Gamma-ray Burst 170531B

(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:     Wed 31 May 17 22:09:32 UT
NOTICE_TYPE:     Swift-BAT GRB Position
TRIGGER_NUM:     755354,   Seg_Num: 0
GRB_RA:          286.895d {+19h 07m 35s} (J2000),
                 287.145d {+19h 08m 35s} (current),
                 286.175d {+19h 04m 42s} (1950)
GRB_DEC:         -16.426d {-16d 25' 34"} (J2000),
                 -16.398d {-16d 23' 52"} (current),
                 -16.506d {-16d 30' 19"} (1950)
GRB_ERROR:       3.00 [arcmin radius, statistical only]
GRB_INTEN:       48238 [cnts]    Image_Peak=800 [image_cnts]
TRIGGER_DUR:     12.160 [sec] 
TRIGGER_INDEX:   478     E_range: 25-100 keV
BKG_INTEN:       237632 [cnts]
BKG_TIME:        79236.85 SOD {22:00:36.85} UT
BKG_DUR:         64 [sec]
GRB_DATE:        17904 TJD;   151 DOY;   17/05/31
GRB_TIME:        79329.20 SOD {22:02:09.20} UT
GRB_PHI:         -150.27 [deg]
GRB_THETA:        20.42 [deg]
SOLN_STATUS:     0x3
RATE_SIGNIF:     10.04 [sigma]
IMAGE_SIGNIF:    9.47 [sigma]
MERIT_PARAMS:     +1  +0  +0  +4  +2  +0  +0  +0 +80  +0 
SUN_POSTN:        69.04d {+04h 36m 10s}  +22.04d {+22d 02' 20"}
SUN_DIST:        143.68 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
MOON_POSTN:      155.45d {+10h 21m 49s}  +10.63d {+10d 37' 51"}
MOON_DIST:       132.78 [deg]
MOON_ILLUM:      44 [%]
GAL_COORDS:       19.93,-10.90 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      286.28,  6.06 [deg] ecliptic lon,lat of the burst (or transient)
COMMENTS:        SWIFT-BAT GRB Coordinates.  
COMMENTS:        This Notice was delayed by more than 60 sec past the end of the trigger integration interval;  
COMMENTS:        probably due to it occurring during a Malindi downlink session.  
COMMENTS:        This is a rate trigger.  
COMMENTS:        A point_source was found.  
COMMENTS:        This does not match any source in the on-board catalog.  
COMMENTS:        This does not match any source in the ground catalog.  
COMMENTS:        This is a GRB.  
COMMENTS:        This trigger occurred at longitude,latitude = 11.45,-6.82 [deg].

red DSS finding chart ps-file

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 31 May 17 22:10:20 UT
NOTICE_TYPE:     Swift-XRT Processed Image
TRIGGER_NUM:     755354,   Seg_Num: 0
GRB_RA:          286.8833d {+19h 07m 31.9s} (J2000),
                 287.1337d {+19h 08m 32.0s} (current),
                 286.1639d {+19h 04m 39.3s} (1950)
GRB_DEC:         -16.4181d {-16d 25' 05.1"} (J2000),
                 -16.3897d {-16d 23' 23.0"} (current),
                 -16.4973d {-16d 29' 50.2"} (1950)
GRB_ERROR:       4.7 [arcsec, radius, statistical plus systematic]
GRB_INTEN:       86 [cnts]
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  79470.10 SOD {22:04:30.10} UT,   140.9 [sec] since BAT Trigger Time
CENTROID_X:      360.67,  raw= 361  [pixels]
CENTROID_Y:      331.24,  raw= 331  [pixels]
ROLL:            74.95 [deg]
GAIN:            1
MODE:            3, Long Image mode
WAVEFORM:        134
EXPO_TIME:       2.50 [sec]
GRB_POS_XRT_Y:   77.43
GRB_POS_XRT_Z:   127.94
IMAGE_URL:       sw00755354000msxps_rw.img
SUN_POSTN:        69.04d {+04h 36m 10s}  +22.04d {+22d 02' 20"}
SUN_DIST:        143.68 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
MOON_POSTN:      155.46d {+10h 21m 51s}  +10.63d {+10d 37' 44"}
MOON_DIST:       132.76 [deg]
MOON_ILLUM:      44 [%]
GAL_COORDS:       19.93,-10.88 [deg] galactic lon,lat of the burst
ECL_COORDS:      286.27,  6.07 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Processed Image.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 31 May 17 22:10:08 UT
NOTICE_TYPE:     Swift-XRT Position
TRIGGER_NUM:     755354,   Seg_Num: 0
GRB_RA:          286.8833d {+19h 07m 31.99s} (J2000),
                 287.1337d {+19h 08m 32.09s} (current),
                 286.1639d {+19h 04m 39.34s} (1950)
GRB_DEC:         -16.4181d {-16d 25' 05.1"} (J2000),
                 -16.3897d {-16d 23' 23.0"} (current),
                 -16.4973d {-16d 29' 50.2"} (1950)
GRB_ERROR:       5.0 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       2.73e-09 [erg/cm2/sec]
GRB_SIGNIF:      9.27 [sigma]
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  79470.10 SOD {22:04:30.10} UT,   140.9 [sec] since BAT Trigger Time
TAM[0-3]:        327.64 237.21 261.57 243.48
AMPLIFIER:       2
WAVEFORM:        134
SUN_POSTN:        69.04d {+04h 36m 10s}  +22.04d {+22d 02' 20"}
SUN_DIST:        143.68 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
MOON_POSTN:      155.46d {+10h 21m 50s}  +10.63d {+10d 37' 45"}
MOON_DIST:       132.76 [deg]
MOON_ILLUM:      44 [%]
GAL_COORDS:       19.93,-10.88 [deg] galactic lon,lat of the burst
ECL_COORDS:      286.27,  6.07 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Coordinates.  
