Gamma-ray Burst 170531A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN/BACODINE POSITION NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 31 May 17 20:08:49 UT
NOTICE_TYPE:     Swift-BAT GRB Position
TRIGGER_NUM:     755343,   Seg_Num: 0
GRB_RA:          121.261d {+08h 05m 03s} (J2000),
                 121.464d {+08h 05m 51s} (current),
                 120.680d {+08h 02m 43s} (1950)
GRB_DEC:         -14.033d {-14d 01' 57"} (J2000),
                 -14.083d {-14d 05' 00"} (current),
                 -13.890d {-13d 53' 22"} (1950)
GRB_ERROR:       3.00 [arcmin radius, statistical only]
GRB_INTEN:       0 [cnts]    Image_Peak=1748 [image_cnts]
TRIGGER_DUR:     64.000 [sec] 
TRIGGER_INDEX:   20000     E_range: 15-50 keV
BKG_INTEN:       0 [cnts]
BKG_TIME:        0.00 SOD {00:00:00.00} UT
BKG_DUR:         0 [sec]
GRB_DATE:        17904 TJD;   151 DOY;   17/05/31
GRB_TIME:        72452.85 SOD {20:07:32.85} UT
GRB_PHI:         -132.24 [deg]
GRB_THETA:        15.98 [deg]
SOLN_STATUS:     0x13
RATE_SIGNIF:     0.00 [sigma]
IMAGE_SIGNIF:    7.14 [sigma]
MERIT_PARAMS:     +1  +0  +0  +6  +1  -3  +0  +1 -45  +0 
SUN_POSTN:        68.96d {+04h 35m 50s}  +22.03d {+22d 01' 38"}
SUN_DIST:         62.87 [deg]   Sun_angle= -3.5 [hr] (East of Sun)
MOON_POSTN:      154.36d {+10h 17m 27s}  +10.94d {+10d 56' 17"}
MOON_DIST:        41.11 [deg]
MOON_ILLUM:      43 [%]
GAL_COORDS:      234.05,  9.32 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      127.15,-33.53 [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 = 2.64,-16.87 [deg].

red DSS finding chart ps-file

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 31 May 17 20:10:04 UT
NOTICE_TYPE:     Swift-XRT Position
TRIGGER_NUM:     755343,   Seg_Num: 0
GRB_RA:          121.2535d {+08h 05m 00.84s} (J2000),
                 121.4559d {+08h 05m 49.41s} (current),
                 120.6723d {+08h 02m 41.34s} (1950)
GRB_DEC:         -14.0230d {-14d 01' 22.7"} (J2000),
                 -14.0734d {-14d 04' 24.4"} (current),
                 -13.8798d {-13d 52' 47.1"} (1950)
GRB_ERROR:       7.4 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       2.92e-10 [erg/cm2/sec]
GRB_SIGNIF:      4.58 [sigma]
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  72575.54 SOD {20:09:35.54} UT,   122.7 [sec] since BAT Trigger Time
TAM[0-3]:        327.67 237.21 261.65 243.49
AMPLIFIER:       2
WAVEFORM:        134
SUN_POSTN:        68.96d {+04h 35m 50s}  +22.03d {+22d 01' 39"}
SUN_DIST:         62.86 [deg]   Sun_angle= -3.5 [hr] (East of Sun)
MOON_POSTN:      154.37d {+10h 17m 30s}  +10.93d {+10d 56' 05"}
MOON_DIST:        41.12 [deg]
MOON_ILLUM:      43 [%]
GAL_COORDS:      234.03,  9.32 [deg] galactic lon,lat of the burst
ECL_COORDS:      127.14,-33.52 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Coordinates.  
