Gamma-ray Burst 250530C

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN/BACODINE POSITION NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Fri 30 May 25 18:59:13 UT
NOTICE_TYPE:     Swift-BAT GRB Position
TRIGGER_NUM:     1319125,   Seg_Num: 0
GRB_RA:           55.143d {+03h 40m 34s} (J2000),
                  55.421d {+03h 41m 41s} (current),
                  54.597d {+03h 38m 23s} (1950)
GRB_DEC:         -22.459d {-22d 27' 32"} (J2000),
                 -22.379d {-22d 22' 42"} (current),
                 -22.619d {-22d 37' 08"} (1950)
GRB_ERROR:       3.00 [arcmin radius, statistical only]
GRB_INTEN:       5335 [cnts]    Image_Peak=193 [image_cnts]
TRIGGER_DUR:     2.048 [sec] 
TRIGGER_INDEX:   262     E_range: 25-100 keV
BKG_INTEN:       51488 [cnts]
BKG_TIME:        68292.30 SOD {18:58:12.30} UT
BKG_DUR:         24 [sec]
GRB_DATE:        20825 TJD;   150 DOY;   25/05/30
GRB_TIME:        68322.89 SOD {18:58:42.89} UT
GRB_PHI:         -156.55 [deg]
GRB_THETA:        23.91 [deg]
SOLN_STATUS:     0x83
RATE_SIGNIF:     13.45 [sigma]
IMAGE_SIGNIF:    6.76 [sigma]
MERIT_PARAMS:     +1  +0  +0  +1  +2  -3  +0  +0 -69  +0 
SUN_POSTN:        67.95d {+04h 31m 49s}  +21.89d {+21d 53' 20"}
SUN_DIST:         45.92 [deg]   Sun_angle= 0.8 [hr] (West of Sun)
MOON_POSTN:      122.49d {+08h 09m 58s}  +24.34d {+24d 20' 08"}
MOON_DIST:        80.13 [deg]
MOON_ILLUM:      18 [%]
GAL_COORDS:      215.53,-51.51 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       45.84,-40.70 [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:        Since the IMAGE_SIGNIF is less than 7 sigma, this is a questionable detection.  
COMMENTS:        This trigger occurred at longitude,latitude = 228.46,-14.42 [deg].

red DSS finding chart ps-file

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Fri 30 May 25 19:00:38 UT
NOTICE_TYPE:     Swift-XRT Nack-Position
TRIGGER_NUM:     1319125,   Seg_Num: 0
POINT_RA:         55.140d {+03h 40m 34s} (J2000)
POINT_DEC:       -22.453d {-22d 27' 09"} (J2000)
IMG_START_DATE:  20825 TJD;   150 DOY;   25/05/30
IMG_START_TIME:  68429.81 SOD {19:00:29.81} UT,   106.9 [sec] since BAT Trigger Time
COUNTS:          1    Min_needed= 20
STD_DEV:         0.00   Max_StdDev_for_Good=28.44 [arcsec]
PH2_ITER:        0    Max_iter_allowed= 4
ERROR_CODE:      1
COMMENTS:        SWIFT-XRT Nack Position.  
COMMENTS:        No source found in the image.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Fri 30 May 25 19:03:34 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     1319125,   Seg_Num: 0
POINT_RA:         55.140d {+03h 40m 34s} (J2000)
POINT_DEC:       -22.453d {-22d 27' 09"} (J2000)
POINT_ROLL:      16.603d
IMG_START_DATE:  20825 TJD;   150 DOY;   25/05/30
IMG_START_TIME:  68434.29 SOD {19:00:34.29} UT,   111.4 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        3.197
N_STARS:         21
X_OFFSET:        453 [pixels]
Y_OFFSET:        589 [pixels]
X_MAX:           1412 [pixels]
Y_MAX:           1548 [pixels]
DET_THRESH:      15
PHOTO_THRESH:    8
SL_URL:          sw01319125000msufc0111.fits
SUN_POSTN:        67.96d {+04h 31m 49s}  +21.89d {+21d 53' 21"}
SUN_DIST:         45.92 [deg]   Sun_angle= 0.8 [hr] (West of Sun)
MOON_POSTN:      122.54d {+08h 10m 09s}  +24.32d {+24d 19' 29"}
MOON_DIST:        80.16 [deg]
MOON_ILLUM:      18 [%]
GAL_COORDS:      215.52,-51.51 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       45.83,-40.70 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Fri 30 May 25 19:03:50 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     1319125,   Seg_Num: 0
POINT_RA:         55.140d {+03h 40m 34s} (J2000)
POINT_DEC:       -22.453d {-22d 27' 09"} (J2000)
POINT_ROLL:      16.603d
IMG_START_DATE:  20825 TJD;   150 DOY;   25/05/30
IMG_START_TIME:  68434.29 SOD {19:00:34.29} UT,   111.4 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        3.197
N_STARS:         21
X_OFFSET:        453 [pixels]
Y_OFFSET:        589 [pixels]
X_MAX:           1412 [pixels]
Y_MAX:           1548 [pixels]
DET_THRESH:      15
PHOTO_THRESH:    8
SL_URL:          sw01319125000msufc0111.fits
SUN_POSTN:        67.96d {+04h 31m 50s}  +21.89d {+21d 53' 21"}
SUN_DIST:         45.92 [deg]   Sun_angle= 0.8 [hr] (West of Sun)
MOON_POSTN:      122.54d {+08h 10m 09s}  +24.32d {+24d 19' 27"}
MOON_DIST:        80.16 [deg]
MOON_ILLUM:      18 [%]
GAL_COORDS:      215.52,-51.51 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       45.83,-40.70 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Source List.  
