Gamma-ray Burst 180818B

(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:     Sat 18 Aug 18 12:32:40 UT
NOTICE_TYPE:     Swift-BAT GRB Position
TRIGGER_NUM:     853882,   Seg_Num: 0
GRB_RA:          104.235d {+06h 56m 56s} (J2000),
                 104.556d {+06h 58m 13s} (current),
                 103.373d {+06h 53m 29s} (1950)
GRB_DEC:         +39.329d {+39d 19' 43"} (J2000),
                 +39.303d {+39d 18' 10"} (current),
                 +39.395d {+39d 23' 42"} (1950)
GRB_ERROR:       3.00 [arcmin radius, statistical only]
GRB_INTEN:       0 [cnts]    Image_Peak=2460 [image_cnts]
TRIGGER_DUR:     128.000 [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:        18348 TJD;   230 DOY;   18/08/18
GRB_TIME:        45018.55 SOD {12:30:18.55} UT
GRB_PHI:          -8.70 [deg]
GRB_THETA:        29.40 [deg]
SOLN_STATUS:     0x20000013
RATE_SIGNIF:     0.00 [sigma]
IMAGE_SIGNIF:    10.17 [sigma]
MERIT_PARAMS:     +1  +0  +0  +7  +1  +0  +0  +0 -68  +0 
SUN_POSTN:       147.79d {+09h 51m 10s}  +13.01d {+13d 00' 36"}
SUN_DIST:         46.22 [deg]   Sun_angle= 2.9 [hr] (West of Sun)
MOON_POSTN:      236.92d {+15h 47m 40s}  -14.91d {-14d 54' 41"}
MOON_DIST:       131.82 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:      177.24, 17.79 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      101.44, 16.45 [deg] ecliptic lon,lat of the burst (or transient)
COMMENTS:        SWIFT-BAT GRB Coordinates.  
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 = 327.42,20.48 [deg].  
COMMENTS:           
COMMENTS:        NOTE: This BAT event is temporally(81.0<100sec) coincident with the FERMI_GBM event (trignum=556288142).

red DSS finding chart ps-file

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Sat 18 Aug 18 12:29:21 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      45
TRIGGER_NUM:     556288142
GRB_RA:           71.617d {+04h 46m 28s} (J2000),
                  71.930d {+04h 47m 43s} (current),
                  70.776d {+04h 43m 06s} (1950)
GRB_DEC:         +37.300d {+37d 17' 60"} (J2000),
                 +37.332d {+37d 19' 57"} (current),
                 +37.210d {+37d 12' 37"} (1950)
GRB_ERROR:       33.95 [deg radius, statistical plus systematic]
GRB_INTEN:       43 [cnts/sec]
DATA_SIGNIF:     4.70 [sigma]
INTEG_TIME:      4.096 [sec]
GRB_DATE:        18348 TJD;   230 DOY;   18/08/18
GRB_TIME:        44937.24 SOD {12:28:57.24} UT
GRB_PHI:          62.00 [deg]
GRB_THETA:       100.00 [deg]
DATA_TIME_SCALE: 4.0960 [sec]
HARD_RATIO:      1.53
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      67%  GRB
2nd_MOST_LIKELY:  20%  Solar Flare
DETECTORS:       0,0,1, 0,0,1, 0,0,0, 0,0,0, 0,0,
SUN_POSTN:       147.79d {+09h 51m 10s}  +13.01d {+13d 00' 39"}
SUN_DIST:         70.99 [deg]   Sun_angle= 5.1 [hr] (West of Sun)
MOON_POSTN:      236.89d {+15h 47m 33s}  -14.90d {-14d 54' 15"}
MOON_DIST:       153.90 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:      166.01, -5.21 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       74.96, 14.82 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2018/bn180818520/quicklook/glg_lc_medres34_bn180818520.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 6.72,-2.77 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Sat 18 Aug 18 12:29:36 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      57
