systematics).
The time-averaged spectrum from T-9.98 to T+91.72 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.49 +- 0.14. The fluence in the 15-150 keV band is 1.1 +- 0.1 x 10^-06 erg/cm2. The 1-sec peak photon flux measured from T+2.29 sec in the 15-150 keV band is 0.8 +- 0.1 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/1400467
GCN Circular #42083
N. Pankov (HSE, IKI), A. Pozanenko (IKI), R. Ya. Inasaridze (AbAO) report on behalf of IKI-GRB-FuN:
We performed optical observations of the field of GRB 251001B detected by Fermi/GBM (The Fermi GBM team et. al, GCN 42038), and Swift/BAT (Beardmore et. al, GCN 42039) with the AS-32 0.7m telescope of the Abastumani Observatory (AbAO). The R-filter observations began on 2025-10-01 22:37 UT, i.e. ~0.35 days since trigger and consisted of 55x30 s exposures. The optical source at z = 2.162 (Sanchez-Ramirez et. al, GCN 42053) found by GOTO (Gompertz et. al, GCN 42040) and also observed by (An et. al, GCN 42041; Strausbaugh & Cucchiara, GCN 42043; O'Neill et. al, GCN 42046; de Ugarte Postigo et. al, GCN 42050; Ma et. al, GCN 42051; Sanchez-Ramirez et. al, GCN 42053; Schneider et. al, GCN 42054; Breeveld et. al, GCN 42055) is not detected in the co-add image of 51x30 s. Preliminary photometry and observation details are presented below:
```
Date       UT start  t-T0         Exp.  Filter OT  Err. UL
                     (mid, days)  (s)                   (3sigma)
2025-10-01 22:37:48  0.35641      51x30 R      n/d n/d  19.9
```
The photometry was calibrated using nearby stars from USNO-B1.0 (R2 magnitudes) and not corrected for the Galactic extinction.
Ref. stars USNO-B1.0
```
RA Dec R2
42.63445 -23.02161 14.13
42.77496 -23.05166 13.47
```
GCN Circular #42097
Rosa L. Becerra (U Roma), Benjamin Schneider (LAM), Camila Angulo (UNAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (OCA), 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), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Antonio de Ugarte Postigo (LAM):
We reobserved the field of the GRB 251001B (Fermi GBM team, GCN Circ. 42038; Beardmore et al. GCN Circ. 42039) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-10-03 12:00 to 12:19 UTC (from 45.7 to 46.0 hours after the trigger) and obtained 16 minutes of simultaneous exposure in the r and z filters.
The data were reduced and coadded with custom software 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 previously reported by Gompertz et al. (GCN Circ. 42040), An et al. (GCN Circ. 42041), Strausbaugh & Cucchiara (GCN Circ. 42043), O'Neill et al. (GCN Circ. 42046), de Ugarte Postigo et al. (GCN Circ. 42050), Ma et al. (GCN Circ. 42051), Sanchez-Ramirez et al. (GCN Circ. 42053), Schneider et al. (GCN Circ. 42054), and Breeveld et al. (GCN Circ. 42055) at preliminary magnitudes of:
r = 22.28 +/- 0.14.
z = 22.08 +/- 0.41 (with a 3-sigma detection limit of z = 21.85).
By comparing the optical data reported by the aforementioned facilities and COLIBRÍ, we estimate a spectral index of approximately -0.5. This shallow decay is consistent with the temporal evolution observed in the X-ray band after 10ks from the trigger (https://www.swift.ac.uk/xrt_curves/01400467/).
Further observations are planned.
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.

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Jochen Greiner, last update: 03-Oct-2025 [Disclaimer]