Gamma-ray Burst 250818A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

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Results of Observations

GCN/BACODINE POSITION NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 18 Aug 25 01:31:48 UT
NOTICE_TYPE:     Swift-BAT GRB Position
TRIGGER_NUM:     1343270,   Seg_Num: 0
GRB_RA:          213.833d {+14h 15m 20s} (J2000),
                 214.290d {+14h 17m 10s} (current),
                 212.949d {+14h 11m 48s} (1950)
GRB_DEC:         -58.007d {-58d 00' 26"} (J2000),
                 -58.126d {-58d 07' 31"} (current),
                 -57.775d {-57d 46' 29"} (1950)
GRB_ERROR:       3.00 [arcmin radius, statistical only]
GRB_INTEN:       3088 [cnts]    Image_Peak=152 [image_cnts]
TRIGGER_DUR:     1.024 [sec] 
TRIGGER_INDEX:   147     E_range: 25-100 keV
BKG_INTEN:       18091 [cnts]
BKG_TIME:        5467.83 SOD {01:31:07.83} UT
BKG_DUR:         8 [sec]
GRB_DATE:        20905 TJD;   230 DOY;   25/08/18
GRB_TIME:        5482.74 SOD {01:31:22.74} UT
GRB_PHI:          86.16 [deg]
GRB_THETA:        27.61 [deg]
SOLN_STATUS:     0x20000003
RATE_SIGNIF:     15.68 [sigma]
IMAGE_SIGNIF:    8.69 [sigma]
MERIT_PARAMS:     +1  +0  +0  +0  +2  -1  +0  +1  -1  +0 
SUN_POSTN:       147.65d {+09h 50m 36s}  +13.06d {+13d 03' 30"}
SUN_DIST:         89.31 [deg]   Sun_angle= -4.4 [hr] (East of Sun)
MOON_POSTN:       78.32d {+05h 13m 17s}  +28.29d {+28d 17' 10"}
MOON_DIST:       137.46 [deg]
MOON_ILLUM:      29 [%]
GAL_COORDS:      313.88,  3.07 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      234.10,-41.36 [deg] ecliptic lon,lat of the burst (or transient)
COMMENTS:        SWIFT-BAT GRB Coordinates.  
COMMENTS:        This is a rate trigger.  
COMMENTS:        A point_source was found.  
COMMENTS:        This does not match any source in the on-board catalog.  
COMMENTS:        This does not match any source in the ground catalog.  
COMMENTS:        This is a GRB.  
COMMENTS:        This trigger occurred at longitude,latitude = 247.15,-19.96 [deg].  
COMMENTS:           
COMMENTS:        NOTE: This BAT event is temporally(1.0<100sec) coincident with the FERMI_GBM event (trignum=777173487).

red DSS finding chart ps-file

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Mon 18 Aug 25 01:31:51 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      45
TRIGGER_NUM:     777173487
GRB_RA:          177.750d {+11h 51m 00s} (J2000),
                 178.060d {+11h 52m 14s} (current),
                 177.153d {+11h 48m 37s} (1950)
GRB_DEC:         -74.183d {-74d 10' 59"} (J2000),
                 -74.326d {-74d 19' 32"} (current),
                 -73.905d {-73d 54' 18"} (1950)
GRB_ERROR:       16.70 [deg radius, statistical plus systematic]
GRB_INTEN:       50 [cnts/sec]
DATA_SIGNIF:     5.50 [sigma]
INTEG_TIME:      4.096 [sec]
GRB_DATE:        20905 TJD;   230 DOY;   25/08/18
GRB_TIME:        5483.00 SOD {01:31:23.00} UT
GRB_PHI:         150.00 [deg]
GRB_THETA:       160.00 [deg]
DATA_TIME_SCALE: 4.0960 [sec]
HARD_RATIO:      0.05
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      98%  GRB
2nd_MOST_LIKELY:   2%  Generic SGR
DETECTORS:       0,0,0, 0,0,0, 0,1,0, 0,1,0, 0,0,
SUN_POSTN:       147.65d {+09h 50m 36s}  +13.06d {+13d 03' 30"}
SUN_DIST:         89.46 [deg]   Sun_angle= -2.0 [hr] (East of Sun)
MOON_POSTN:       78.32d {+05h 13m 17s}  +28.29d {+28d 17' 10"}
MOON_DIST:       119.77 [deg]
MOON_ILLUM:      29 [%]
GAL_COORDS:      298.77,-11.80 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      233.86,-62.50 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250818063/quicklook/glg_lc_medres34_bn250818063.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 301.83,14.90 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.  
COMMENTS:           
COMMENTS:        NOTE: This GBM event is temporally(1.0<100sec) coincident with the SWIFT_BAT event (trignum=1343270).