TRIGGER_NUM:     556288142
GRB_RA:           90.410d {+06h 01m 38s} (J2000),
                  90.767d {+06h 03m 04s} (current),
                  89.452d {+05h 57m 49s} (1950)
GRB_DEC:         +48.740d {+48d 44' 24"} (J2000),
                 +48.739d {+48d 44' 20"} (current),
                 +48.740d {+48d 44' 23"} (1950)
GRB_ERROR:       18.46 [deg radius, statistical only]
DATA_SIGNIF:     5.70 [sigma]
DATA_INTERVAL:   4.096 [sec]
GRB_DATE:        18348 TJD;   230 DOY;   18/08/18
GRB_TIME:        44937.24 SOD {12:28:57.24} UT
GRB_PHI:          44.00 [deg]
GRB_THETA:       102.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   4153 (Gnd S/W Version number)
SUN_POSTN:       147.79d {+09h 51m 10s}  +13.01d {+13d 00' 39"}
SUN_DIST:         58.74 [deg]   Sun_angle= 3.8 [hr] (West of Sun)
MOON_POSTN:      236.89d {+15h 47m 34s}  -14.91d {-14d 54' 17"}
MOON_DIST:       136.26 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:      164.20, 12.49 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:       90.30, 25.30 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2018/bn180818520/quicklook/glg_lc_medres34_bn180818520.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_556288142.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.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Sat 18 Aug 18 12:34:17 UT
NOTICE_TYPE:     Swift-XRT Image
TRIGGER_NUM:     853882,   Seg_Num: 0
GRB_RA:          104.2242d {+06h 56m 53.8s} (J2000),
                 104.5451d {+06h 58m 10.8s} (current),
                 103.3620d {+06h 53m 26.8s} (1950)
GRB_DEC:         +39.3165d {+39d 18' 59.3"} (J2000),
                 +39.2907d {+39d 17' 26.6"} (current),
                 +39.3829d {+39d 22' 58.3"} (1950)
GRB_ERROR:       5.0 [arcsec, radius, statistical plus systematic]
GRB_INTEN:       65 [cnts]
IMG_START_DATE:  18348 TJD;   230 DOY;   18/08/18
IMG_START_TIME:  45222.44 SOD {12:33:42.44} UT,   203.9 [sec] since BAT Trigger Time
CENTROID_X:      336.56,  raw= 337  [pixels]
CENTROID_Y:      286.48,  raw= 286  [pixels]
ROLL:            112.18 [deg]
GAIN:            1
MODE:            3, Long Image mode
WAVEFORM:        134
EXPO_TIME:       2.50 [sec]
GRB_POS_XRT_Y:   -28.09
GRB_POS_XRT_Z:   71.10
IMAGE_URL:       sw00853882000msxps_rw.img
SUN_POSTN:       147.79d {+09h 51m 10s}  +13.01d {+13d 00' 35"}
SUN_DIST:         46.23 [deg]   Sun_angle= 2.9 [hr] (West of Sun)
MOON_POSTN:      236.93d {+15h 47m 43s}  -14.92d {-14d 54' 54"}
MOON_DIST:       131.85 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:      177.25, 17.78 [deg] galactic lon,lat of the burst
ECL_COORDS:      101.43, 16.44 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Image.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Sat 18 Aug 18 12:34:19 UT
NOTICE_TYPE:     Swift-XRT Processed Image
TRIGGER_NUM:     853882,   Seg_Num: 0
GRB_RA:          104.2242d {+06h 56m 53.8s} (J2000),
                 104.5451d {+06h 58m 10.8s} (current),
                 103.3620d {+06h 53m 26.8s} (1950)
GRB_DEC:         +39.3165d {+39d 18' 59.3"} (J2000),
                 +39.2907d {+39d 17' 26.6"} (current),
                 +39.3829d {+39d 22' 58.3"} (1950)
GRB_ERROR:       5.0 [arcsec, radius, statistical plus systematic]
GRB_INTEN:       65 [cnts]
IMG_START_DATE:  18348 TJD;   230 DOY;   18/08/18