COMMENTS:        The XRT position is 0.82 arcmin from the BAT position.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 31 May 17 22:10:29 UT
NOTICE_TYPE:     Swift-BAT GRB Lightcurve
TRIGGER_NUM:     755354,   Seg_Num: 0
GRB_RA:          286.895d {+19h 07m 35s} (J2000),
                 287.145d {+19h 08m 35s} (current),
                 286.175d {+19h 04m 42s} (1950)
GRB_DEC:         -16.426d {-16d 25' 34"} (J2000),
                 -16.398d {-16d 23' 52"} (current),
                 -16.506d {-16d 30' 19"} (1950)
GRB_DATE:        17904 TJD;   151 DOY;   17/05/31
GRB_TIME:        79329.20 SOD {22:02:09.20} UT
TRIGGER_INDEX:   478
GRB_PHI:         -150.27 [deg]
GRB_THETA:        20.42 [deg]
DELTA_TIME:      44.00 [sec]
TRIGGER_DUR:     12.160 [sec] 
SOLN_STATUS:     0x3
RATE_SIGNIF:     10.04 [sigma]
IMAGE_SIGNIF:    9.47 [sigma]
LC_URL:          sw00755354000msb.lc
SUN_POSTN:        69.04d {+04h 36m 10s}  +22.04d {+22d 02' 20"}
SUN_DIST:        143.68 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
MOON_POSTN:      155.46d {+10h 21m 51s}  +10.63d {+10d 37' 42"}
MOON_DIST:       132.77 [deg]
MOON_ILLUM:      44 [%]
GAL_COORDS:       19.93,-10.90 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      286.28,  6.06 [deg] ecliptic lon,lat of the burst (or transient)
COMMENTS:        SWIFT-BAT GRB Lightcurve.  
COMMENTS:           
COMMENTS:        The next comments were copied from the BAT_POS Notice:  
COMMENTS:        This is a rate trigger.  
COMMENTS:        A point_source was found.  
COMMENTS:        This does not match any source in the on-board catalog.  
COMMENTS:        This does not match any source in the ground catalog.  
COMMENTS:        This is a GRB.  
COMMENTS:        This trigger occurred at longitude,latitude = 11.45,-6.82 [deg].

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 31 May 17 22:10:17 UT
NOTICE_TYPE:     Swift-XRT Image
TRIGGER_NUM:     755354,   Seg_Num: 0
GRB_RA:          286.8833d {+19h 07m 31.9s} (J2000),
                 287.1337d {+19h 08m 32.0s} (current),
                 286.1639d {+19h 04m 39.3s} (1950)
GRB_DEC:         -16.4181d {-16d 25' 05.1"} (J2000),
                 -16.3897d {-16d 23' 23.0"} (current),
                 -16.4973d {-16d 29' 50.2"} (1950)
GRB_ERROR:       4.7 [arcsec, radius, statistical plus systematic]
GRB_INTEN:       86 [cnts]
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  79470.10 SOD {22:04:30.10} UT,   140.9 [sec] since BAT Trigger Time
CENTROID_X:      360.67,  raw= 361  [pixels]
CENTROID_Y:      331.24,  raw= 331  [pixels]
ROLL:            74.95 [deg]
GAIN:            1
MODE:            3, Long Image mode
WAVEFORM:        134
EXPO_TIME:       2.50 [sec]
GRB_POS_XRT_Y:   77.43
GRB_POS_XRT_Z:   127.94
IMAGE_URL:       sw00755354000msxps_rw.img
SUN_POSTN:        69.04d {+04h 36m 10s}  +22.04d {+22d 02' 20"}
SUN_DIST:        143.68 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
MOON_POSTN:      155.46d {+10h 21m 51s}  +10.63d {+10d 37' 44"}
MOON_DIST:       132.76 [deg]
MOON_ILLUM:      44 [%]
GAL_COORDS:       19.93,-10.88 [deg] galactic lon,lat of the burst
ECL_COORDS:      286.27,  6.07 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Image.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 31 May 17 22:11:21 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     755354,   Seg_Num: 0
POINT_RA:        286.925d {+19h 07m 42s} (J2000)
POINT_DEC:       -16.429d {-16d 25' 44"} (J2000)
POINT_ROLL:      74.948d
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  79481.62 SOD {22:04:41.62} UT,   152.4 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        3.617
N_STARS:         190
X_OFFSET:        182 [pixels]
Y_OFFSET:        535 [pixels]
X_MAX:           1141 [pixels]
Y_MAX:           1494 [pixels]
DET_THRESH:      16
PHOTO_THRESH:    9
SL_URL:          sw00755354000msufc0152.fits
SUN_POSTN:        69.04d {+04h 36m 11s}  +22.04d {+22d 02' 20"}
SUN_DIST:        143.65 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
MOON_POSTN:      155.47d {+10h 21m 53s}  +10.63d {+10d 37' 34"}
MOON_DIST:       132.79 [deg]
MOON_ILLUM:      44 [%]
GAL_COORDS:       19.94,-10.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      286.31,  6.06 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 31 May 17 22:11:29 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     755354,   Seg_Num: 0
POINT_RA:        286.925d {+19h 07m 42s} (J2000)
POINT_DEC:       -16.429d {-16d 25' 44"} (J2000)
POINT_ROLL:      74.948d
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  79481.62 SOD {22:04:41.62} UT,   152.4 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        3.617
N_STARS:         190
X_OFFSET:        182 [pixels]
Y_OFFSET:        535 [pixels]
X_MAX:           1141 [pixels]
Y_MAX:           1494 [pixels]
DET_THRESH:      16
PHOTO_THRESH:    9
SL_URL:          sw00755354000msufc0152.fits
SUN_POSTN:        69.04d {+04h 36m 11s}  +22.04d {+22d 02' 20"}
SUN_DIST:        143.65 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
MOON_POSTN:      155.47d {+10h 21m 53s}  +10.63d {+10d 37' 33"}
MOON_DIST:       132.79 [deg]
MOON_ILLUM:      44 [%]
GAL_COORDS:       19.94,-10.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      286.31,  6.06 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Source List.  
