Gamma-ray Burst 250331C

(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 31 Mar 25 04:43:33 UT
NOTICE_TYPE:     Swift-BAT GRB Position
TRIGGER_NUM:     1299967,   Seg_Num: 0
GRB_RA:           80.993d {+05h 23m 58s} (J2000),
                  81.407d {+05h 25m 38s} (current),
                  80.174d {+05h 20m 42s} (1950)
GRB_DEC:         +33.029d {+33d 01' 43"} (J2000),
                 +33.050d {+33d 03' 00"} (current),
                 +32.983d {+32d 58' 59"} (1950)
GRB_ERROR:       3.00 [arcmin radius, statistical only]
GRB_INTEN:       0 [cnts]    Image_Peak=1712 [image_cnts]
TRIGGER_DUR:     64.000 [sec] 
TRIGGER_INDEX:   20000     E_range: 15-50 keV
BKG_INTEN:       0 [cnts]
BKG_TIME:        0.00 SOD {00:00:00.00} UT
BKG_DUR:         0 [sec]
GRB_DATE:        20765 TJD;    90 DOY;   25/03/31
GRB_TIME:        16660.65 SOD {04:37:40.65} UT
GRB_PHI:         159.60 [deg]
GRB_THETA:        13.13 [deg]
SOLN_STATUS:     0x13
RATE_SIGNIF:     0.00 [sigma]
IMAGE_SIGNIF:    7.76 [sigma]
MERIT_PARAMS:     +1  +0  +0  +6  +1  -2  +0  +1 -30  +0 
SUN_POSTN:         9.86d {+00h 39m 27s}   +4.25d {+04d 14' 45"}
SUN_DIST:         72.24 [deg]   Sun_angle= -4.8 [hr] (East of Sun)
MOON_POSTN:       32.00d {+02h 08m 01s}  +16.30d {+16d 17' 56"}
MOON_DIST:        47.42 [deg]
MOON_ILLUM:      5 [%]
GAL_COORDS:      174.05, -1.68 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       82.35,  9.83 [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 trig
ger integration interval;  
COMMENTS:        probably due to it occurring during a Malindi downlink session.  
COMMENTS:        This is an image trigger.  (The RATE_SIGNIF & BKG_{INTEN, TIME, DUR}
 are undefined.)  
COMMENTS:        A point_source was found.  
COMMENTS:        This does not match any source in the on-board catalog.  
COMMENTS:        This does not match any source in the ground catalog.  
COMMENTS:        This is a GRB.  
COMMENTS:        This trigger occurred at longitude,latitude = 138.01,15.37 [deg].

red DSS finding chart ps-file

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 31 Mar 25 04:44:30 UT
NOTICE_TYPE:     Swift-XRT Position
TRIGGER_NUM:     1299967,   Seg_Num: 0
GRB_RA:           80.9582d {+05h 23m 49.96s} (J2000),
                  81.3721d {+05h 25m 29.30s} (current),
                  80.1391d {+05h 20m 33.37s} (1950)
GRB_DEC:         +33.0639d {+33d 03' 50.0"} (J2000),
                 +33.0855d {+33d 05' 07.7"} (current),
                 +33.0182d {+33d 01' 05.4"} (1950)
GRB_ERROR:       5.5 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       1.04e-09 [erg/cm2/sec]
GRB_SIGNIF:      6.24 [sigma]
IMG_START_DATE:  20765 TJD;    90 DOY;   25/03/31
IMG_START_TIME:  16791.55 SOD {04:39:51.55} UT,   130.9 [sec] since BAT Trigger Time
TAM[0-3]:        327.66 237.19 261.54 243.75
AMPLIFIER:       2
WAVEFORM:        134
SUN_POSTN:         9.86d {+00h 39m 27s}   +4.25d {+04d 14' 45"}
SUN_DIST:         72.22 [deg]   Sun_angle= -4.8 [hr] (East of Sun)
MOON_POSTN:       32.01d {+02h 08m 03s}  +16.30d {+16d 18' 11"}
MOON_DIST:        47.39 [deg]
MOON_ILLUM:      5 [%]
GAL_COORDS:      174.00, -1.69 [deg] galactic lon,lat of the burst
ECL_COORDS:       82.32,  9.87 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Coordinates.  