IMG_START_TIME:  45222.44 SOD {12:33:42.44} UT,   203.9 [sec] since BAT Trigger Time
CENTROID_X:      336.56,  raw= 337  [pixels]
CENTROID_Y:      286.48,  raw= 286  [pixels]
ROLL:            112.18 [deg]
GAIN:            1
MODE:            3, Long Image mode
WAVEFORM:        134
EXPO_TIME:       2.50 [sec]
GRB_POS_XRT_Y:   -28.09
GRB_POS_XRT_Z:   71.10
IMAGE_URL:       sw00853882000msxps_rw.img
SUN_POSTN:       147.79d {+09h 51m 10s}  +13.01d {+13d 00' 35"}
SUN_DIST:         46.23 [deg]   Sun_angle= 2.9 [hr] (West of Sun)
MOON_POSTN:      236.93d {+15h 47m 44s}  -14.92d {-14d 54' 54"}
MOON_DIST:       131.85 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:      177.25, 17.78 [deg] galactic lon,lat of the burst
ECL_COORDS:      101.43, 16.44 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Processed Image.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Sat 18 Aug 18 12:35:15 UT
NOTICE_TYPE:     Swift-BAT GRB Lightcurve
TRIGGER_NUM:     853882,   Seg_Num: 0
GRB_RA:          104.235d {+06h 56m 56s} (J2000),
                 104.556d {+06h 58m 13s} (current),
                 103.373d {+06h 53m 29s} (1950)
GRB_DEC:         +39.329d {+39d 19' 43"} (J2000),
                 +39.303d {+39d 18' 10"} (current),
                 +39.395d {+39d 23' 42"} (1950)
GRB_DATE:        18348 TJD;   230 DOY;   18/08/18
GRB_TIME:        45018.55 SOD {12:30:18.55} UT
TRIGGER_INDEX:   20000
GRB_PHI:          -8.70 [deg]
GRB_THETA:        29.40 [deg]
DELTA_TIME:      0.00 [sec]
TRIGGER_DUR:     128.000 [sec] 
SOLN_STATUS:     0x13
RATE_SIGNIF:     0.00 [sigma]
IMAGE_SIGNIF:    10.17 [sigma]
LC_URL:          sw00853882000msb.lc
SUN_POSTN:       147.79d {+09h 51m 10s}  +13.01d {+13d 00' 34"}
SUN_DIST:         46.22 [deg]   Sun_angle= 2.9 [hr] (West of Sun)
MOON_POSTN:      236.94d {+15h 47m 45s}  -14.92d {-14d 55' 02"}
MOON_DIST:       131.84 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:      177.24, 17.79 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      101.44, 16.45 [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 = 327.42,20.48 [deg].

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Sat 18 Aug 18 12:34:07 UT
NOTICE_TYPE:     Swift-XRT Position
TRIGGER_NUM:     853882,   Seg_Num: 0
GRB_RA:          104.2242d {+06h 56m 53.80s} (J2000),
                 104.5451d {+06h 58m 10.82s} (current),
                 103.3620d {+06h 53m 26.87s} (1950)
GRB_DEC:         +39.3165d {+39d 18' 59.3"} (J2000),
                 +39.2907d {+39d 17' 26.6"} (current),
                 +39.3829d {+39d 22' 58.3"} (1950)
GRB_ERROR:       5.4 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       1.30e-09 [erg/cm2/sec]
GRB_SIGNIF:      8.06 [sigma]
IMG_START_DATE:  18348 TJD;   230 DOY;   18/08/18
IMG_START_TIME:  45222.44 SOD {12:33:42.44} UT,   203.9 [sec] since BAT Trigger Time
TAM[0-3]:        327.65 237.19 261.63 243.51
AMPLIFIER:       2
WAVEFORM:        134
SUN_POSTN:       147.79d {+09h 51m 10s}  +13.01d {+13d 00' 35"}
SUN_DIST:         46.23 [deg]   Sun_angle= 2.9 [hr] (West of Sun)
MOON_POSTN:      236.93d {+15h 47m 43s}  -14.91d {-14d 54' 53"}
MOON_DIST:       131.85 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:      177.25, 17.78 [deg] galactic lon,lat of the burst
ECL_COORDS:      101.43, 16.44 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Coordinates.  