COMMENTS:        All 4 attachments are included.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 31 May 17 22:13:20 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     755354,   Seg_Num: 0
POINT_RA:        286.925d {+19h 07m 42s} (J2000)
POINT_DEC:       -16.429d {-16d 25' 44"} (J2000)
ROLL:            74.948d
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  79481.62 SOD {22:04:41.62} UT,   152.4 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     517961100
X_OFFSET:        502 [pixels]
Y_OFFSET:        855 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       662
Y_GRB_POS:       1015
BINNING_INDEX:   1
IM_URL:          sw00755354000msuni0172.fits
SUN_POSTN:        69.05d {+04h 36m 11s}  +22.04d {+22d 02' 21"}
SUN_DIST:        143.65 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
MOON_POSTN:      155.49d {+10h 21m 57s}  +10.62d {+10d 37' 16"}
MOON_DIST:       132.77 [deg]
MOON_ILLUM:      44 [%]
GAL_COORDS:       19.94,-10.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      286.31,  6.06 [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:     Wed 31 May 17 22:13:33 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     755354,   Seg_Num: 0
POINT_RA:        286.925d {+19h 07m 42s} (J2000)
POINT_DEC:       -16.429d {-16d 25' 44"} (J2000)
ROLL:            74.948d
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  79481.62 SOD {22:04:41.62} UT,   152.4 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     517961100
X_OFFSET:        502 [pixels]
Y_OFFSET:        855 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       662
Y_GRB_POS:       1015
BINNING_INDEX:   1
IM_URL:          sw00755354000msuni0172.fits
SUN_POSTN:        69.05d {+04h 36m 11s}  +22.04d {+22d 02' 21"}
SUN_DIST:        143.65 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
MOON_POSTN:      155.49d {+10h 21m 58s}  +10.62d {+10d 37' 14"}
MOON_DIST:       132.77 [deg]
MOON_ILLUM:      44 [%]
GAL_COORDS:       19.94,-10.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      286.31,  6.06 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Image.  
COMMENTS:        The GRB Position came from the XRT Position Command.  
COMMENTS:        The image has 2x2 binning (compression).  
COMMENTS:        All 4 attachments are included.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 31 May 17 22:18:12 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     755354,   Seg_Num: 0
POINT_RA:        286.924d {+19h 07m 42s} (J2000)
POINT_DEC:       -16.430d {-16d 25' 49"} (J2000)
POINT_ROLL:      74.951d
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  79639.68 SOD {22:07:19.68} UT,   310.5 [sec] since BAT Trigger Time
FILTER:          7, U
BKG_MEAN:        1.165
N_STARS:         190
X_OFFSET:        182 [pixels]
Y_OFFSET:        535 [pixels]
X_MAX:           1141 [pixels]
Y_MAX:           1494 [pixels]
DET_THRESH:      9
PHOTO_THRESH:    4
SL_URL:          sw00755354000msufc0310.fits
SUN_POSTN:        69.05d {+04h 36m 12s}  +22.04d {+22d 02' 23"}
SUN_DIST:        143.65 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
MOON_POSTN:      155.53d {+10h 22m 08s}  +10.61d {+10d 36' 31"}
MOON_DIST:       132.73 [deg]
MOON_ILLUM:      44 [%]
GAL_COORDS:       19.94,-10.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      286.31,  6.06 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 31 May 17 22:18:29 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     755354,   Seg_Num: 0
POINT_RA:        286.924d {+19h 07m 42s} (J2000)
POINT_DEC:       -16.430d {-16d 25' 49"} (J2000)
POINT_ROLL:      74.951d
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  79639.68 SOD {22:07:19.68} UT,   310.5 [sec] since BAT Trigger Time
FILTER:          7, U
BKG_MEAN:        1.165
N_STARS:         190
X_OFFSET:        182 [pixels]
Y_OFFSET:        535 [pixels]
X_MAX:           1141 [pixels]
Y_MAX:           1494 [pixels]
DET_THRESH:      9
PHOTO_THRESH:    4
SL_URL:          sw00755354000msufc0310.fits
SUN_POSTN:        69.05d {+04h 36m 12s}  +22.04d {+22d 02' 23"}
SUN_DIST:        143.65 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
MOON_POSTN:      155.54d {+10h 22m 09s}  +10.61d {+10d 36' 28"}
MOON_DIST:       132.73 [deg]
MOON_ILLUM:      44 [%]
GAL_COORDS:       19.94,-10.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      286.31,  6.06 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Source List.  