COMMENTS:        All 4 attachments are included.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Fri 30 May 25 19:04:58 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     1319125,   Seg_Num: 0
POINT_RA:         55.140d {+03h 40m 34s} (J2000)
POINT_DEC:       -22.453d {-22d 27' 09"} (J2000)
ROLL:            16.603d
IMG_START_DATE:  20825 TJD;   150 DOY;   25/05/30
IMG_START_TIME:  68434.29 SOD {19:00:34.29} UT,   111.4 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     770324469
X_OFFSET:        772 [pixels]
Y_OFFSET:        908 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       932
Y_GRB_POS:       1068
BINNING_INDEX:   1
IM_URL:          sw01319125000msuni0147.fits
SUN_POSTN:        67.96d {+04h 31m 50s}  +21.89d {+21d 53' 22"}
SUN_DIST:         45.92 [deg]   Sun_angle= 0.8 [hr] (West of Sun)
MOON_POSTN:      122.55d {+08h 10m 12s}  +24.32d {+24d 19' 17"}
MOON_DIST:        80.17 [deg]
MOON_ILLUM:      18 [%]
GAL_COORDS:      215.52,-51.51 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       45.83,-40.70 [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:     Fri 30 May 25 19:05:07 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     1319125,   Seg_Num: 0
POINT_RA:         55.140d {+03h 40m 34s} (J2000)
POINT_DEC:       -22.453d {-22d 27' 09"} (J2000)
ROLL:            16.603d
IMG_START_DATE:  20825 TJD;   150 DOY;   25/05/30
IMG_START_TIME:  68434.29 SOD {19:00:34.29} UT,   111.4 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     770324469
X_OFFSET:        772 [pixels]
Y_OFFSET:        908 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       932
Y_GRB_POS:       1068
BINNING_INDEX:   1
IM_URL:          sw01319125000msuni0147.fits
SUN_POSTN:        67.96d {+04h 31m 50s}  +21.89d {+21d 53' 22"}
SUN_DIST:         45.92 [deg]   Sun_angle= 0.8 [hr] (West of Sun)
MOON_POSTN:      122.55d {+08h 10m 13s}  +24.32d {+24d 19' 15"}
MOON_DIST:        80.17 [deg]
MOON_ILLUM:      18 [%]
GAL_COORDS:      215.52,-51.51 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       45.83,-40.70 [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:     Fri 30 May 25 19:04:33 UT
NOTICE_TYPE:     Swift-XRT Position
TRIGGER_NUM:     1319125,   Seg_Num: 0
GRB_RA:           55.1692d {+03h 40m 40.60s} (J2000),
                  55.4468d {+03h 41m 47.24s} (current),
                  54.6232d {+03h 38m 29.55s} (1950)
GRB_DEC:         -22.4482d {-22d 26' 53.5"} (J2000),
                 -22.3677d {-22d 22' 03.6"} (current),
                 -22.6083d {-22d 36' 29.8"} (1950)
GRB_ERROR:       3.7 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       1.00e-10 [erg/cm2/sec]
GRB_SIGNIF:      10.00 [sigma]