COMMENTS:        The XRT position is 0.74 arcmin from the BAT position.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 31 May 17 20:10:17 UT
NOTICE_TYPE:     Swift-XRT Processed Image
TRIGGER_NUM:     755343,   Seg_Num: 0
GRB_RA:          121.2535d {+08h 05m 00.8s} (J2000),
                 121.4559d {+08h 05m 49.4s} (current),
                 120.6723d {+08h 02m 41.3s} (1950)
GRB_DEC:         -14.0230d {-14d 01' 22.7"} (J2000),
                 -14.0734d {-14d 04' 24.4"} (current),
                 -13.8798d {-13d 52' 47.1"} (1950)
GRB_ERROR:       7.2 [arcsec, radius, statistical plus systematic]
GRB_INTEN:       21 [cnts]
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  72575.54 SOD {20:09:35.54} UT,   122.7 [sec] since BAT Trigger Time
CENTROID_X:      255.26,  raw= 255  [pixels]
CENTROID_Y:      292.91,  raw= 293  [pixels]
ROLL:            304.31 [deg]
GAIN:            1
MODE:            3, Long Image mode
WAVEFORM:        134
EXPO_TIME:       2.50 [sec]
GRB_POS_XRT_Y:   -12.93
GRB_POS_XRT_Z:   -120.53
IMAGE_URL:       sw00755343000msxps_rw.img
SUN_POSTN:        68.96d {+04h 35m 50s}  +22.03d {+22d 01' 39"}
SUN_DIST:         62.86 [deg]   Sun_angle= -3.5 [hr] (East of Sun)
MOON_POSTN:      154.38d {+10h 17m 30s}  +10.93d {+10d 56' 03"}
MOON_DIST:        41.12 [deg]
MOON_ILLUM:      43 [%]
GAL_COORDS:      234.03,  9.32 [deg] galactic lon,lat of the burst
ECL_COORDS:      127.14,-33.52 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Processed Image.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 31 May 17 20:10:15 UT
NOTICE_TYPE:     Swift-XRT Image
TRIGGER_NUM:     755343,   Seg_Num: 0
GRB_RA:          121.2535d {+08h 05m 00.8s} (J2000),
                 121.4559d {+08h 05m 49.4s} (current),
                 120.6723d {+08h 02m 41.3s} (1950)
GRB_DEC:         -14.0230d {-14d 01' 22.7"} (J2000),
                 -14.0734d {-14d 04' 24.4"} (current),
                 -13.8798d {-13d 52' 47.1"} (1950)
GRB_ERROR:       7.2 [arcsec, radius, statistical plus systematic]
GRB_INTEN:       21 [cnts]
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  72575.54 SOD {20:09:35.54} UT,   122.7 [sec] since BAT Trigger Time
CENTROID_X:      255.26,  raw= 255  [pixels]
CENTROID_Y:      292.91,  raw= 293  [pixels]
ROLL:            304.31 [deg]
GAIN:            1
MODE:            3, Long Image mode
WAVEFORM:        134
EXPO_TIME:       2.50 [sec]
GRB_POS_XRT_Y:   -12.93
GRB_POS_XRT_Z:   -120.53
IMAGE_URL:       sw00755343000msxps_rw.img
SUN_POSTN:        68.96d {+04h 35m 50s}  +22.03d {+22d 01' 39"}
SUN_DIST:         62.86 [deg]   Sun_angle= -3.5 [hr] (East of Sun)
MOON_POSTN:      154.37d {+10h 17m 30s}  +10.93d {+10d 56' 04"}
MOON_DIST:        41.12 [deg]
MOON_ILLUM:      43 [%]
GAL_COORDS:      234.03,  9.32 [deg] galactic lon,lat of the burst
ECL_COORDS:      127.14,-33.52 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Image.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 31 May 17 20:11:31 UT
NOTICE_TYPE:     Swift-BAT GRB Lightcurve
TRIGGER_NUM:     755343,   Seg_Num: 0
GRB_RA:          121.261d {+08h 05m 03s} (J2000),
                 121.464d {+08h 05m 51s} (current),
                 120.680d {+08h 02m 43s} (1950)
GRB_DEC:         -14.033d {-14d 01' 57"} (J2000),
                 -14.083d {-14d 05' 00"} (current),
                 -13.890d {-13d 53' 22"} (1950)
GRB_DATE:        17904 TJD;   151 DOY;   17/05/31
GRB_TIME:        72452.85 SOD {20:07:32.85} UT
TRIGGER_INDEX:   20000
GRB_PHI:         -132.24 [deg]
GRB_THETA:        15.98 [deg]
DELTA_TIME:      0.00 [sec]
TRIGGER_DUR:     64.000 [sec] 
SOLN_STATUS:     0x13
RATE_SIGNIF:     0.00 [sigma]
IMAGE_SIGNIF:    7.14 [sigma]
LC_URL:          sw00755343000msb.lc
SUN_POSTN:        68.96d {+04h 35m 50s}  +22.03d {+22d 01' 39"}
SUN_DIST:         62.87 [deg]   Sun_angle= -3.5 [hr] (East of Sun)
MOON_POSTN:      154.39d {+10h 17m 33s}  +10.93d {+10d 55' 52"}
MOON_DIST:        41.13 [deg]
MOON_ILLUM:      43 [%]
GAL_COORDS:      234.05,  9.32 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      127.15,-33.53 [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 = 2.64,-16.87 [deg].

GCN Circular #21170
P.A. Evans (U Leicester), S. D. Barthelmy (GSFC), H. A. Krimm (NSF/USRA), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL), M. H. Siegel (PSU) and T. N. Ukwatta (LANL) report on behalf of the Swift Team:
At 20:07:32 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 170531A (trigger=755343). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 121.261, -14.033 which is
```
   RA(J2000)  =  08h 05m 03s
   Dec(J2000) = -14d 01' 57"
```
with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for image triggers, there is nothing significant in the real-time light curve. We note that this trigger occurred while coming out of the SAA.
The XRT began observing the field at 20:09:35.5 UT, 122.6 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 121.25380, -14.02292 which is equivalent to:
```
   RA(J2000)  = 08h 05m 00.91s
   Dec(J2000) = -14d 01' 22.5"
```
with an uncertainty of 4.5 arcseconds (radius, 90% containment). This location is 44 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 2.92e-10 erg cm^-2 s^-1 (0.2-10 keV).