COMMENTS:        All 4 attachments are included.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 31 May 17 22:18:50 UT
NOTICE_TYPE:     Swift-XRT Position UPDATE
TRIGGER_NUM:     755354,   Seg_Num: 0
GRB_RA:          286.8835d {+19h 07m 32.04s} (J2000),
                 287.1339d {+19h 08m 32.14s} (current),
                 286.1641d {+19h 04m 39.39s} (1950)
GRB_DEC:         -16.4192d {-16d 25' 09.1"} (J2000),
                 -16.3908d {-16d 23' 27.0"} (current),
                 -16.4984d {-16d 29' 54.1"} (1950)
GRB_ERROR:       2.4 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       1.00e-10 [erg/cm2/sec]
GRB_SIGNIF:      10.00 [sigma]
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  79609.00 SOD {22:06:49.00} UT,   279.8 [sec] since BAT Trigger Time
TAM[0-3]:        100.00 100.00 100.00 100.00
AMPLIFIER:       1
WAVEFORM:        31
SUN_POSTN:        69.05d {+04h 36m 12s}  +22.04d {+22d 02' 23"}
SUN_DIST:        143.69 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
MOON_POSTN:      155.54d {+10h 22m 09s}  +10.61d {+10d 36' 25"}
MOON_DIST:       132.68 [deg]
MOON_ILLUM:      44 [%]
GAL_COORDS:       19.93,-10.88 [deg] galactic lon,lat of the burst
ECL_COORDS:      286.27,  6.07 [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:     Wed 31 May 17 22:19:36 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     755354,   Seg_Num: 0
POINT_RA:        286.924d {+19h 07m 42s} (J2000)
POINT_DEC:       -16.430d {-16d 25' 49"} (J2000)
ROLL:            74.951d
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  79639.68 SOD {22:07:19.68} UT,   310.5 [sec] since BAT Trigger Time
FILTER:          7, U
EXPOSURE_ID:     517961258
X_OFFSET:        502 [pixels]
Y_OFFSET:        855 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       662
Y_GRB_POS:       1015
BINNING_INDEX:   1
IM_URL:          sw00755354000msuni0330.fits
SUN_POSTN:        69.05d {+04h 36m 12s}  +22.04d {+22d 02' 23"}
SUN_DIST:        143.66 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
MOON_POSTN:      155.55d {+10h 22m 11s}  +10.61d {+10d 36' 18"}
MOON_DIST:       132.71 [deg]
MOON_ILLUM:      44 [%]
GAL_COORDS:       19.94,-10.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      286.31,  6.06 [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:     Wed 31 May 17 22:19:46 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     755354,   Seg_Num: 0
POINT_RA:        286.924d {+19h 07m 42s} (J2000)
POINT_DEC:       -16.430d {-16d 25' 49"} (J2000)
ROLL:            74.951d
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  79639.68 SOD {22:07:19.68} UT,   310.5 [sec] since BAT Trigger Time
FILTER:          7, U
EXPOSURE_ID:     517961258
X_OFFSET:        502 [pixels]
Y_OFFSET:        855 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       662
Y_GRB_POS:       1015
BINNING_INDEX:   1
IM_URL:          sw00755354000msuni0330.fits
SUN_POSTN:        69.05d {+04h 36m 12s}  +22.04d {+22d 02' 23"}
SUN_DIST:        143.66 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
MOON_POSTN:      155.55d {+10h 22m 11s}  +10.60d {+10d 36' 17"}
MOON_DIST:       132.71 [deg]
MOON_ILLUM:      44 [%]
GAL_COORDS:       19.94,-10.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      286.31,  6.06 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Image.  
COMMENTS:        The GRB Position came from the XRT Position Command.  
COMMENTS:        The image has 2x2 binning (compression).  
COMMENTS:        All 4 attachments are included.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 31 May 17 22:24:15 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     755354,   Seg_Num: 0
POINT_RA:        286.924d {+19h 07m 42s} (J2000)
POINT_DEC:       -16.430d {-16d 25' 49"} (J2000)
POINT_ROLL:      74.952d
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  80193.11 SOD {22:16:33.11} UT,   863.9 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        3.584
N_STARS:         190
X_OFFSET:        0 [pixels]
Y_OFFSET:        295 [pixels]
X_MAX:           1439 [pixels]
Y_MAX:           1734 [pixels]
DET_THRESH:      16
PHOTO_THRESH:    9
SL_URL:          sw00755354000msufc0863.fits
SUN_POSTN:        69.05d {+04h 36m 13s}  +22.04d {+22d 02' 25"}
SUN_DIST:        143.66 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
MOON_POSTN:      155.59d {+10h 22m 21s}  +10.59d {+10d 35' 35"}
MOON_DIST:       132.67 [deg]
MOON_ILLUM:      44 [%]
GAL_COORDS:       19.94,-10.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      286.31,  6.06 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN Circular #21171
P.A. Evans (U Leicester), A. P. Beardmore (U Leicester), D. N. Burrows (PSU), A. Cholden-Brown (PSU), A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester) and D. M. Palmer (LANL) report on behalf of the Swift Team:
At 22:02:09 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 170531B (trigger=755354). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 286.895, -16.426 which is
```
   RA(J2000) = 19h 07m 35s
   Dec(J2000) = -16d 25' 34"
```
with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of at least 50 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~6 sec after the trigger.
The XRT began observing the field at 22:04:30.1 UT, 140.9 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source with an enhanced position: RA, Dec 286.8835, -16.4196 which is equivalent to:
```
   RA(J2000)  = 19h 07m 32.03s
   Dec(J2000) = -16d 25' 10.4"
```
with an uncertainty of 2.3 arcseconds (radius, 90% containment). This location is 45 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.
A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 1.41 x 10^21 cm^-2 (Willingale et al. 2013).
The initial flux in the 2.5 s image was 2.73e-09 erg cm^-2 s^-1 (0.2-10 keV).
UVOT took a finding chart exposure of 150 seconds with the White filter starting 152 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. Because of the density of catalogued stars, further analysis is required to report an upper limit for any afterglow in the sub-image. 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 expected extinction corresponding to E(B-V) of 0.19.
Burst Advocate for this burst is P.A. Evans (pae9 AT star.le.ac.uk). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/too.html.)