IMG_START_DATE:  20825 TJD;   150 DOY;   25/05/30
IMG_START_TIME:  68438.00 SOD {19:00:38.00} UT,   115.1 [sec] since BAT Trigger Time
TAM[0-3]:        100.00 100.00 100.00 100.00
AMPLIFIER:       1
WAVEFORM:        31
SUN_POSTN:        67.96d {+04h 31m 50s}  +21.89d {+21d 53' 21"}
SUN_DIST:         45.91 [deg]   Sun_angle= 0.8 [hr] (West of Sun)
MOON_POSTN:      122.55d {+08h 10m 11s}  +24.32d {+24d 19' 21"}
MOON_DIST:        80.14 [deg]
MOON_ILLUM:      18 [%]
GAL_COORDS:      215.53,-51.48 [deg] galactic lon,lat of the burst
ECL_COORDS:       45.87,-40.70 [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:     Fri 30 May 25 19:11:40 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     1319125,   Seg_Num: 0
POINT_RA:         55.142d {+03h 40m 34s} (J2000)
POINT_DEC:       -22.453d {-22d 27' 12"} (J2000)
POINT_ROLL:      16.602d
IMG_START_DATE:  20825 TJD;   150 DOY;   25/05/30
IMG_START_TIME:  68593.53 SOD {19:03:13.53} UT,   270.6 [sec] since BAT Trigger Time
FILTER:          7, U
BKG_MEAN:        0.810
N_STARS:         70
X_OFFSET:        213 [pixels]
Y_OFFSET:        349 [pixels]
X_MAX:           1652 [pixels]
Y_MAX:           1788 [pixels]
DET_THRESH:      8
PHOTO_THRESH:    3
SL_URL:          sw01319125000msufc0270.fits
SUN_POSTN:        67.96d {+04h 31m 51s}  +21.89d {+21d 53' 24"}
SUN_DIST:         45.92 [deg]   Sun_angle= 0.8 [hr] (West of Sun)
MOON_POSTN:      122.62d {+08h 10m 28s}  +24.30d {+24d 18' 16"}
MOON_DIST:        80.22 [deg]
MOON_ILLUM:      18 [%]
GAL_COORDS:      215.52,-51.51 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       45.84,-40.70 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Fri 30 May 25 19:11:54 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     1319125,   Seg_Num: 0
POINT_RA:         55.142d {+03h 40m 34s} (J2000)
POINT_DEC:       -22.453d {-22d 27' 12"} (J2000)
POINT_ROLL:      16.602d
IMG_START_DATE:  20825 TJD;   150 DOY;   25/05/30
IMG_START_TIME:  68593.53 SOD {19:03:13.53} UT,   270.6 [sec] since BAT Trigger Time
FILTER:          7, U
BKG_MEAN:        0.810
N_STARS:         70
X_OFFSET:        213 [pixels]
Y_OFFSET:        349 [pixels]
X_MAX:           1652 [pixels]
Y_MAX:           1788 [pixels]
DET_THRESH:      8
PHOTO_THRESH:    3
SL_URL:          sw01319125000msufc0270.fits
SUN_POSTN:        67.96d {+04h 31m 51s}  +21.89d {+21d 53' 24"}
SUN_DIST:         45.92 [deg]   Sun_angle= 0.8 [hr] (West of Sun)
MOON_POSTN:      122.62d {+08h 10m 29s}  +24.30d {+24d 18' 14"}
MOON_DIST:        80.22 [deg]
MOON_ILLUM:      18 [%]
GAL_COORDS:      215.52,-51.51 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       45.84,-40.70 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Source List.  