COMMENTS:        The XRT position is 2.76 arcmin from the BAT position.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 31 Mar 25 04:44:40 UT
NOTICE_TYPE:     Swift-XRT Image
TRIGGER_NUM:     1299967,   Seg_Num: 0
GRB_RA:           80.9582d {+05h 23m 49.9s} (J2000),
                  81.3721d {+05h 25m 29.3s} (current),
                  80.1391d {+05h 20m 33.3s} (1950)
GRB_DEC:         +33.0639d {+33d 03' 50.0"} (J2000),
                 +33.0855d {+33d 05' 07.7"} (current),
                 +33.0182d {+33d 01' 05.4"} (1950)
GRB_ERROR:       5.4 [arcsec, radius, statistical plus systematic]
GRB_INTEN:       39 [cnts]
IMG_START_DATE:  20765 TJD;    90 DOY;   25/03/31
IMG_START_TIME:  16791.55 SOD {04:39:51.55} UT,   130.9 [sec] since BAT Trigger Time
CENTROID_X:      286.24,  raw= 286  [pixels]
CENTROID_Y:      240.44,  raw= 240  [pixels]
ROLL:            263.12 [deg]
GAIN:            1
MODE:            3, Long Image mode
WAVEFORM:        134
EXPO_TIME:       2.50 [sec]
GRB_POS_XRT_Y:   -137.33
GRB_POS_XRT_Z:   -59.76
IMAGE_URL:       sw01299967000msxps_rw.img
SUN_POSTN:         9.86d {+00h 39m 27s}   +4.25d {+04d 14' 46"}
SUN_DIST:         72.22 [deg]   Sun_angle= -4.8 [hr] (East of Sun)
MOON_POSTN:       32.02d {+02h 08m 04s}  +16.30d {+16d 18' 13"}
MOON_DIST:        47.39 [deg]
MOON_ILLUM:      5 [%]
GAL_COORDS:      174.00, -1.69 [deg] galactic lon,lat of the burst
ECL_COORDS:       82.32,  9.87 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Image.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 31 Mar 25 04:44:42 UT
NOTICE_TYPE:     Swift-XRT Processed Image
TRIGGER_NUM:     1299967,   Seg_Num: 0
GRB_RA:           80.9582d {+05h 23m 49.9s} (J2000),
                  81.3721d {+05h 25m 29.3s} (current),
                  80.1391d {+05h 20m 33.3s} (1950)
GRB_DEC:         +33.0639d {+33d 03' 50.0"} (J2000),
                 +33.0855d {+33d 05' 07.7"} (current),
                 +33.0182d {+33d 01' 05.4"} (1950)
GRB_ERROR:       5.4 [arcsec, radius, statistical plus systematic]
GRB_INTEN:       39 [cnts]
IMG_START_DATE:  20765 TJD;    90 DOY;   25/03/31
IMG_START_TIME:  16791.55 SOD {04:39:51.55} UT,   130.9 [sec] since BAT Trigger Time
CENTROID_X:      286.24,  raw= 286  [pixels]
CENTROID_Y:      240.44,  raw= 240  [pixels]
ROLL:            263.12 [deg]
GAIN:            1
MODE:            3, Long Image mode
WAVEFORM:        134
EXPO_TIME:       2.50 [sec]
GRB_POS_XRT_Y:   -137.33
GRB_POS_XRT_Z:   -59.76
IMAGE_URL:       sw01299967000msxps_rw.img
SUN_POSTN:         9.86d {+00h 39m 27s}   +4.25d {+04d 14' 46"}
SUN_DIST:         72.22 [deg]   Sun_angle= -4.8 [hr] (East of Sun)
MOON_POSTN:       32.02d {+02h 08m 04s}  +16.30d {+16d 18' 14"}
MOON_DIST:        47.39 [deg]
MOON_ILLUM:      5 [%]
GAL_COORDS:      174.00, -1.69 [deg] galactic lon,lat of the burst
ECL_COORDS:       82.32,  9.87 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Processed Image.

GCN Circular #39959
S. B. Cenko (GSFC), C. Gronwall (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team:
At 04:37:40 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250331C (trigger=1299967). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 80.993, +33.029 which is
```
   RA(J2000) = 05h 23m 58s
   Dec(J2000) = +33d 01' 43"
```
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 25 sec. The peak count rate was ~800 counts/sec (15-350 keV), at ~1 sec after the trigger.