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Mon 18 Aug 25 01:32:10 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      57
TRIGGER_NUM:     777173487
GRB_RA:          188.550d {+12h 34m 12s} (J2000),
                 188.961d {+12h 35m 51s} (current),
                 187.761d {+12h 31m 03s} (1950)
GRB_DEC:         -75.230d {-75d 13' 47"} (J2000),
                 -75.371d {-75d 22' 15"} (current),
                 -74.954d {-74d 57' 15"} (1950)
GRB_ERROR:       6.90 [deg radius, statistical only]
DATA_SIGNIF:     6.40 [sigma]
DATA_INTERVAL:   4.096 [sec]
GRB_DATE:        20905 TJD;   230 DOY;   25/08/18
GRB_TIME:        5483.00 SOD {01:31:23.00} UT
GRB_PHI:         149.00 [deg]
GRB_THETA:       157.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   4173 (Gnd S/W Version number)
SUN_POSTN:       147.65d {+09h 50m 36s}  +13.06d {+13d 03' 30"}
SUN_DIST:         91.94 [deg]   Sun_angle= -2.8 [hr] (East of Sun)
MOON_POSTN:       78.32d {+05h 13m 18s}  +28.29d {+28d 17' 11"}
MOON_DIST:       122.47 [deg]
MOON_ILLUM:      29 [%]
GAL_COORDS:      301.81,-12.40 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      238.99,-60.70 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250818063/quicklook/glg_lc_medres34_bn250818063.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_777173487.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file will not be created/available until ~15 min after the trigger.  
COMMENTS:        The POS_MAP_URL file will not be created/available until ~1.5 min after the notice.  
COMMENTS:           
COMMENTS:        NOTE: This GBM event is temporally(1.0<100sec) coincident with the SWIFT_BAT event (trignum=1343270).

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Mon 18 Aug 25 01:33:18 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      0
TRIGGER_NUM:     777173487
GRB_RA:          191.630d {+12h 46m 31s} (J2000),
                 192.050d {+12h 48m 12s} (current),
                 190.822d {+12h 43m 17s} (1950)
GRB_DEC:         -72.190d {-72d 11' 23"} (J2000),
                 -72.330d {-72d 19' 46"} (current),
                 -71.917d {-71d 55' 00"} (1950)
GRB_ERROR:       5.20 [deg radius, statistical only]
DATA_SIGNIF:     7.60 [sigma]
DATA_INTERVAL:   9.216 [sec]
GRB_DATE:        20905 TJD;   230 DOY;   25/08/18
GRB_TIME:        5483.00 SOD {01:31:23.00} UT
GRB_PHI:         141.00 [deg]
GRB_THETA:       157.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:       147.65d {+09h 50m 37s}  +13.06d {+13d 03' 29"}
SUN_DIST:         90.23 [deg]   Sun_angle= -3.0 [hr] (East of Sun)
MOON_POSTN:       78.34d {+05h 13m 21s}  +28.29d {+28d 17' 14"}
MOON_DIST:       123.99 [deg]
MOON_ILLUM:      29 [%]
GAL_COORDS:      302.55, -9.32 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      235.46,-58.10 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250818063/quicklook/glg_lc_medres34_bn250818063.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_777173487.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        This is likely a Long GRB.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file will not be created/available until ~15 min after the trigger.  
COMMENTS:        The POS_MAP_URL file will not be created/available until ~1.5 min after the notice.  
COMMENTS:           
COMMENTS:        NOTE: This GBM event is temporally(1.0<100sec) coincident with the SWIFT_BAT event (trignum=1343270).