COMMENTS:        All 4 attachments are included.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Fri 30 May 25 19:12:04 UT
NOTICE_TYPE:     Swift-XRT Position UPDATE
TRIGGER_NUM:     1319125,   Seg_Num: 0
GRB_RA:           55.1701d {+03h 40m 40.82s} (J2000),
                  55.4477d {+03h 41m 47.45s} (current),
                  54.6241d {+03h 38m 29.77s} (1950)
GRB_DEC:         -22.4481d {-22d 26' 53.1"} (J2000),
                 -22.3676d {-22d 22' 03.3"} (current),
                 -22.6082d {-22d 36' 29.5"} (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:  20825 TJD;   150 DOY;   25/05/30
IMG_START_TIME:  68438.00 SOD {19:00:38.00} UT,   115.1 [sec] since BAT Trigger Time
TAM[0-3]:        100.00 100.00 100.00 100.00
AMPLIFIER:       1
WAVEFORM:        31
SUN_POSTN:        67.96d {+04h 31m 51s}  +21.89d {+21d 53' 24"}
SUN_DIST:         45.91 [deg]   Sun_angle= 0.8 [hr] (West of Sun)
MOON_POSTN:      122.62d {+08h 10m 29s}  +24.30d {+24d 18' 13"}
MOON_DIST:        80.19 [deg]
MOON_ILLUM:      18 [%]
GAL_COORDS:      215.53,-51.48 [deg] galactic lon,lat of the burst
ECL_COORDS:       45.87,-40.70 [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:     Fri 30 May 25 19:13:16 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     1319125,   Seg_Num: 0
POINT_RA:         55.142d {+03h 40m 34s} (J2000)
POINT_DEC:       -22.453d {-22d 27' 12"} (J2000)
ROLL:            16.602d
IMG_START_DATE:  20825 TJD;   150 DOY;   25/05/30
IMG_START_TIME:  68593.53 SOD {19:03:13.53} UT,   270.6 [sec] since BAT Trigger Time
FILTER:          7, U
EXPOSURE_ID:     770324629
X_OFFSET:        772 [pixels]
Y_OFFSET:        908 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       932
Y_GRB_POS:       1068
BINNING_INDEX:   1
IM_URL:          sw01319125000msuni0306.fits
SUN_POSTN:        67.96d {+04h 31m 51s}  +21.89d {+21d 53' 25"}
SUN_DIST:         45.92 [deg]   Sun_angle= 0.8 [hr] (West of Sun)
MOON_POSTN:      122.63d {+08h 10m 32s}  +24.30d {+24d 18' 02"}
MOON_DIST:        80.23 [deg]
MOON_ILLUM:      18 [%]
GAL_COORDS:      215.52,-51.51 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       45.84,-40.70 [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:     Fri 30 May 25 19:13:02 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     1319125,   Seg_Num: 0
POINT_RA:         55.142d {+03h 40m 34s} (J2000)
POINT_DEC:       -22.453d {-22d 27' 12"} (J2000)
ROLL:            16.602d
IMG_START_DATE:  20825 TJD;   150 DOY;   25/05/30
IMG_START_TIME:  68593.53 SOD {19:03:13.53} UT,   270.6 [sec] since BAT Trigger Time
FILTER:          7, U
EXPOSURE_ID:     770324629
X_OFFSET:        772 [pixels]
Y_OFFSET:        908 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       932
Y_GRB_POS:       1068
BINNING_INDEX:   1
IM_URL:          sw01319125000msuni0306.fits
SUN_POSTN:        67.96d {+04h 31m 51s}  +21.89d {+21d 53' 25"}
SUN_DIST:         45.92 [deg]   Sun_angle= 0.8 [hr] (West of Sun)
MOON_POSTN:      122.63d {+08h 10m 32s}  +24.30d {+24d 18' 04"}
MOON_DIST:        80.22 [deg]
MOON_ILLUM:      18 [%]
GAL_COORDS:      215.52,-51.51 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:       45.84,-40.70 [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 Circular #40585
R. Caputo (GSFC), R. Caputo (NASA/GSFC), R. Gupta (NASA GSFC), J. A. Kennea (PSU), M. J. Moss (GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL), T. M. Parsotan (GSFC), C. Salvaggio (INAF-OAB), T. Sbarrato (INAF-OAB) and B. Sbarufatti (INAF-OAB) report on behalf of the Neil Gehrels Swift Observatory Team:
At 18:58:42 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250530C (trigger=1319125). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 55.143, -22.459 which is
```
   RA(J2000) = 3h 40m 34s
   Dec(J2000) = -22d 27' 32"
```
with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). Due to a telemetry gap, the immediately-available BAT light curve does not start until T+8s. This shows possible activity out to T+40 s. The count rate was ~200 counts/sec (15-350 keV), at the start of available light curve. A more complete description of the emission, including the trigger time, will require the full downlinked data from the next ground pass.
The XRT began observing the field at 19:00:29.8 UT, 106.9 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 55.17029, -22.44825 which is equivalent to:
```
   RA(J2000)  = 03h 40m 40.87s
   Dec(J2000) = -22d 26' 53.7"
```
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This location is 98 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 in excess of the Galactic value (1.70 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.6 (+2.00/-1.76) x 10^21 cm^-2 (90% confidence).
UVOT took a finding chart exposure of 150 seconds with the White filter starting 111 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 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 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 0.025.
Burst Advocate for this burst is R. Caputo (regina.caputo 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 Circular #40586
J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester) report on behalf of the Swift-XRT team.