COMMENTS:        The XRT position is 0.88 arcmin from the BAT position.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Sat 18 Aug 18 12:38:00 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     853882,   Seg_Num: 0
POINT_RA:        104.252d {+06h 57m 00s} (J2000)
POINT_DEC:       +39.316d {+39d 18' 57"} (J2000)
POINT_ROLL:      112.183d
IMG_START_DATE:  18348 TJD;   230 DOY;   18/08/18
IMG_START_TIME:  45231.10 SOD {12:33:51.10} UT,   212.5 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        5.399
N_STARS:         61
X_OFFSET:        360 [pixels]
Y_OFFSET:        584 [pixels]
X_MAX:           1319 [pixels]
Y_MAX:           1543 [pixels]
DET_THRESH:      20
PHOTO_THRESH:    11
SL_URL:          sw00853882000msufc0212.fits
SUN_POSTN:       147.80d {+09h 51m 11s}  +13.01d {+13d 00' 32"}
SUN_DIST:         46.21 [deg]   Sun_angle= 2.9 [hr] (West of Sun)
MOON_POSTN:      236.96d {+15h 47m 51s}  -14.92d {-14d 55' 23"}
MOON_DIST:       131.86 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:      177.25, 17.80 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      101.45, 16.44 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Sat 18 Aug 18 12:38:21 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     853882,   Seg_Num: 0
POINT_RA:        104.252d {+06h 57m 00s} (J2000)
POINT_DEC:       +39.316d {+39d 18' 57"} (J2000)
POINT_ROLL:      112.183d
IMG_START_DATE:  18348 TJD;   230 DOY;   18/08/18
IMG_START_TIME:  45231.10 SOD {12:33:51.10} UT,   212.5 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        5.399
N_STARS:         61
X_OFFSET:        360 [pixels]
Y_OFFSET:        584 [pixels]
X_MAX:           1319 [pixels]
Y_MAX:           1543 [pixels]
DET_THRESH:      20
PHOTO_THRESH:    11
SL_URL:          sw00853882000msufc0212.fits
SUN_POSTN:       147.80d {+09h 51m 11s}  +13.01d {+13d 00' 32"}
SUN_DIST:         46.21 [deg]   Sun_angle= 2.9 [hr] (West of Sun)
MOON_POSTN:      236.97d {+15h 47m 52s}  -14.92d {-14d 55' 26"}
MOON_DIST:       131.86 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:      177.25, 17.80 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      101.45, 16.44 [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:     Sat 18 Aug 18 12:39:28 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     853882,   Seg_Num: 0
POINT_RA:        104.252d {+06h 57m 00s} (J2000)
POINT_DEC:       +39.316d {+39d 18' 57"} (J2000)
ROLL:            112.183d
IMG_START_DATE:  18348 TJD;   230 DOY;   18/08/18
IMG_START_TIME:  45231.10 SOD {12:33:51.10} UT,   212.5 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     556288452
X_OFFSET:        704 [pixels]
Y_OFFSET:        1003 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       864
Y_GRB_POS:       1163
BINNING_INDEX:   1
IM_URL:          sw00853882000msuni0234.fits
SUN_POSTN:       147.80d {+09h 51m 11s}  +13.01d {+13d 00' 31"}
SUN_DIST:         46.21 [deg]   Sun_angle= 2.9 [hr] (West of Sun)
MOON_POSTN:      236.98d {+15h 47m 54s}  -14.93d {-14d 55' 34"}
MOON_DIST:       131.87 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:      177.25, 17.80 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      101.45, 16.44 [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:     Sat 18 Aug 18 12:39:38 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     853882,   Seg_Num: 0
POINT_RA:        104.252d {+06h 57m 00s} (J2000)
POINT_DEC:       +39.316d {+39d 18' 57"} (J2000)
ROLL:            112.183d
IMG_START_DATE:  18348 TJD;   230 DOY;   18/08/18
IMG_START_TIME:  45231.10 SOD {12:33:51.10} UT,   212.5 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     556288452
X_OFFSET:        704 [pixels]
Y_OFFSET:        1003 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       864
Y_GRB_POS:       1163