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Mon 18 Aug 25 01:40:50 UT
NOTICE_TYPE:     Fermi-GBM Final Position
RECORD_NUM:      0
TRIGGER_NUM:     777173487
GRB_RA:          232.730d {+15h 30m 55s} (J2000),
                 233.282d {+15h 33m 08s} (current),
                 231.661d {+15h 26m 39s} (1950)
GRB_DEC:         -62.920d {-62d 55' 11"} (J2000),
                 -63.006d {-63d 00' 20"} (current),
                 -62.749d {-62d 44' 57"} (1950)
GRB_ERROR:       7.78 [deg radius, statistical only]
GRB_DATE:        20905 TJD;   230 DOY;   25/08/18
GRB_TIME:        5483.00 SOD {01:31:23.00} UT
GRB_PHI:         131.00 [deg]
GRB_THETA:       140.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:       147.66d {+09h 50m 38s}  +13.06d {+13d 03' 23"}
SUN_DIST:         99.65 [deg]   Sun_angle= -5.7 [hr] (East of Sun)
MOON_POSTN:       78.42d {+05h 13m 40s}  +28.29d {+28d 17' 35"}
MOON_DIST:       141.64 [deg]
MOON_ILLUM:      29 [%]
GAL_COORDS:      319.98, -5.45 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      248.12,-42.28 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250818063/quicklook/glg_lc_medres34_bn250818063.gif
LOC_URL:         http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250818063/quicklook/glg_locplot_all_bn250818063.png
COMMENTS:        Fermi-GBM Final Position.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file should be available by the time this FINAL notice is produced.  
COMMENTS:        This notice was generated completely by automated pipeline processing.  
COMMENTS:        This is likely a Long GRB.  
COMMENTS:           
COMMENTS:        NOTE: This GBM event is temporally(1.0<100sec) coincident with the SWIFT_BAT event (trignum=1343270).

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 18 Aug 25 01:33:32 UT
NOTICE_TYPE:     Swift-XRT Image
TRIGGER_NUM:     1343270,   Seg_Num: 0
GRB_RA:          213.8002d {+14h 15m 12.0s} (J2000),
                 214.2567d {+14h 17m 01.6s} (current),
                 212.9157d {+14h 11m 39.7s} (1950)
GRB_DEC:         -58.0134d {-58d 00' 48.2"} (J2000),
                 -58.1316d {-58d 07' 53.9"} (current),
                 -57.7809d {-57d 46' 51.0"} (1950)
GRB_ERROR:       4.7 [arcsec, radius, statistical plus systematic]
GRB_INTEN:       134 [cnts]
IMG_START_DATE:  20905 TJD;   230 DOY;   25/08/18
IMG_START_TIME:  5580.62 SOD {01:33:00.62} UT,   97.9 [sec] since BAT Trigger Time
CENTROID_X:      260.72,  raw= 261  [pixels]
CENTROID_Y:      316.93,  raw= 317  [pixels]
ROLL:            296.94 [deg]
GAIN:            2
MODE:            2, Short Image mode
WAVEFORM:        134
EXPO_TIME:       0.10 [sec]
GRB_POS_XRT_Y:   42.98
GRB_POS_XRT_Z:   -119.93
IMAGE_URL:       sw01343270000msxps_rw.img
SUN_POSTN:       147.65d {+09h 50m 37s}  +13.06d {+13d 03' 29"}
SUN_DIST:         89.29 [deg]   Sun_angle= -4.4 [hr] (East of Sun)
MOON_POSTN:       78.34d {+05h 13m 21s}  +28.29d {+28d 17' 15"}
MOON_DIST:       137.43 [deg]
MOON_ILLUM:      29 [%]
GAL_COORDS:      313.86,  3.07 [deg] galactic lon,lat of the burst
ECL_COORDS:      234.08,-41.37 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Image.  
COMMENTS:        The object found at this position is either a very bright burst or a cosmic ray hit.  
COMMENTS:        Examine the XRT Image to differentiate (CRs are much more compact); see examples at:  
COMMENTS:        http://www.swift.psu.edu/xrt/XRT_Postage_Stamp_Image_Photo_Gallery.htm .