Using 2203 s of XRT Photon Counting mode data and 5 UVOT images for GRB 250530C, 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 = 55.17085, -22.44801 which is equivalent to:
```
RA (J2000): 03h 40m 41.00s
Dec (J2000): -22d 26' 52.8"
```
with an uncertainty of 2.0 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 #40587
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 (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 250530C ( R. Caputo et al., GCN 40585) errorbox 31007 sec after notice time and 31055 sec after trigger time at 2025-05-31 03:36:18 UT, with upper limit up to 18.6 mag. The observations began at zenith distance = 79 deg. The sun altitude is -23.6 deg.
The galactic latitude b = -51 deg., longitude l = 216 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2887090
We obtain a following upper limits.
```
Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

   31086 |         MASTER-SAAO |   C |    60 | 18.6 |        
   31086 |             MASTER- |   C |    60 | 13.1 |        
```
Filter C is a clear (unfiltred) band.
The observation and reduction will continue. The message may be cited.
GCN Circular #40589
M.A. Williams (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), S. Dichiara (PSU), J.A. Kennea (PSU) and P.A. Evans report on behalf of the Swift-XRT team:
We have analysed 7.7 ks of XRT data for GRB 250530C, from 90 s to 52.0 ks after the trigger. The data comprise 9 s in Windowed Timing (WT) mode (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=0.80 (+/-0.04).
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.55 (+0.20, -0.19). The best-fitting absorption column is 2.8 (+1.1, -0.9) x 10^21 cm^-2, in excess of the Galactic value of 1.7 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.0 x 10^-11 (6.1 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
```
Total column:	     2.8 (+1.1, -0.9) x 10^21 cm^-2
Galactic foreground: 1.7 x 10^20 cm^-2
Excess significance: 4.7 sigma
Photon index:	     1.55 (+0.20, -0.19)
```
If the light curve continues to decay with a power-law decay index of 0.80, the count rate at T+24 hours will be 0.012 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 6.0 x 10^-13 (7.3 x 10^-13) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01319125.
This circular is an official product of the Swift-XRT team.
GCN Circular #40591
I. Perez-Garcia, E. Fernandez-Garcia, S.-Y. Wu, M. D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy, and A. J. Castro-Tirado (IAA-CSIC, Granada), C. Perez del Pulgar (Univ. of Malaga), L. Hernandez-Garcia (Univ. of Valparaiso), M. Gritsevich (Univ. of Helsinki), Y.-D. Hu (GXU), B.-B. Zhang (Nanjing Univ.), and A. Maury (Space Obs., San Pedro de Atacama), on behalf of a larger collaboration, report:
Following the detection of GRB 250530C by Swift (Osborne et al., GCNC [40586](https://gcn.nasa.gov/circulars/40586)), the 0.6m BOOTES-7 robotic telescope at Space Observatory (San Pedro de Atacama, Chile) automatically responded to this high-energy event starting on May 21, 09:42:59 UT (i.e., 14.73 hour after detection), observing at low elevation (airmass ≈ 5). No optical transient is detected down to 16.4 mag (clear filter). On a coadd (11) of images at 09:51:43 UT (mid exposure time; i.e. 14.88 post burst), nothing is detected down to 18.2 mag.
We would like to thank the staff at San Pedro de Atacama Space Observatory for their excellent support.
GCN Circular #40609
D. Sadaula (GSFC/UMBC), S. D. Barthelmy (GSFC), R. Caputo (NASA/GSFC), R. Gupta (GSFC), H. A. Krimm (NSF), S. Laha (GSFC/UMBC), A. Y. Lien (U Tampa), C. B. Markwardt (GSFC), M. J. Moss (GSFC), D. M. Palmer (LANL), T. Parsotan (GSFC), T. Sakamoto (AGU) (i.e. the Swift-BAT team):
Using the data set from T-240 to T+885 sec from the recent telemetry downlink, we report further analysis of BAT GRB 250530C (trigger #1319125) (Caputo, et al., GCN Circ. 40585). The BAT ground-calculated position is RA, Dec = 55.146, -22.457 deg which is
```
   RA(J2000)  =  03h 40m 35.1s 
   Dec(J2000) = -22d 27' 26.1" 
```
with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment). The partial coding was 96%.
The mask-weighted light curve displays a complex structure with multiple dim pulses. The T90 (15-350 keV) is 42.86 +- 6.04 sec (estimated error including systematics).
The time-averaged spectrum from T-29.77 to T+21.26 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.32 +- 0.17. The fluence in the 15-150 keV band is 8.5 +- 0.9 x 10^-07 erg/cm2. The 1-sec peak photon flux measured from T-0.28 sec in the 15-150 keV band is 0.8 +- 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/1319125

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Jochen Greiner, last update: 02-Jun-2025 [Disclaimer]