BINNING_INDEX:   1
IM_URL:          sw00853882000msuni0234.fits
SUN_POSTN:       147.80d {+09h 51m 11s}  +13.01d {+13d 00' 30"}
SUN_DIST:         46.21 [deg]   Sun_angle= 2.9 [hr] (West of Sun)
MOON_POSTN:      236.98d {+15h 47m 55s}  -14.93d {-14d 55' 36"}
MOON_DIST:       131.87 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:      177.25, 17.80 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      101.45, 16.44 [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 #23149
F. E. Marshall (NASA/GSFC), V. D'Elia (ASDC), S. W. K Emery (UCL-MSSL), J. A. Kennea (PSU), M. J. Moss (George Washington University), K. L. Page (U Leicester) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team:
At 12:30:18 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180818B (trigger=853882). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 104.235, +39.329 which is
```
   RA(J2000) = 06h 56m 56s
   Dec(J2000) = +39d 19' 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 130 sec, starting ~100 sec before the trigger time during the previous spacecraft slew. The peak count rate was ~1000 counts/sec (15-350 keV), at ~-90 sec after the trigger.
The XRT began observing the field at 12:33:42.4 UT, 203.8 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 104.22468, 39.31662 which is equivalent to:
```
   RA(J2000)  = 06h 56m 53.92s
   Dec(J2000) = +39d 18' 59.8"
```
with an uncertainty of 4.4 arcseconds (radius, 90% containment). This location is 53 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 http://www.swift.ac.uk/sper. We cannot determine whether the source is fading at the present time. No spectrum from the promptly downlinked event data is yet available to determine the column density.
The initial flux in the 2.5 s image was 1.30e-09 erg cm^-2 s^-1 (0.2-10 keV).
UVOT took a finding chart exposure of 150 seconds with the White filter starting 212 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 0.09.
Burst Advocate for this burst is F. E. Marshall (marshall AT milkyway.gsfc.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/too.html.)

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Sat 18 Aug 18 13:01:55 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     853882,   Seg_Num: 0
POINT_RA:        104.252d {+06h 57m 01s} (J2000)
POINT_DEC:       +39.318d {+39d 19' 04"} (J2000)
POINT_ROLL:      112.185d
IMG_START_DATE:  18348 TJD;   230 DOY;   18/08/18
IMG_START_TIME:  45878.23 SOD {12:44:38.23} UT,   859.7 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        6.811
N_STARS:         190
X_OFFSET:        144 [pixels]
Y_OFFSET:        443 [pixels]
X_MAX:           1583 [pixels]
Y_MAX:           1882 [pixels]
DET_THRESH:      22
PHOTO_THRESH:    14
SL_URL:          sw00853882000msufc0859.fits
SUN_POSTN:       147.81d {+09h 51m 15s}  +13.00d {+13d 00' 12"}
SUN_DIST:         46.22 [deg]   Sun_angle= 2.9 [hr] (West of Sun)
MOON_POSTN:      237.17d {+15h 48m 41s}  -14.98d {-14d 58' 30"}
MOON_DIST:       132.04 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:      177.25, 17.80 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      101.45, 16.44 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Sat 18 Aug 18 13:02:13 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     853882,   Seg_Num: 0
POINT_RA:        104.252d {+06h 57m 01s} (J2000)
POINT_DEC:       +39.318d {+39d 19' 04"} (J2000)
POINT_ROLL:      112.185d
IMG_START_DATE:  18348 TJD;   230 DOY;   18/08/18
IMG_START_TIME:  45878.23 SOD {12:44:38.23} UT,   859.7 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        6.811
N_STARS:         190
X_OFFSET:        144 [pixels]
Y_OFFSET:        443 [pixels]
X_MAX:           1583 [pixels]
Y_MAX:           1882 [pixels]
DET_THRESH:      22
PHOTO_THRESH:    14