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 18 Aug 25 01:33:22 UT
NOTICE_TYPE:     Swift-XRT Position
TRIGGER_NUM:     1343270,   Seg_Num: 0
GRB_RA:          213.8002d {+14h 15m 12.04s} (J2000),
                 214.2567d {+14h 17m 01.61s} (current),
                 212.9157d {+14h 11m 39.76s} (1950)
GRB_DEC:         -58.0134d {-58d 00' 48.2"} (J2000),
                 -58.1316d {-58d 07' 53.9"} (current),
                 -57.7809d {-57d 46' 51.0"} (1950)
GRB_ERROR:       4.9 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       1.09e-07 [erg/cm2/sec]
GRB_SIGNIF:      11.57 [sigma]
IMG_START_DATE:  20905 TJD;   230 DOY;   25/08/18
IMG_START_TIME:  5580.62 SOD {01:33:00.62} UT,   97.9 [sec] since BAT Trigger Time
TAM[0-3]:        327.66 237.14 261.55 243.76
AMPLIFIER:       2
WAVEFORM:        134
SUN_POSTN:       147.65d {+09h 50m 37s}  +13.06d {+13d 03' 29"}
SUN_DIST:         89.29 [deg]   Sun_angle= -4.4 [hr] (East of Sun)
MOON_POSTN:       78.34d {+05h 13m 21s}  +28.29d {+28d 17' 15"}
MOON_DIST:       137.43 [deg]
MOON_ILLUM:      29 [%]
GAL_COORDS:      313.86,  3.07 [deg] galactic lon,lat of the burst
ECL_COORDS:      234.08,-41.37 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Coordinates.  
COMMENTS:        The XRT position is 1.11 arcmin from the BAT position.  
COMMENTS:        The object found at this position is either a very bright burst or a cosmic ray hit.  
COMMENTS:        Examine the XRT Image to differentiate (CRs are much more compact); see examples at:  
COMMENTS:        http://www.swift.psu.edu/xrt/XRT_Postage_Stamp_Image_Photo_Gallery.htm .

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 18 Aug 25 01:33:35 UT
NOTICE_TYPE:     Swift-XRT Processed Image
TRIGGER_NUM:     1343270,   Seg_Num: 0
GRB_RA:          213.8002d {+14h 15m 12.0s} (J2000),
                 214.2567d {+14h 17m 01.6s} (current),
                 212.9157d {+14h 11m 39.7s} (1950)
GRB_DEC:         -58.0134d {-58d 00' 48.2"} (J2000),
                 -58.1316d {-58d 07' 53.9"} (current),
                 -57.7809d {-57d 46' 51.0"} (1950)
GRB_ERROR:       4.7 [arcsec, radius, statistical plus systematic]
GRB_INTEN:       134 [cnts]
IMG_START_DATE:  20905 TJD;   230 DOY;   25/08/18
IMG_START_TIME:  5580.62 SOD {01:33:00.62} UT,   97.9 [sec] since BAT Trigger Time
CENTROID_X:      260.72,  raw= 261  [pixels]
CENTROID_Y:      316.93,  raw= 317  [pixels]
ROLL:            296.94 [deg]
GAIN:            2
MODE:            2, Short Image mode
WAVEFORM:        134
EXPO_TIME:       0.10 [sec]
GRB_POS_XRT_Y:   42.98
GRB_POS_XRT_Z:   -119.93
IMAGE_URL:       sw01343270000msxps_rw.img
SUN_POSTN:       147.65d {+09h 50m 37s}  +13.06d {+13d 03' 29"}
SUN_DIST:         89.29 [deg]   Sun_angle= -4.4 [hr] (East of Sun)
MOON_POSTN:       78.34d {+05h 13m 21s}  +28.29d {+28d 17' 15"}
MOON_DIST:       137.43 [deg]
MOON_ILLUM:      29 [%]
GAL_COORDS:      313.86,  3.07 [deg] galactic lon,lat of the burst
ECL_COORDS:      234.08,-41.37 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Processed Image.  
COMMENTS:        The object found at this position is either a very bright burst or a cosmic ray hit.  
COMMENTS:        Examine the XRT Image to differentiate (CRs are much more compact); see examples at:  
COMMENTS:        http://www.swift.psu.edu/xrt/XRT_Postage_Stamp_Image_Photo_Gallery.htm .