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 31 May 17 22:24:32 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     755354,   Seg_Num: 0
POINT_RA:        286.924d {+19h 07m 42s} (J2000)
POINT_DEC:       -16.430d {-16d 25' 49"} (J2000)
POINT_ROLL:      74.952d
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  80193.11 SOD {22:16:33.11} UT,   863.9 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        3.584
N_STARS:         190
X_OFFSET:        0 [pixels]
Y_OFFSET:        295 [pixels]
X_MAX:           1439 [pixels]
Y_MAX:           1734 [pixels]
DET_THRESH:      16
PHOTO_THRESH:    9
SL_URL:          sw00755354000msufc0863.fits
SUN_POSTN:        69.05d {+04h 36m 13s}  +22.04d {+22d 02' 25"}
SUN_DIST:        143.66 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
MOON_POSTN:      155.59d {+10h 22m 22s}  +10.59d {+10d 35' 33"}
MOON_DIST:       132.67 [deg]
MOON_ILLUM:      44 [%]
GAL_COORDS:       19.94,-10.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      286.31,  6.06 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Processed Source List.  
COMMENTS:        All 4 attachments are included.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 31 May 17 22:25:51 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     755354,   Seg_Num: 0
POINT_RA:        286.924d {+19h 07m 42s} (J2000)
POINT_DEC:       -16.430d {-16d 25' 49"} (J2000)
ROLL:            74.952d
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  80193.11 SOD {22:16:33.11} UT,   863.9 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     517961812
X_OFFSET:        559 [pixels]
Y_OFFSET:        854 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       719
Y_GRB_POS:       1014
BINNING_INDEX:   1
IM_URL:          sw00755354000msuni0883.fits
SUN_POSTN:        69.05d {+04h 36m 13s}  +22.04d {+22d 02' 25"}
SUN_DIST:        143.66 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
MOON_POSTN:      155.60d {+10h 22m 25s}  +10.59d {+10d 35' 21"}
MOON_DIST:       132.66 [deg]
MOON_ILLUM:      44 [%]
GAL_COORDS:       19.94,-10.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      286.31,  6.06 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Image.  
COMMENTS:        The GRB Position came from the Window Position in the Mode Command.  
COMMENTS:        The image has 2x2 binning (compression).

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 31 May 17 22:26:00 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     755354,   Seg_Num: 0
POINT_RA:        286.924d {+19h 07m 42s} (J2000)
POINT_DEC:       -16.430d {-16d 25' 49"} (J2000)
ROLL:            74.952d
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  80193.11 SOD {22:16:33.11} UT,   863.9 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     517961812
X_OFFSET:        559 [pixels]
Y_OFFSET:        854 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       719
Y_GRB_POS:       1014
BINNING_INDEX:   1
IM_URL:          sw00755354000msuni0883.fits
SUN_POSTN:        69.05d {+04h 36m 13s}  +22.04d {+22d 02' 25"}
SUN_DIST:        143.66 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
MOON_POSTN:      155.60d {+10h 22m 25s}  +10.59d {+10d 35' 19"}
MOON_DIST:       132.66 [deg]
MOON_ILLUM:      44 [%]
GAL_COORDS:       19.94,-10.92 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      286.31,  6.06 [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 #21173
V. Lipunov, V.Kornilov, N.Tyurina, E.Gorbovskoy, D.Kuvshinov, A.V.Krylov, I.Gorbunov, P.Balanutsa, A.Kuznetsov, V.V.Chazov, D. Vlasenko Lomonosov Moscow State University, Sternberg Astronomical Institut of MSU
A. Tlatov, V.Senik, A.V. Parhomenko, D. Dormidontov Kislovodsk Solar Station of the Pulkovo Observatory
N.M.Budnev, O.Gres, O. Ershova Irkutsk State University
A.Gabovich, V.Yurkov, Yu.Sergienko Blagoveschensk Educational State University, Blagoveschensk
D.Buckley, S. Potter, A.Kniazev, M.Kotze South African Astronomical Observatory
R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres The Instituto de Astrofisica de Canarias
R.Podesta, C.Lopez, F. Podesta Observatorio Astronomico Felix Aguilar (OAFA), National University of San Juan, Argentina
H. Levato, C. Saffe Instituto de Ciencias Astronomicas,de la Tierra y del Espacio (ICATE), San Juan, Argentina
MASTER-Kislovodsk robotic telescope (2x4 square degrees, MASTER-Net: http://observ.pereplet.ru) was pointed to the GRB170531B (Evans et al., GCN #21171) 12 sec after notice time and 452 sec after trigger time on 2017-05-31 22:09:44 UT. There is marginally seen optical transient at the position 19h 07m 32.03s -16d 25' 10.4" with m_OT~19.0+-0.5
The 5-sigma upper limit is about 18.5mag The message may be cited.
====================================================================
The observations made on zenit distance = 64 degrees, galaxy latitude b = -11 degree. The moon (44 % bright part) below the horizon (The Moon altitude is -6 degree). The sun altitude is -22.7 degree. The object can be observed till sunrise at 2017-06-01 01:30:50 .
The message may be cited.
GCN Circular #21174
A. Pozanenko (IKI), D. Chestnov (ISON), A. Novichonok (ISON), A. Mokhnatkin (ISON) report on behalf of larger GRB follow-up collaboration:
We observed the field of the GRB 170531B (Evans et al.,GCN 21171) with with Zeiss-1000/Koshka telescope starting on May 31 (UT) 22:32:32, i.e. ~30 minutes after burst onset. We took several images in filter R with exposures of 180 s. Near the border of XRT error circle (Evans et al.,GCN 21171) we detect the source in coordinates (J2000) 19:07:31.67 -16:25:09.3 with uncertainity of 0.5 arcsec in both coordinates. The source is probably coincedent with the object reported by (Lipunov et al., GCN 21172). The source is fading from R=19.1 at (UT) 22:32:32 down to R=19.5 at (UT) 23:30:56. Most probably the source is an afterglow of GRB 170531B.