The XRT began observing the field at 04:39:51.5 UT, 130.9 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 80.95740, 33.06310 which is equivalent to:
```
   RA(J2000)  = 05h 23m 49.78s
   Dec(J2000) = +33d 03' 47.2"
```
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 163 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 6.49 x 10^21 cm^-2 (Willingale et al. 2013).
The initial flux in the 2.5 s image was 1.04e-09 erg cm^-2 s^-1 (0.2-10 keV).
UVOT took a finding chart exposure of 150 seconds with the White filter starting 139 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers none of the XRT error circle. 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 large, but uncertain, extinction expected.
Burst Advocate for this burst is S. B. Cenko (brad.cenko AT nasa.gov). 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 31 Mar 25 05:01:10 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     1299967,   Seg_Num: 0
POINT_RA:         80.983d {+05h 23m 56s} (J2000)
POINT_DEC:       +33.029d {+33d 01' 44"} (J2000)
POINT_ROLL:      263.120d
IMG_START_DATE:  20765 TJD;    90 DOY;   25/03/31
IMG_START_TIME:  17535.64 SOD {04:52:15.64} UT,   875.0 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        5.385
N_STARS:         190
X_OFFSET:        468 [pixels]
Y_OFFSET:        494 [pixels]
X_MAX:           1907 [pixels]
Y_MAX:           1933 [pixels]
DET_THRESH:      20
PHOTO_THRESH:    11
SL_URL:          sw01299967000msufc0874.fits
SUN_POSTN:         9.87d {+00h 39m 30s}   +4.25d {+04d 15' 02"}
SUN_DIST:         72.22 [deg]   Sun_angle= -4.8 [hr] (East of Sun)
MOON_POSTN:       32.18d {+02h 08m 43s}  +16.37d {+16d 22' 24"}
MOON_DIST:        47.23 [deg]
MOON_ILLUM:      5 [%]
GAL_COORDS:      174.04, -1.69 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       82.34,  9.83 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 31 Mar 25 05:01:28 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     1299967,   Seg_Num: 0
POINT_RA:         80.983d {+05h 23m 56s} (J2000)
POINT_DEC:       +33.029d {+33d 01' 44"} (J2000)
POINT_ROLL:      263.120d
IMG_START_DATE:  20765 TJD;    90 DOY;   25/03/31
IMG_START_TIME:  17535.64 SOD {04:52:15.64} UT,   875.0 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        5.385
N_STARS:         190
X_OFFSET:        468 [pixels]
Y_OFFSET:        494 [pixels]
X_MAX:           1907 [pixels]
Y_MAX:           1933 [pixels]
DET_THRESH:      20
PHOTO_THRESH:    11
SL_URL:          sw01299967000msufc0874.fits
SUN_POSTN:         9.87d {+00h 39m 30s}   +4.25d {+04d 15' 02"}
SUN_DIST:         72.22 [deg]   Sun_angle= -4.8 [hr] (East of Sun)
MOON_POSTN:       32.18d {+02h 08m 44s}  +16.37d {+16d 22' 28"}
MOON_DIST:        47.23 [deg]
MOON_ILLUM:      5 [%]
GAL_COORDS:      174.04, -1.69 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       82.34,  9.83 [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 31 Mar 25 05:01:48 UT
NOTICE_TYPE:     Swift-XRT Position UPDATE
TRIGGER_NUM:     1299967,   Seg_Num: 0
GRB_RA:           80.9562d {+05h 23m 49.48s} (J2000),
                  81.3701d {+05h 25m 28.82s} (current),
                  80.1371d {+05h 20m 32.89s} (1950)
GRB_DEC:         +33.0631d {+33d 03' 47.1"} (J2000),
                 +33.0847d {+33d 05' 04.8"} (current),
                 +33.0174d {+33d 01' 02.5"} (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:  20765 TJD;    90 DOY;   25/03/31
IMG_START_TIME:  16895.00 SOD {04:41:35.00} UT,   234.3 [sec] since BAT Trigger Time
TAM[0-3]:        100.00 100.00 100.00 100.00
AMPLIFIER:       1
WAVEFORM:        31
SUN_POSTN:         9.87d {+00h 39m 30s}   +4.25d {+04d 15' 02"}
SUN_DIST:         72.20 [deg]   Sun_angle= -4.8 [hr] (East of Sun)
MOON_POSTN:       32.19d {+02h 08m 45s}  +16.38d {+16d 22' 33"}
MOON_DIST:        47.21 [deg]
MOON_ILLUM:      5 [%]
GAL_COORDS:      174.00, -1.69 [deg] galactic lon,lat of the burst
ECL_COORDS:       82.32,  9.86 [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:     Mon 31 Mar 25 05:02:45 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     1299967,   Seg_Num: 0
POINT_RA:         80.983d {+05h 23m 56s} (J2000)