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 18 Aug 25 01:39:20 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     1343270,   Seg_Num: 0
POINT_RA:        213.866d {+14h 15m 28s} (J2000)
POINT_DEC:       -58.018d {-58d 01' 03"} (J2000)
POINT_ROLL:      296.939d
IMG_START_DATE:  20905 TJD;   230 DOY;   25/08/18
IMG_START_TIME:  5645.15 SOD {01:34:05.15} UT,   162.4 [sec] since BAT Trigger Time
FILTER:          7, U
BKG_MEAN:        0.525
N_STARS:         190
X_OFFSET:        0 [pixels]
Y_OFFSET:        0 [pixels]
X_MAX:           2047 [pixels]
Y_MAX:           2047 [pixels]
DET_THRESH:      7
PHOTO_THRESH:    3
SL_URL:          sw01343270000msufc0162.fits
SUN_POSTN:       147.66d {+09h 50m 37s}  +13.06d {+13d 03' 24"}
SUN_DIST:         89.32 [deg]   Sun_angle= -4.4 [hr] (East of Sun)
MOON_POSTN:       78.40d {+05h 13m 36s}  +28.29d {+28d 17' 31"}
MOON_DIST:       137.43 [deg]
MOON_ILLUM:      29 [%]
GAL_COORDS:      313.89,  3.05 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      234.13,-41.36 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 18 Aug 25 01:39:38 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     1343270,   Seg_Num: 0
POINT_RA:        213.866d {+14h 15m 28s} (J2000)
POINT_DEC:       -58.018d {-58d 01' 03"} (J2000)
POINT_ROLL:      296.939d
IMG_START_DATE:  20905 TJD;   230 DOY;   25/08/18
IMG_START_TIME:  5645.15 SOD {01:34:05.15} UT,   162.4 [sec] since BAT Trigger Time
FILTER:          7, U
BKG_MEAN:        0.525
N_STARS:         190
X_OFFSET:        0 [pixels]
Y_OFFSET:        0 [pixels]
X_MAX:           2047 [pixels]
Y_MAX:           2047 [pixels]
DET_THRESH:      7
PHOTO_THRESH:    3
SL_URL:          sw01343270000msufc0162.fits
SUN_POSTN:       147.66d {+09h 50m 38s}  +13.06d {+13d 03' 24"}
SUN_DIST:         89.32 [deg]   Sun_angle= -4.4 [hr] (East of Sun)
MOON_POSTN:       78.41d {+05h 13m 37s}  +28.29d {+28d 17' 32"}
MOON_DIST:       137.43 [deg]
MOON_ILLUM:      29 [%]
GAL_COORDS:      313.89,  3.05 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      234.13,-41.36 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Source List.  
COMMENTS:        All 4 attachments are included.

GCN Circular #41402
The Fermi GBM team reports the detection of a likely LONG GRB
At 01:31:22 UT on 18 Aug 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250818A (trigger 777173487.99771 / 250818063).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 232.7, Dec = -62.9 (J2000 degrees, equivalent to J2000 15h 30m, -62d 53'), with a statistical uncertainty of 7.8 degrees.
The angle from the Fermi LAT boresight is 140.0 degrees.
The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250818063/quicklook/glg_skymap_all_bn250818063.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/bn250818063/quicklook/glg_healpix_all_bn250818063.fit
The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250818063/quicklook/glg_lc_medres34_bn250818063.gif

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 18 Aug 25 01:42:24 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     1343270,   Seg_Num: 0
POINT_RA:        213.866d {+14h 15m 28s} (J2000)
POINT_DEC:       -58.018d {-58d 01' 03"} (J2000)
ROLL:            296.939d
IMG_START_DATE:  20905 TJD;   230 DOY;   25/08/18
IMG_START_TIME:  5645.15 SOD {01:34:05.15} UT,   162.4 [sec] since BAT Trigger Time
FILTER:          7, U
EXPOSURE_ID:     777173681
X_OFFSET:        965 [pixels]
Y_OFFSET:        684 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       1125
Y_GRB_POS:       844
BINNING_INDEX:   1
IM_URL:          sw01343270000msuni0199.fits
SUN_POSTN:       147.66d {+09h 50m 38s}  +13.06d {+13d 03' 22"}
SUN_DIST:         89.32 [deg]   Sun_angle= -4.4 [hr] (East of Sun)
MOON_POSTN:       78.44d {+05h 13m 45s}  +28.29d {+28d 17' 39"}
MOON_DIST:       137.42 [deg]
MOON_ILLUM:      29 [%]
GAL_COORDS:      313.89,  3.05 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      234.13,-41.36 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Image.  
COMMENTS:        The GRB Position came from the XRT Position Command.  
COMMENTS:        The image has 2x2 binning (compression).