SL_URL:          sw00853882000msufc0859.fits
SUN_POSTN:       147.81d {+09h 51m 15s}  +13.00d {+13d 00' 12"}
SUN_DIST:         46.22 [deg]   Sun_angle= 2.9 [hr] (West of Sun)
MOON_POSTN:      237.17d {+15h 48m 42s}  -14.98d {-14d 58' 32"}
MOON_DIST:       132.04 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:      177.25, 17.80 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      101.45, 16.44 [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:     Sat 18 Aug 18 13:03:34 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     853882,   Seg_Num: 0
POINT_RA:        104.252d {+06h 57m 01s} (J2000)
POINT_DEC:       +39.318d {+39d 19' 04"} (J2000)
ROLL:            112.185d
IMG_START_DATE:  18348 TJD;   230 DOY;   18/08/18
IMG_START_TIME:  45878.23 SOD {12:44:38.23} UT,   859.7 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     556289099
X_OFFSET:        703 [pixels]
Y_OFFSET:        1002 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       863
Y_GRB_POS:       1162
BINNING_INDEX:   1
IM_URL:          sw00853882000msuni0881.fits
SUN_POSTN:       147.81d {+09h 51m 15s}  +13.00d {+13d 00' 11"}
SUN_DIST:         46.22 [deg]   Sun_angle= 2.9 [hr] (West of Sun)
MOON_POSTN:      237.19d {+15h 48m 45s}  -14.98d {-14d 58' 42"}
MOON_DIST:       132.06 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:      177.25, 17.80 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      101.45, 16.44 [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:     Sat 18 Aug 18 13:02:33 UT
NOTICE_TYPE:     Swift-XRT Position UPDATE
TRIGGER_NUM:     853882,   Seg_Num: 0
GRB_RA:          104.2253d {+06h 56m 54.07s} (J2000),
                 104.5462d {+06h 58m 11.09s} (current),
                 103.3631d {+06h 53m 27.14s} (1950)
GRB_DEC:         +39.3153d {+39d 18' 55.0"} (J2000),
                 +39.2895d {+39d 17' 22.3"} (current),
                 +39.3817d {+39d 22' 54.0"} (1950)
GRB_ERROR:       2.3 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       1.00e-10 [erg/cm2/sec]
GRB_SIGNIF:      10.00 [sigma]
IMG_START_DATE:  18348 TJD;   230 DOY;   18/08/18
IMG_START_TIME:  45436.00 SOD {12:37:16.00} UT,   417.4 [sec] since BAT Trigger Time
TAM[0-3]:        100.00 100.00 100.00 100.00
AMPLIFIER:       1
WAVEFORM:        31
SUN_POSTN:       147.81d {+09h 51m 15s}  +13.00d {+13d 00' 12"}
SUN_DIST:         46.24 [deg]   Sun_angle= 2.9 [hr] (West of Sun)
MOON_POSTN:      237.18d {+15h 48m 42s}  -14.98d {-14d 58' 34"}
MOON_DIST:       132.07 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:      177.25, 17.78 [deg] galactic lon,lat of the burst
ECL_COORDS:      101.43, 16.44 [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 0.11% < P(seren) < 0.58%.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Sat 18 Aug 18 13:03:43 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     853882,   Seg_Num: 0
POINT_RA:        104.252d {+06h 57m 01s} (J2000)
POINT_DEC:       +39.318d {+39d 19' 04"} (J2000)
ROLL:            112.185d
IMG_START_DATE:  18348 TJD;   230 DOY;   18/08/18
IMG_START_TIME:  45878.23 SOD {12:44:38.23} UT,   859.7 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     556289099
X_OFFSET:        703 [pixels]
Y_OFFSET:        1002 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       863
Y_GRB_POS:       1162
BINNING_INDEX:   1
IM_URL:          sw00853882000msuni0881.fits
SUN_POSTN:       147.81d {+09h 51m 15s}  +13.00d {+13d 00' 11"}
SUN_DIST:         46.22 [deg]   Sun_angle= 2.9 [hr] (West of Sun)
MOON_POSTN:      237.19d {+15h 48m 45s}  -14.98d {-14d 58' 44"}
MOON_DIST:       132.06 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:      177.25, 17.80 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      101.45, 16.44 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Image.  