GCN Circular #21175
J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester) report on behalf of the Swift-XRT team.
Using 1160 s of XRT Photon Counting mode data and 2 UVOT images for GRB 170531B, 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 = 286.88369, -16.41911 which is equivalent to:
```
RA (J2000): 19h 07m 32.09s
Dec (J2000): -16d 25' 08.8"
```
with an uncertainty of 1.8 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 #21177
A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), L. Izzo (IAA-CSIC),=20 D.A. Kann (IAA-CSIC), C.C. Thoene (IAA-CSIC), Z. Cano=20 (IAA-CSIC), J.P.U. Fynbo (DARK/NBI), D. Garcia Alvarez=20 (GTC, IAC) report on behalf of a larger collaboration:
We observed the afterglow of GRB 170531B (Evans et al. GCN=20 21171, Lipunov et al. GCN 21173, Pozanenko et al. GCN 21174)=20 using OSIRIS at the 10.4 m GTC telescope in La Palma (Spain).=20 Observation consisted in 3x900s exposures using the R1000B=20 grism, covering the range between 3700 and 7880 AA. The mean=20 epoch of the combined spectrum is 03:09:29 UT (5.12 hr after the=20 GRB onset). Additionally, g, r, i, and z-band observations were=20 obtained. We note that the coordinates of the afterglow are=20 19:07:32.09, -16:25:05.8 (J2000 +/-0.5=E2=80=9D), which places the GRB=20 approximately 5=E2=80=9D away from those of the source identified by=20 Lipunov et al. (GCN 21173) and Pozanenko et al. (GCN 21174).=20 The afterglow can be also detected in the early Swift/UVOT=20 images. In the acquisition image we measure a magnitude of=20 r_AB =3D 21.9 for the afterglow, as compared to PANSTARRS=20 catalogue field stars.
The afterglow spectrum shows continuum throughout the=20 wavelength range and several strong absorption features=20 corresponding to Lyman-alpha, SiII, OI, CII, SiIV, CIV, AlII and AlII=20 at a common redshift of 2.366, which we identify as the redshift of=20 the GRB.
GCN Circular #21178
P. Schady and T. Kruehler (both MPE) report
We observed the field of GRB 170531B (Swift trigger 755354; Evans et al., GCN #21171) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla Observatory (Chile).
Observations started at 03:55 UT on 01-06-2017, 5.9 hours the GRB trigger. They were performed at an average seeing of 1.3" and at an average airmass of 1.3.
We detect a point source within the 1.8" enhanced Swift/XRT error circle (Osborne et al., GCN #21175), and consistent with the afterglow position reported by Ugarte Postigo et al. (GCN #21177).
Based on a total exposure of 54 min in g'r'i'z' and 45 min in JHK, we estimate preliminary magnitudes (all in AB) of
```
g' = 22.65 +/- 0.04
r' = 22.15 +/- 0.04
i' = 21.99 +/- 0.05
z' = 21.84 +/- 0.06
J = 21.8 +/- 0.3
H = 21.5 +/- 0.3
K > 19.9
```
Given magnitudes are calibrated against PanSTARRS and 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V)=0.16 mag in the direction of the burst (Schlafly & Finkbeiner 2011).
We acknowledge the excellent support provided by the Chilean observer at the telescope, and the support astronomer, Sam Kim, in obtaining these data.
GCN Circular #21179
N.Tyurina, V. Lipunov, V.Kornilov, E.Gorbovskoy, I.Gorbunov, D.Kuvshinov, A.V.Krylov, P.Balanutsa, A.Kuznetsov, V.V.Chazov, D. Vlasenko Lomonosov Moscow State University, Sternberg Astronomical Institut of MSU
A. Tlatov, V.Senik, A.V. Parhomenko, D. Dormidontov Kislovodsk Solar Station of the Pulkovo Observatory
N.M.Budnev, O.Gres, O. Ershova Irkutsk State University
A.Gabovich, V.Yurkov, Yu.Sergienko Blagoveschensk Educational State University, Blagoveschensk
D.Buckley, S. Potter, A.Kniazev, M.Kotze South African Astronomical Observatory
R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres The Instituto de Astrofisica de Canarias
R.Podesta, C.Lopez, F. Podesta Observatorio Astronomico Felix Aguilar (OAFA), National University of San Juan, Argentina
H. Levato, C. Saffe Instituto de Ciencias Astronomicas,de la Tierra y del Espacio (ICATE), San Juan, Argentina
MASTER-Kislovodsk robotic telescope (2x4 square degrees, MASTER-Net: http://observ.pereplet.ru) was pointed to the GRB170531B (Evans et al., GCN #21171) 12 sec after notice time and 452 sec after trigger time on 2017-05-31 22:09:44 UT (Lipunov et al., GCN #21173).
```
Id 	    Date      time   Exp  Proc.type     Limit* 	Tube  OT   Err
                        UT    sec                5sigma

1240587  2017-05-31 22:09:44  90 Alert (SWIFT)	 18.3   EAST  18.3  0.5
1240589  2017-05-31 22:11:23 110 Alert (SWIFT)   18.4   EAST  18.4  0.5
1240591  2017-05-31 22:13:24 130 Alert (SWIFT)   18.5   EAST  18.4  0.5
1240593  2017-05-31 22:15:44 160 Alert (SWIFT)   18.5   EAST  18.5  0.5
1240596  2017-05-31 22:18:35 180 Alert (SWIFT)   18.5   EAST  18.5  0.5
1240652  2017-05-31 22:09:44 490 SumAlert(SWIFT) 19.1 	EAST  18.3  0.2 
1240597  2017-05-31 22:21:59 180 Alert (SWIFT) 	 18.3   EAST   no    -
1240599  2017-05-31 22:25:24 180 Alert (SWIFT) 	 18.5   EAST   no    - 
.................................................................