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 18 Aug 25 01:42:34 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     1343270,   Seg_Num: 0
POINT_RA:        213.866d {+14h 15m 28s} (J2000)
POINT_DEC:       -58.018d {-58d 01' 03"} (J2000)
ROLL:            296.939d
IMG_START_DATE:  20905 TJD;   230 DOY;   25/08/18
IMG_START_TIME:  5645.15 SOD {01:34:05.15} UT,   162.4 [sec] since BAT Trigger Time
FILTER:          7, U
EXPOSURE_ID:     777173681
X_OFFSET:        965 [pixels]
Y_OFFSET:        684 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       1125
Y_GRB_POS:       844
BINNING_INDEX:   1
IM_URL:          sw01343270000msuni0199.fits
SUN_POSTN:       147.66d {+09h 50m 38s}  +13.06d {+13d 03' 21"}
SUN_DIST:         89.32 [deg]   Sun_angle= -4.4 [hr] (East of Sun)
MOON_POSTN:       78.44d {+05h 13m 45s}  +28.29d {+28d 17' 40"}
MOON_DIST:       137.42 [deg]
MOON_ILLUM:      29 [%]
GAL_COORDS:      313.89,  3.05 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      234.13,-41.36 [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 Circular #41403
S. B. Cenko (GSFC), C. Gronwall (PSU), R. Gupta (NASA GSFC), H. A. Krimm (NSF), A. Y. Lien (U Tampa), 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 01:31:22 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250818A (trigger=1343270). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 213.833, -58.007 which is
```
   RA(J2000) = 14h 15m 20s
   Dec(J2000) = -58d 00' 26"
```
with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). Due to a communications gap, only the BAT light curve after T+8s is available. This later lightcurve shows a complex structure out to at least T+150s. The peak count rate in the available lightcurve was ~5500 counts/sec (15-350 keV), at ~80 sec after the trigger.
The XRT began observing the field at 01:33:00.6 UT, 97.9 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 213.8002, -58.0134 which is equivalent to:
```
   RA(J2000)  = 14h 15m 12.05s
   Dec(J2000) = -58d 00' 48.2"
```
with an uncertainty of 4.9 arcseconds (radius, 90% containment). This location is 66 arcseconds from the BAT onboard position, within the BAT error circle. Despite the onboard localisation, no X-ray source was detected in 45 s of promptly-downlinked data, suggesting that the initial centroid may equally have been a cosmic ray. This position should therefore be treated with caution.
The initial flux in the 0.1 s image was 1.09e-07 erg cm^-2 s^-1 (0.2-10 keV).
UVOT took a finding chart exposure of 250 seconds with the U filter starting 162 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. Because of the density of catalogued stars, further analysis is required to report an upper limit for any afterglow in the region. 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 Circular #41411
C. de Barra (University College Dublin) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 01:31:23.00 UT on 18 August 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250818A (trigger 777173487/250818063) which was also detected by Swift BAT (S. B. Cenko et al. 2025, GCN 41403). The Fermi GBM on-ground location is consistent with the Swift BAT position.
The angle from the Fermi LAT boresight is 146 degrees.
The GBM light curve consists of multiple emission episodes with a duration (T90) of about 101 s (50-300 keV). The time-averaged spectrum from T0-9.2 to T0+94.2 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -0.83 +/- 0.14 and the cutoff energy, parameterized as Epeak, is 83 +/- 5 keV.
The event fluence (10-1000 keV) in this time interval is (1.62 +/- 0.06)E-05 erg/cm^2. The 1-sec peak photon flux measured starting from T0+78 s in the 10-1000 keV band is 8.8 +/- 0.7 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 #41413
J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester) report on behalf of the Swift-XRT team.
Using 431 s of XRT Photon Counting mode data and 2 UVOT images for GRB 250818A, 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 = 213.80012, -58.01403 which is equivalent to:
```
RA (J2000): 14h 15m 12.03s
Dec (J2000): -58d 00' 50.5"
```
with an uncertainty of 2.4 arcsec (radius, 90% confidence).
This position may be improved as more data are received. The latest position can be viewed at 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 #41416
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 250818A 162 s after the BAT trigger (Cenko et al., GCN Circ. 41403). No optical afterglow consistent with the XRT position (Osborn et al., GCN Circ. 41403) 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