COMMENTS:        The GRB Position came from the Window Position in the Mode Command.  
COMMENTS:        The image has 2x2 binning (compression).  
COMMENTS:        All 4 attachments are included.

GCN Circular #23151
A.P. Beardmore, P.A. Evans, M.R. Goad and J.P. Osborne (U. Leicester) report on behalf of the Swift-XRT team.
Using 1716 s of XRT Photon Counting mode data and 2 UVOT images for GRB 180818B, 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 = 104.22470, +39.31571 which is equivalent to:
```
RA (J2000): 06h 56m 53.93s
Dec (J2000): +39d 18' 56.6"
```
with an uncertainty of 1.7 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 #23152
N. P. M. Kuin (UCL-MSSL) and F. E. Marshall (NASA/GSFC) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 180818B 212 s after the BAT trigger (Marshall et al., GCN Circ. 23149). No optical afterglow consistent with the XRT position (Beardmore et al. GCN Circ. 23151) 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           212          362          147         >19.9
white              212        11082         1482         >21.1
v                  369        12690         1148         >19.8
b                  467         6919          471         >19.9
u                  442         6714          469         >19.6
w1                 418         6509          468         >19.9
m2                 393         6304          471         >20.5
w2                 517        11988         1160         >20.3
```
The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.09 in the direction of the burst (Schlegel et al. 1998).
GCN Circular #23153
P. Veres (UAH), C. Meegan (UAH) and A. von Kienlin (MPE) report on behalf of the Fermi GBM Team:
"At 12:28:57.24 UT on 18 August 2018, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 180818B (trigger 556288142 / 180818520). which was also detected by the Swift/BAT (Marshall et al., GCN 23149). The GBM on-ground location is consistent with the Swift position.
The angle from the Fermi LAT boresight at the GBM trigger time is 89 degrees.
The GBM light curve consists of two separate emission episodes with a duration (T90) of about 169 s (50-300 keV). The time-averaged spectrum from T0-16 s to T0+125 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is 1.8 +/- 1.32 and the cutoff energy, parameterized as Epeak, is 78 +/- 9 keV.
The event fluence (10-1000 keV) in this time interval is (4.7 +/- 0.7)E-7 erg/cm^2. The 1-sec peak photon flux measured starting from T0-2.4 s in the 10-1000 keV band is 2.1 +/- 0.2 ph/s/cm^2.
The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."
GCN Circular #23155
J. R. Cummings (CPI), S. D. Barthelmy (GSFC), H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), T. Sakamoto (AGU), M. Stamatikos (OSU), T. N. Ukwatta (LANL) (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 180818B (trigger #853882) (Marshall et al., GCN Circ. 23149). The BAT ground-calculated position is RA, Dec = 104.227, 39.310 deg which is
```
  RA(J2000)  =  06h 56m 54.5s
  Dec(J2000) = +39d 18' 35.1"
```
with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 89%.
The mask-weighted light curve shows two pulses. The first pulse starts at at ~T-100 s, peaks at ~T-80 s, and ends at ~T-50 s. The second pulse starts at ~T+10 s, peaks at ~T+30 s, and ends at ~T+60 s. T90 (15-350 keV) is 134.4 +- 9.8 sec (estimated error including systematics). However, note that the burst came into the BAT FOV at ~T-128 s, and thus there might be additional burst emission beforehand.