1240629  2017-05-31 23:16:43 180 Alert (SWIFT) 	 18.5   EAST   no    -
```
* - unfiltered magnitude with respect to USNO B1.0 stars reference stars m=0.2B + 0.8R
We refind OT preliminary astrometric position (Lipunov et al., GCN #21173) which was done visually. Our refind position are:
RA,DEC = 19 07 32.19 -16 25 06.9
3 sigma error = (+/-)2 arcsec (the source is marginaly visible). Our pixel is 2 arcsec.
The offset from X-ray Swift position (Osborne at all) is 1.9 arcsec .
The position of the Postigo et al. ( GCN 21177) is inside our error box (offset ~ 1.9 arcsec).
The possible OT position Pozanenko et al.( GCN #21174) offset ~ 8.2 arc sec.
The OT detection image is available.
The message may be cited.
====================================================================
The observations made on zenit distance = 64 degrees, galaxy latitude b = -11 degree. The moon (44 % bright part) below the horizon (The Moon altitude is -6 degree). The sun altitude is -22.7 degree. The object can be observed till sunrise at 2017-06-01 01:30:50 .
The message may be cited.
GCN Circular #21182
P. D'Avanzo (INAF-OAB), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), K.L. Page (U. Leicester), B. Mingo (U. Leicester), A.P. Beardmore (U. Leicester), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA) and P.A. Evans report on behalf of the Swift-XRT team:
We have analysed 6.8 ks of XRT data for GRB 170531B (Evans et al. GCN Circ. 21171), from 129 s to 52.1 ks after the BAT trigger. The data comprise 303 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 enhanced XRT position for this burst was given by Osborne et al. (GCN Circ. 21175).
The late-time light curve (from T0+5.5 ks) can be modelled with a power-law decay with a decay index of alpha=1.18 (+0.25, -0.20).
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.45 (+/-0.04). The best-fitting absorption column is 6.5 (+2.7, -2.5) x 10^21 cm^-2, at a redshift of 2.366, in addition to the Galactic value of 1.4 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.07 (+0.13, -0.12) 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 3.4 x 10^-11 (4.7 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
```
Galactic foreground: 1.4 x 10^21 cm^-2
Intrinsic column:    1.4 (+2.2, -0.0) x 10^21 cm^-2 at z=2.366
Photon index:	     2.07 (+0.13, -0.12)
```
If the light curve continues to decay with a power-law decay index of 1.18, the count rate at T+24 hours will be 3.3 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x 10^-13 (1.6 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/00755354.
This circular is an official product of the Swift-XRT team.
GCN Circular #21183
A. Cucchiara, D. Morris (U. of Virgin Islands), C. Guidorzi (U. Ferrara), reports on behalf of a larger collaboration:
"On June 1.00 UT (T_0 +2.0h) we began observing the center of the field of GRB 170531B (Evans et al. GCN 211171) using the Las Cumbres Observatory 1m Sutherland facility. We performed a series of 10x60s observations in R and I band for a total of 10 minutes on sky in each filter. Seeing conditions were poor.
We identified no optical counterpart within the center of Swift-XRT refined position (Evans et al. GCN 21175), or at the location of the identified counterpart (Lipunov et al. GCN 21173, Pozanenko et al. GCN 21174, Schady et al. GCN 21178) at the following 3-sigma limits:
```
R > 19.9 mag
I > 19.6 mag
```
These magnitude are calibrated against nearby USNO-B1 sources, and are not corrected for Galactic extinction."
GCN Circular #21186
T. Sakamoto (AGU), S. D. Barthelmy (GSFC), J. R. Cummings (CPI), P. A. Evans (U Leicester), H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), D. M. Palmer (LANL), 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 170531B (trigger #755354) (Evans et al., GCN Circ. 21171). The BAT ground-calculated position is RA, Dec = 286.901, -16.414 deg which is
```
  RA(J2000)  =  19h 07m 36.3s
  Dec(J2000) = -16d 24' 51.6"
```
with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment). The partial coding was 96%.
The mask-weighted light curve shows a multi-peaked structure that starts at ~T0 and ends at ~T+180 s. T90 (15-350 keV) is 164.13 +- 8.82 sec (estimated error including systematics).
The time-averaged spectrum from T+0.55 to T+181.67 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.95 +- 0.14. The fluence in the 15-150 keV band is 2.0 +- 0.2 x 10^-6 erg/cm2. The 1-sec peak photon flux measured from T+42.71 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 http://gcn.gsfc.nasa.gov/notices_s/755354/BA/
GCN Circular #21187
B. E. Cobb (GWU), reports:
Using the ANDICAM instrument on the 1.3m telescope at CTIO, we obtained optical/IR imaging of the error region of GRB 170531B (GCN 21171, Evans et al.) at two epochs (with mid-exposure times of 2017-06-01 04:22 UT & 08:19 UT). For each epoch, several dithered images were obtained with total summed exposure times amounting to 36 minutes in I and 30 minutes in J.
No source is detected at the position of the X-ray/optical afterglow (GCN 21175, Osborne et al.; GCN 21174, Pozanenko et al.; GCN 21177, de Ugarte Postigo et al.; GCN 21178, Schady & Kruehler; GCN 21179, Tyurina et al.) to the approximate 3-sigma limiting magnitudes listed below:
```
mid-exposure
time post-burst
(hours)                I-limit    J-limit
 6.33056             >20.8    >18.5
10.27333            >21.2    >18.7
```
(Optical photometry is calibrated against USNO-B1.0 stars and IR photometry is calibrated against 2MASS stars in the field.)