u (fc)             162          412          246         >20.3
b                  418          438           19         >18.6
v                32212        32641          429.        >20.2
```
The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 1.602 in the direction of the burst (Schlegel et al. 1998).
GCN Circular #41423
S. Laha (GSFC/UMBC), S. D. Barthelmy (GSFC), S. B. Cenko (GSFC), R. Gupta (GSFC), H. A. Krimm (NSF), 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-721 to T+482 sec from the recent telemetry downlink, we report further analysis of BAT GRB 250818A (trigger #1343270) (Cenko et al., GCN Circ. 41403). The BAT ground-calculated position is RA, Dec = 213.816, -58.010 deg which is
```
   RA(J2000)  =  14h 15m 15.8s
   Dec(J2000) = -58d 00' 36.5"
```
with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment). The partial coding was 44%.
The mask-weighted light curve shows a multi-peaked structure that starts at ~T-15 s and ends at ~T+140 s. The main peak occurs at ~T+78 s. T90 (15-350 keV) is 106.86 +- 3.79 sec (estimated error including systematics).
The time-averaged spectrum from T-15.92 to T+141.43 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.85 +- 0.04. The fluence in the 15-150 keV band is 1.2 +- 0.03 x 10^-05 erg/cm2. The 1-sec peak photon flux measured from T+77.63 sec in the 15-150 keV band is 5.5 +- 0.3 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/1343270/
GCN Circular #41425
D.N. Burrows (PSU), M.A. Williams (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA), S. Campana (INAF-OAB) and P.A. Evans report on behalf of the Swift-XRT team:
We have analysed 5.5 ks of XRT data for GRB 250818A, from 86 s to 83.2 ks after the trigger. The data comprise 247 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 late-time light curve (from T0+4.4 ks) can be modelled with a power-law decay with a decay index of alpha=1.15 (+/-0.04).
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 2.18 (+/-0.06). The best-fitting absorption column is 1.63 (+/-0.08) x 10^22 cm^-2, in excess of the Galactic value of 6.8 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.76 (+0.17, -0.16) and a best-fitting absorption column of 1.69 (+0.25, -0.23) x 10^22 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 6.6 x 10^-11 (1.2 x 10^-10) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
```
Total column:	     1.69 (+0.25, -0.23) x 10^22 cm^-2
Galactic foreground: 6.8 x 10^21 cm^-2
Excess significance: 7.2 sigma
Photon index:	     1.76 (+0.17, -0.16)
```
The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/01343270.
This circular is an official product of the Swift-XRT team.

GCN Circular #41427
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-OAFA robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Argentina (OAFA observatory of San Juan National University) was pointed to the Swift GRB 250818A ( S. B. Cenko et al., GCN 41403) errorbox 30 sec after notice time and 64 sec after trigger time at 2025-08-18 01:32:26 UT, with upper limit up to 18.4 mag. The observations began at zenith distance = 52 deg. The sun altitude is -43.8 deg.
The galactic latitude b = 3 deg., longitude l = 314 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=2971328
We obtain a following upper limits.

Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

      69 |         MASTER-OAFA |   C |    10 | 17.6 |        
      90 |         MASTER-OAFA |   C |    20 | 17.4 |        
     116 |         MASTER-OAFA |   C |    20 | 17.6 |        
     148 |         MASTER-OAFA |   C |    30 | 18.0 |        
     184 |         MASTER-OAFA |   C |    30 | 18.1 |        
     225 |         MASTER-OAFA |   C |    40 | 18.3 |        
     276 |         MASTER-OAFA |   C |    50 | 18.1 |        
     337 |         MASTER-OAFA |   C |    60 | 18.4 |        