POINT_DEC:       +33.029d {+33d 01' 44"} (J2000)
ROLL:            263.120d
IMG_START_DATE:  20765 TJD;    90 DOY;   25/03/31
IMG_START_TIME:  17535.64 SOD {04:52:15.64} UT,   875.0 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     765089570
X_OFFSET:        1027 [pixels]
Y_OFFSET:        1053 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       1187
Y_GRB_POS:       1213
BINNING_INDEX:   1
IM_URL:          sw01299967000msuni0910.fits
SUN_POSTN:         9.87d {+00h 39m 30s}   +4.25d {+04d 15' 03"}
SUN_DIST:         72.22 [deg]   Sun_angle= -4.8 [hr] (East of Sun)
MOON_POSTN:       32.20d {+02h 08m 47s}  +16.38d {+16d 22' 48"}
MOON_DIST:        47.21 [deg]
MOON_ILLUM:      5 [%]
GAL_COORDS:      174.04, -1.69 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       82.34,  9.83 [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 31 Mar 25 05:02:58 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     1299967,   Seg_Num: 0
POINT_RA:         80.983d {+05h 23m 56s} (J2000)
POINT_DEC:       +33.029d {+33d 01' 44"} (J2000)
ROLL:            263.120d
IMG_START_DATE:  20765 TJD;    90 DOY;   25/03/31
IMG_START_TIME:  17535.64 SOD {04:52:15.64} UT,   875.0 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     765089570
X_OFFSET:        1027 [pixels]
Y_OFFSET:        1053 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       1187
Y_GRB_POS:       1213
BINNING_INDEX:   1
IM_URL:          sw01299967000msuni0910.fits
SUN_POSTN:         9.87d {+00h 39m 30s}   +4.25d {+04d 15' 03"}
SUN_DIST:         72.22 [deg]   Sun_angle= -4.8 [hr] (East of Sun)
MOON_POSTN:       32.20d {+02h 08m 48s}  +16.38d {+16d 22' 51"}
MOON_DIST:        47.21 [deg]
MOON_ILLUM:      5 [%]
GAL_COORDS:      174.04, -1.69 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       82.34,  9.83 [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 31 Mar 25 04:48:13 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     1299967,   Seg_Num: 0
POINT_RA:         80.982d {+05h 23m 56s} (J2000)
POINT_DEC:       +33.027d {+33d 01' 39"} (J2000)
POINT_ROLL:      263.120d
IMG_START_DATE:  20765 TJD;    90 DOY;   25/03/31
IMG_START_TIME:  16800.14 SOD {04:40:00.14} UT,   139.5 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        6.402
N_STARS:         158
X_OFFSET:        416 [pixels]
Y_OFFSET:        576 [pixels]
X_MAX:           1375 [pixels]
Y_MAX:           1535 [pixels]
DET_THRESH:      22
PHOTO_THRESH:    13
SL_URL:          sw01299967000msufc0139.fits
SUN_POSTN:         9.87d {+00h 39m 28s}   +4.25d {+04d 14' 49"}
SUN_DIST:         72.23 [deg]   Sun_angle= -4.8 [hr] (East of Sun)
MOON_POSTN:       32.05d {+02h 08m 12s}  +16.32d {+16d 19' 07"}
MOON_DIST:        47.36 [deg]
MOON_ILLUM:      5 [%]
GAL_COORDS:      174.04, -1.69 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       82.34,  9.83 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 31 Mar 25 04:45:41 UT
NOTICE_TYPE:     Swift-BAT GRB Lightcurve
TRIGGER_NUM:     1299967,   Seg_Num: 0
GRB_RA:           80.993d {+05h 23m 58s} (J2000),
                  81.407d {+05h 25m 38s} (current),
                  80.174d {+05h 20m 42s} (1950)
GRB_DEC:         +33.029d {+33d 01' 43"} (J2000),
                 +33.050d {+33d 03' 00"} (current),
                 +32.983d {+32d 58' 59"} (1950)
GRB_DATE:        20765 TJD;    90 DOY;   25/03/31
GRB_TIME:        16660.65 SOD {04:37:40.65} UT
TRIGGER_INDEX:   20000
GRB_PHI:         159.60 [deg]
GRB_THETA:        13.13 [deg]
DELTA_TIME:      0.00 [sec]
TRIGGER_DUR:     64.000 [sec] 
SOLN_STATUS:     0x13
RATE_SIGNIF:     0.00 [sigma]
IMAGE_SIGNIF:    7.76 [sigma]
LC_URL:          sw01299967000msb.lc
SUN_POSTN:         9.86d {+00h 39m 27s}   +4.25d {+04d 14' 47"}
SUN_DIST:         72.24 [deg]   Sun_angle= -4.8 [hr] (East of Sun)
MOON_POSTN:       32.03d {+02h 08m 06s}  +16.31d {+16d 18' 29"}
MOON_DIST:        47.40 [deg]
MOON_ILLUM:      5 [%]
GAL_COORDS:      174.05, -1.68 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       82.35,  9.83 [deg] ecliptic lon,lat of the burst (or transient)
COMMENTS:        SWIFT-BAT GRB Lightcurve.  