The time-averaged spectrum from T-93.58 to T+62.74 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.77 +- 0.12. The fluence in the 15-150 keV band is 2.3 +- 0.2 x 10^-6 erg/cm2. The 1-sec peak photon flux measured from T-82.11 sec in the 15-150 keV band is 1.2 +- 0.2 ph/cm2/sec. All the quoted errors are at the 90% confidence level.
The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/853882/BA/
GCN Circular #23156
A. D'Ai (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), A. Tohuvavohu (PSU), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester), S.L. Gibson (U. Leicester), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB) and F.E. Marshall report on behalf of the Swift-XRT team:
We have analysed 8.1 ks of XRT data for GRB 180818B (Marshall et al. GCN Circ. 23149), from 210 s to 52.8 ks after the BAT trigger. The data comprise 205 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The enhanced XRT position for this burst was given by Beardmore et al. (GCN Circ. 23151).
The late-time light curve (from T0+4.5 ks) can be modelled with a power-law decay with a decay index of alpha=0.86 (+/-0.07).
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 3.47 (+0.12, -0.11). The best-fitting absorption column is 2.89 (+0.25, -0.23) x 10^21 cm^-2, in excess of the Galactic value of 1.3 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.77 (+0.07, -0.06) and a best-fitting absorption column consistent with the Galactic value. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.8 x 10^-11 (4.5 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
```
Total column:	     1.27 (+0.11, -0.00) x 10^21 cm^-2
Galactic foreground: 1.3 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.77 (+0.07, -0.06)
```
If the light curve continues to decay with a power-law decay index of 0.86, the count rate at T+24 hours will be 0.030 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x 10^-12 (1.3 x 10^-12) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00853882.
This circular is an official product of the Swift-XRT team.
GCN Circular #23161
Y.-D. Hu, A. Sota, V. Casanova and A.J. Castro-Tirado (IAA-CSIC Granada), on behalf of a larger collaboration, report:
We observed the field of the Swift/Fermi burst GRB 180818B (Marshall et al. GCNC 23149, Veres et al. GCNC 23153), with the 150-cm telescope of the Observatorio de Sierra Nevada (OSN) near Granada, Spain.
Two I-band observations were taken on Aug 19 and 21 (starting at 03:40 and 04:19 UT respectively, i.e. ~15.2 h and ~63.8 h after trigger) with total exposure times of 2100s and 1200s respectively. Within the enhanced Swift/XRT position (Beardmore et al. GCNC 23151), an optical source was detected on the first epoch with an I-band magnitude of 21.6, which faded by at least 0.5 mag on the 2nd epoch observation. We propose this source to be the optical afterglow to GRB 180818B.
The above mentioned magnitude is calibrated against the USNO-B1 catalog and is not corrected for Galactic extinction in the direction of the GRB. Further observations are encouraged.
GCN Circular #23177
E. Mazaeva (IKI), A. Pozanenko (IKI), A. Kusakin (FAPHI), I. Reva (FAPHI), A. Volnova (IKI), M. Krugov (FAPHI) report on behalf of larger GRB follow-up collaboration:
We observed the field of GRB 180818B (Marshall et al., GCN 23149; Veres et al., GCN 23153) with Zeiss-1000 1-m telescope of Tien Shan Astronomical Observatory starting on August 08 (UT) 22:20:38. We obtained several images in R-filter. Within enhanced XRT Swift-XRT position (Beardmore et al., GCN 23151) we do not detect any object. In particular we do not detect the afterglow candidate reported in GCN 23161 (Hu et al.) Preliminary photometry of the field is following.
```
Date        UT start t-T0     Filter Exp.   OT   Err.   UL
                      (mid, days)     (s)

2018-08-18  22:20:38 0.42710  R      36*60  n/d  n/d    20.4
```
The photometry is based on several nearby USNO-B1.0 stars.

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Jochen Greiner, last update: 26-Aug-2018 [Disclaimer]