     408 |         MASTER-OAFA |   C |    70 | 18.1 |        
     494 |         MASTER-OAFA |   C |    90 | 18.1 |        
     601 |         MASTER-OAFA |   C |   110 | 18.2 |        
     722 |         MASTER-OAFA |   C |   120 | 18.1 |        
     848 |         MASTER-OAFA |   C |   120 | 18.0 |        
     974 |         MASTER-OAFA |   C |   120 | 18.1 |        
    1100 |         MASTER-OAFA |   C |   120 | 18.1 |        
    1226 |         MASTER-OAFA |   C |   120 | 18.1 |        
    1352 |         MASTER-OAFA |   C |   120 | 18.1 |        
    1478 |         MASTER-OAFA |   C |   120 | 18.1 |        
    1604 |         MASTER-OAFA |   C |   120 | 18.0 |        
    1730 |         MASTER-OAFA |   C |   120 | 18.1 |        
    1856 |         MASTER-OAFA |   C |   120 | 18.3 |        
    1982 |         MASTER-OAFA |   C |   120 | 18.1 |        
    2108 |         MASTER-OAFA |   C |   120 | 18.1 |        
    2234 |         MASTER-OAFA |   C |   120 | 18.1 |        
    2361 |         MASTER-OAFA |   C |   120 | 18.0 |        
    2487 |         MASTER-OAFA |   C |   120 | 18.0 |        
    2613 |         MASTER-OAFA |   C |   120 | 17.9 |        
    2739 |         MASTER-OAFA |   C |   120 | 17.8 |        
    2865 |         MASTER-OAFA |   C |   120 | 17.9 |        
    2992 |         MASTER-OAFA |   C |   120 | 17.8 |        
    3118 |         MASTER-OAFA |   C |   120 | 17.9 |        
    3244 |         MASTER-OAFA |   C |   120 | 18.1 |        
    3370 |         MASTER-OAFA |   C |   120 | 18.1 |        
    3496 |         MASTER-OAFA |   C |   120 | 18.0 |        
    3622 |         MASTER-OAFA |   C |   120 | 18.1 |        
    3748 |         MASTER-OAFA |   C |   120 | 18.0 |        
    3874 |         MASTER-OAFA |   C |   120 | 18.1 |        
    4000 |         MASTER-OAFA |   C |   120 | 18.0 |        
    4126 |         MASTER-OAFA |   C |   120 | 18.0 |        
    4253 |         MASTER-OAFA |   C |   120 | 18.0 |        
    4379 |         MASTER-OAFA |   C |   120 | 18.1 |        
    4505 |         MASTER-OAFA |   C |   120 | 18.1 |        
    4631 |         MASTER-OAFA |   C |   120 | 17.9 |        
    4757 |         MASTER-OAFA |   C |   120 | 18.0 |        
    4884 |         MASTER-OAFA |   C |   120 | 18.0 |        
    5010 |         MASTER-OAFA |   C |   120 | 18.1 |        
    5135 |         MASTER-OAFA |   C |   120 | 18.0 |        
    5261 |         MASTER-OAFA |   C |   120 | 18.1 |        
    5388 |         MASTER-OAFA |   C |   120 | 18.1 |        
    5515 |         MASTER-OAFA |   C |   120 | 18.0 |        
    5640 |         MASTER-OAFA |   C |   120 | 18.0 |

Filter C is a clear (unfiltred) band.
The observation and reduction will continue. The message may be cited.

GCN Circular #41438
Harsha K. H. (IUCAA), Anuraag Arya (IITB), G. Waratkar (IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:
Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long-duration GRB 250818A which was also detected by Fermi/GBM (Fermi GBM Team, GCN Circ. 41402), and Swift/BAT (Cenko et. al., GCN Circ. 41403).
The source was clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2025-08-18 01:32:39.98 UTC. The measured peak count rate associated with the burst is 214 (+77, -0) counts/s above the background in the combined data of all quadrants, with a total of 6518 (+769, -832) counts. The local mean background count rate was 1187 (+3, -3) counts/s. We measure a T90 of 89 (+7, -5) s from the cumulative Veto light curve.
CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.
CZTI GRB detections are reported regularly on the payload site at: http://astrosat.iucaa.in/czti/?q=grb

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Jochen Greiner, last update: 20-Aug-2025 [Disclaimer]