GCN Circular #21188
M. H. Siegel (PSU) and P. A. Evans (U. Leicester) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 170531B 152 s after the BAT trigger (Evans et al., GCN Circ. 21171). We detect a possible afterglow consistent with the position given by GTC (de Ugarte Postigo et al., GCN Circ. 21177) and MASTER (Tyurin et al., GCN CIrc. 21179).
The preliminary UVOT position is:
```
    RA  (J2000) =  19:07:32.10 = 286.88373 (deg.)
    Dec (J2000) = -16:25:05.8  = -16.41829 (deg.)
```
with an estimated uncertainty of 0.1 arc sec. (radius, 90% confidence).
This position is 3.0 arc sec away from the enhanced XRT position (Osborne et al. GCN Circ. 21175). Given the fading, detection by other observatories and lack of matching source in the DSS, we find it likely to be the GRB afterglow.
Preliminary 3-sigma magnitudes 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)         152          302          147         20.50+-0.29
white              590         1532          244         >21.03
v                  640         1581          116         >18.88
b                  566         1508           97         >19.77
u (fc)             310          560          245         >20.07
u                  713         1643           85         >19.17
uvw1               689         1630           97         >18.91
uvm2               664         1606          116         >18.88
uvw2               615         1557          116         >19.14
```
The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.19 in the direction of the burst (Schlegel et al. 1998).
GCN Circular #21189
E. Mazaeva (IKI), A. Mokhnatkin (KIAM), A. Pozanenko (IKI), A. Volnova (IKI), I. Molotov (KIAM) report on behalf of larger GRB follow-up collaboration:
We observed the field of GRB 170531B (Evans et al.,GCN 21171) with K-800 (0.8m) telescope of ISON/Terskol observatory starting on May, 31 (UT) 22:21:31. We obtained several unfiltered images of 40 s exposure. The optical afterglow (Lipunov et al., GCN 21173 Pozanenko et al. GCN 21174) is clearly visible in combined images. Preliminary photometry of the afterglow is following
```
Date       UT start  t-T0   Filter   Exp.  OT   Err. UL
                      (mid, days)     (s)

2017-05-31 22:21:31 0.01810 CR     18*40  18.90 0.10 21.0
2017-05-31 22:35:41 0.02949 CR     21*40  19.10 0.10 21.0
```
Photometry is based on nearby USNO-B1.0 stars
```
USNO-B.1_id R2
0736-0718845 16.77
0735-0729626 17.35
0735-0729622 16.96
0735-0729439 16.59
0735-0729327 17.22
```
GCN Circular #21209
K. Wiersema, N. Tanvir (Leicester), A. Levan (Warwick) report:
We observed the afterglow of GRB 170531B (Evans et al. GCN 21171; Lipunov et al. GCN 21173; Pozanenko et al. GCN 21174) with the 4.2m William Herschel Telescope on La Palma, using the ACAM instrument. We obtained a spectrum, consisting of 4x900 seconds exposure time, starting at 02:55 UT on 1 June 2017. Our wavelength coverage is ~4500 - 9500 A. We detect a small number metal lines (at low S/N) at a redshift consistent with that obtained by de Ugarte-Postigo et al. (GCN 21177).
We warmly thank the staff of ING, in particular Raine Karjalainen, for their assistance.
GCN Circular #21210
E. Mazaeva (IKI), A. Pozanenko (IKI), D. Chestnov (ISON), A. Novichonok (ISON), V. Agletdinov (KIAM), I. Nikolenko (CrAO), A. Volnova (IKI), report on behalf of larger GRB follow-up collaboration:
We report refined analysis of Zeiss-1000/CrAO observations. Results reported below supersede GCN cir. 21174. We observed the field of the GRB 170531B (Evans et al., GCN 21171) with Zeiss-1000/Koshka (CrAO) telescope between May 31 (UT) 22:32:32 and June 01 (UT) 00:58:11. We took several unfiltered images with exposure of 180 s. The afterglow (Lipunov et al. GCN 21173; Pozanenko et al. GCN 21174) is clearly visible in each image in first initial images, and detected later in combined images. The finding chart of the combined image can be found at http://grb.rssi.ru/GRB170531B/GRB170531B_Koshka_170531_sources.png
The coordinates of the afterglow (source A in the finding chart) are (J2000) 19:07:32.10 -16:25:06.0 with uncertainty of 0.5 arcsec in both coordinates. The coordinates are consistent with the position reported by de Ugarte Postigo et al., GCN 21177, Tyurina et al., GCN 21179, and Siegel et al., GCN 21188.
Preliminary photometry of the afterglow can be found in the light curve at http://grb.rssi.ru/GRB170531B/GRB170531B_light_curve_v2.png
The photometry is based on nearby USNO-B1.0 stars, calibrated against to R2 magnitudes
```
USNO-B.1_id  R2
0736-0718845 16.77 
0735-0729622 16.96
0735-0729439 16.59 
0735-0729327 17.22 
```
Due to presence of sources in near vicinity to the afterglow the photometry of wide FOV telescopes might be affected by the sources. In particular our photometry reported in GCN 21189 is apparently biased by the two sources C and D (see the finding chart). We report coordinates and brightness of the nearby sources.
```
Source  coordinates(J2000)          AG distance  filter  mag.   err.
C       19:07:32.34 -16:25:09.1     4.6"         CR      20.46  0.04
D       19:07:31.83 -16:25:05.6     3.9"         CR      20.47  0.04
```

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Jochen Greiner, last update: 07-Jun-2017 [Disclaimer]