COMMENTS:           
COMMENTS:        The next comments were copied from the BAT_POS Notice:  
COMMENTS:        This is an image trigger.  
COMMENTS:        A point_source was found.  
COMMENTS:        This does not match any source in the on-board catalog.  
COMMENTS:        This does not match any source in the ground catalog.  
COMMENTS:        This is a GRB.  
COMMENTS:        This trigger occurred at longitude,latitude = 138.01,15.37 [deg].

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 31 Mar 25 04:49:44 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     1299967,   Seg_Num: 0
POINT_RA:         80.982d {+05h 23m 56s} (J2000)
POINT_DEC:       +33.027d {+33d 01' 39"} (J2000)
ROLL:            263.120d
IMG_START_DATE:  20765 TJD;    90 DOY;   25/03/31
IMG_START_TIME:  16800.14 SOD {04:40:00.14} UT,   139.5 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     765088835
X_OFFSET:        1028 [pixels]
Y_OFFSET:        1054 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       1188
Y_GRB_POS:       1214
BINNING_INDEX:   1
IM_URL:          sw01299967000msuni0175.fits
SUN_POSTN:         9.87d {+00h 39m 28s}   +4.25d {+04d 14' 51"}
SUN_DIST:         72.23 [deg]   Sun_angle= -4.8 [hr] (East of Sun)
MOON_POSTN:       32.07d {+02h 08m 16s}  +16.33d {+16d 19' 30"}
MOON_DIST:        47.35 [deg]
MOON_ILLUM:      5 [%]
GAL_COORDS:      174.04, -1.69 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       82.34,  9.83 [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 31 Mar 25 04:48:27 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     1299967,   Seg_Num: 0
POINT_RA:         80.982d {+05h 23m 56s} (J2000)
POINT_DEC:       +33.027d {+33d 01' 39"} (J2000)
POINT_ROLL:      263.120d
IMG_START_DATE:  20765 TJD;    90 DOY;   25/03/31
IMG_START_TIME:  16800.14 SOD {04:40:00.14} UT,   139.5 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        6.402
N_STARS:         158
X_OFFSET:        416 [pixels]
Y_OFFSET:        576 [pixels]
X_MAX:           1375 [pixels]
Y_MAX:           1535 [pixels]
DET_THRESH:      22
PHOTO_THRESH:    13
SL_URL:          sw01299967000msufc0139.fits
SUN_POSTN:         9.87d {+00h 39m 28s}   +4.25d {+04d 14' 49"}
SUN_DIST:         72.23 [deg]   Sun_angle= -4.8 [hr] (East of Sun)
MOON_POSTN:       32.05d {+02h 08m 13s}  +16.32d {+16d 19' 11"}
MOON_DIST:        47.36 [deg]
MOON_ILLUM:      5 [%]
GAL_COORDS:      174.04, -1.69 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       82.34,  9.83 [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 31 Mar 25 04:49:33 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     1299967,   Seg_Num: 0
POINT_RA:         80.982d {+05h 23m 56s} (J2000)
POINT_DEC:       +33.027d {+33d 01' 39"} (J2000)
ROLL:            263.120d
IMG_START_DATE:  20765 TJD;    90 DOY;   25/03/31
IMG_START_TIME:  16800.14 SOD {04:40:00.14} UT,   139.5 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     765088835
X_OFFSET:        1028 [pixels]
Y_OFFSET:        1054 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       1188
Y_GRB_POS:       1214
BINNING_INDEX:   1
IM_URL:          sw01299967000msuni0175.fits
SUN_POSTN:         9.87d {+00h 39m 28s}   +4.25d {+04d 14' 50"}
SUN_DIST:         72.23 [deg]   Sun_angle= -4.8 [hr] (East of Sun)
MOON_POSTN:       32.06d {+02h 08m 15s}  +16.32d {+16d 19' 27"}
MOON_DIST:        47.35 [deg]
MOON_ILLUM:      5 [%]
GAL_COORDS:      174.04, -1.69 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       82.34,  9.83 [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 Circular #39960
P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:
Using 1.6 ks of promptly downlinked XRT event data for GRB 250331C, we find an enhanced XRT position of the afterglow: RA, Dec: 80.95640, 33.06317 which is equivalent to:
```
   RA (J2000)  = 05 23 49.54
   Dec (J2000) = +33 03 47.4
```
with an uncertainty of 1.9 arcseconds (radius, 90% confidence). Analysis of the promptly available data is online at http://www.swift.ac.uk/sper/1299967.
Position enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans et al. (2009, MNRAS, 397, 1177).
This circular is an official product of the Swift-XRT team.
GCN Circular #39963
Damien Dornic (CPPM), Margarita Pereyra (UNAM), Jean-Grégoire Ducoin (CPPM), Jean-Luc Atteia (IRAP), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Dalya Akl (AUS), Camila Angulo (UNAM), Sarah Antier (OCA), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU), Francis Fortin (IRAP), Ny Avo Rakotondrainibe (LAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report:
We imaged the field of Swift/BAT GRB 250331C (Cenko et al., GCN Circ. 39959) with the DDRAGO wide-field imager on the COLIBRÍ telescope at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico.
We started observing at 2025-03-31 04:43:50 UTC (370 s after the trigger) and obtained a total of 720 s of exposure time in the i filter. The data were coadded with the COLIBRÍ pipeline and analyzed in STDWeb/STDPipe (Karpov 2025), with photometric calibration against Pan-STARRS DR1 and image subtraction against Pan-STARRS DR2. Our photometry is in the AB system and is not corrected for Galactic extinction.
At the XRT localization (Evans et al. 39960), we do not find any optical counterpart to a 3-sigma limiting magnitude of:
i > 22.26
We warmly thank the COLIBRÍ and DDRAGO engineering teams and the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir.
GCN Circular #39977
A.P. Beardmore, P.A. Evans, M.R. Goad and J.P. Osborne (U. Leicester) report on behalf of the Swift-XRT team.
Using 1624 s of XRT Photon Counting mode data and 2 UVOT images for GRB 250331C, 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 = 80.95619, +33.06347 which is equivalent to:
```
RA (J2000): 05h 23m 49.49s
Dec (J2000): +33d 03' 48.5"
```
with an uncertainty of 2.2 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 #39981
V.Lipunov, E.Gorbovskoy, N.Tiurina, P.Balanutsa, , D.Vlasenko, I.Panchenko, A.Kuznetsov, G.Antipov, A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, 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. 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 (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in South Africa (South African Astronomical Observatory) was pointed to the Swift GRB 250331C ( S. B. Cenko et al., GCN 39959) errorbox 48174 sec after notice time and 48531 sec after trigger time at 2025-03-31 18:06:32 UT, with upper limit up to 18.0 mag. The observations began at zenith distance = 75 deg. The sun altitude is -20.0 deg.
The galactic latitude b = -1 deg., longitude l = 174 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2828956
We obtain a following upper limits.
```
Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

   48622 |         MASTER-SAAO |   C |   180 | 17.9 |        
   48821 |         MASTER-SAAO |   C |   180 | 18.0 |        
```
Filter C is a clear (unfiltred) band.
The observation and reduction will continue. The message may be cited.
GCN Circular #39997
M. A. Williams (PSU), S. Dichiara (PSU), J.A. Kennea (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-IASFPA) and P.A. Evans report on behalf of the Swift-XRT team:
We have analysed 3.9 ks of XRT data for GRB 250331C, from 137 s to 62.7 ks after the BAT trigger. The data comprise 95 s in Windowed Timing (WT) mode 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=2.81 (+0.11, -0.10).
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.11 (+0.11, -0.08). The best-fitting absorption column is consistent with the Galactic value of 6.5 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.65 (+0.32, -0.30) and a best-fitting absorption column of 9.0 (+3.4, -2.5) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.9 x 10^-11 (8.6 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
```
Total column:	     9.0 (+3.4, -2.5) x 10^21 cm^-2
Galactic foreground: 6.5 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.65 (+0.32, -0.30)
```
If the light curve continues to decay with a power-law decay index of 2.81, the count rate at T+24 hours will be 3.2 x 10^-7 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.9 x 10^-17 (2.8 x 10^-17) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01299967.
This circular is an official product of the Swift-XRT team.
GCN Circular #40003
M. H. Siegel (PSU) and S. B. Cenko (GSFC) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 250331C 140 s after the BAT trigger (Cenko et al., GCN Circ. 39959). No optical afterglow consistent with the XRT position (Beardmore et al., GCN Circ. 39977) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposure and subsequent exposures are:
```
Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white_FC           140          289          147         >21.1
u_FC               297          547          246         >19.9
white              140         1717          411         >22.3
v                  628         1767          136         >19.1
b                  553         1863          133         >20.1
u                  297         1841          363         >20.1
w1                 676         1817          136         >19.1
m2                 652         1792          136         >19.8
w2                 603         1743          136         >20.0
```
The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.921 in the direction of the burst (Schlegel et al. 1998).
GCN Circular #40013
Giovanni Calapai at Calapai Astronomical Observatory, Massa S. Giorgio, (Messina) Italy Member of: GRB/UAI Gamma Ray Burst Section of Unione Astrofili Italiani.
Report:
We imaged the field of GRB 250331C detected by Swift Burst Alert Telescope (BAT) (Cenko et al. GCN 39959), with the 11 inches Schmidt-Cassegrain (Celestron 11) telescope F/D=6,3.
The observations were started at 2025-03-31 20:59 UT (approximately 16.37 hours after burst) stacking a set of unfiltered CCD image. The observations were made with modest sky conditions and the target low on the horizon.
We co-added 30 exposures of 60 sec each.
```
Start T0+      End T0+     CR lim
16.37 hour    16.92 hour    18.7
```
We did not found any optical counterpart in the error box of the XRTcandidate. Evans GCN 39960
Magnitudes were estimated with the PanSTARRS cat. and are not corrected for galactic dust extinction.
Our observations are consistent with other already reported Dornic et al. (GCN 39963), Lipunov et al. (GCN 39981), Siegel et al. (GCN 40003).
The message may be cited.
GCN Circular #40057
A. Y. Lien (U Tampa), S. D. Barthelmy (GSFC), S. B. Cenko (GSFC), R. Gupta (GSFC), H. A. Krimm (NSF), S. Laha (GSFC/UMBC), C. B. Markwardt (GSFC), M. J. Moss (GSFC), D. M. Palmer (LANL), T. Parsotan (GSFC), D. Sadaula (GSFC/UMBC), T. Sakamoto (AGU) (i.e. the Swift-BAT team):
Using the data set from T-239 to T+963 sec from the recent telemetry downlink, we report further analysis of BAT GRB 250331C (trigger #1299967) (Cenko, et al., GCN Circ. 39959). The BAT ground-calculated position is RA, Dec = 80.963, 33.068 deg which is
```
   RA(J2000)  =  05h 23m 51.0s
   Dec(J2000) = +33d 04' 03.4"
```
with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 100%.
The mask-weighted light curve shows a dim pulse of emission with some possible complex structure. The T90 (15-350 keV) is 70.10 +- 11.70 sec (estimated error including systematics).
The time-averaged spectrum from T-12.59 to T+66.67 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.88 +- 0.23. The fluence in the 15-150 keV band is 6.6 +- 1.0 x 10^-07 erg/cm2. The 1-sec peak photon flux measured from T-3.19 sec in the 15-150 keV band is 0.3 +- 0.2 ph/cm2/sec. All the quoted errors are at the 90% confidence level.
The results of the batgrbproduct analysis are available at https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1299967

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Jochen Greiner, last update: 04-Apr-2025 [Disclaimer]