No UVOT data are available at this time.
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 21:10:56 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     755343,   Seg_Num: 0
POINT_RA:        121.269d {+08h 05m 05s} (J2000)
POINT_DEC:       -13.980d {-13d 58' 49"} (J2000)
POINT_ROLL:      304.399d
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  75974.86 SOD {21:06:14.86} UT
FILTER:          10, White
BKG_MEAN:        6.243
N_STARS:         189
X_OFFSET:        167 [pixels]
Y_OFFSET:        350 [pixels]
X_MAX:           1126 [pixels]
Y_MAX:           1309 [pixels]
DET_THRESH:      21
PHOTO_THRESH:    13
SL_URL:          sw00755343000msufc517957574.fits
SUN_POSTN:        69.00d {+04h 36m 00s}  +22.03d {+22d 01' 60"}
SUN_DIST:         62.81 [deg]   Sun_angle= -3.5 [hr] (East of Sun)
MOON_POSTN:      154.93d {+10h 19m 42s}  +10.78d {+10d 46' 49"}
MOON_DIST:        41.43 [deg]
MOON_ILLUM:      43 [%]
GAL_COORDS:      234.00,  9.36 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      127.15,-33.48 [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 21:11:09 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     755343,   Seg_Num: 0
POINT_RA:        121.269d {+08h 05m 05s} (J2000)
POINT_DEC:       -13.980d {-13d 58' 49"} (J2000)
POINT_ROLL:      304.399d
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  75974.86 SOD {21:06:14.86} UT
FILTER:          10, White
BKG_MEAN:        6.243
N_STARS:         189
X_OFFSET:        167 [pixels]
Y_OFFSET:        350 [pixels]
X_MAX:           1126 [pixels]
Y_MAX:           1309 [pixels]
DET_THRESH:      21
PHOTO_THRESH:    13
SL_URL:          sw00755343000msufc517957574.fits
SUN_POSTN:        69.00d {+04h 36m 00s}  +22.03d {+22d 01' 60"}
SUN_DIST:         62.81 [deg]   Sun_angle= -3.5 [hr] (East of Sun)
MOON_POSTN:      154.93d {+10h 19m 42s}  +10.78d {+10d 46' 47"}
MOON_DIST:        41.43 [deg]
MOON_ILLUM:      43 [%]
GAL_COORDS:      234.00,  9.36 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      127.15,-33.48 [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 21:11:29 UT
NOTICE_TYPE:     Swift-XRT Position UPDATE
TRIGGER_NUM:     755343,   Seg_Num: 0
GRB_RA:          121.2523d {+08h 05m 00.55s} (J2000),
                 121.4547d {+08h 05m 49.12s} (current),
                 120.6711d {+08h 02m 41.06s} (1950)
GRB_DEC:         -14.0227d {-14d 01' 21.7"} (J2000),
                 -14.0731d {-14d 04' 23.3"} (current),
                 -13.8795d {-13d 52' 46.0"} (1950)
GRB_ERROR:       3.5 [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:  72707.00 SOD {20:11:47.00} UT
TAM[0-3]:        100.00 100.00 100.00 100.00
AMPLIFIER:       1
WAVEFORM:        31
SUN_POSTN:        69.00d {+04h 36m 00s}  +22.03d {+22d 01' 60"}
SUN_DIST:         62.82 [deg]   Sun_angle= -3.5 [hr] (East of Sun)
MOON_POSTN:      154.93d {+10h 19m 43s}  +10.78d {+10d 46' 44"}
MOON_DIST:        41.47 [deg]
MOON_ILLUM:      43 [%]
GAL_COORDS:      234.03,  9.32 [deg] galactic lon,lat of the burst
ECL_COORDS:      127.14,-33.52 [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 21:12:28 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     755343,   Seg_Num: 0
POINT_RA:        121.269d {+08h 05m 05s} (J2000)
POINT_DEC:       -13.980d {-13d 58' 49"} (J2000)
ROLL:            304.399d
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  75974.86 SOD {21:06:14.86} UT
FILTER:          10, White
EXPOSURE_ID:     517957594
X_OFFSET:        486 [pixels]
Y_OFFSET:        669 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       646
Y_GRB_POS:       829
BINNING_INDEX:   1
IM_URL:          sw00755343000msuni517957594.fits
SUN_POSTN:        69.00d {+04h 36m 01s}  +22.03d {+22d 02' 00"}
SUN_DIST:         62.81 [deg]   Sun_angle= -3.5 [hr] (East of Sun)
MOON_POSTN:      154.94d {+10h 19m 45s}  +10.78d {+10d 46' 35"}
MOON_DIST:        41.44 [deg]
MOON_ILLUM:      43 [%]
GAL_COORDS:      234.00,  9.36 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      127.15,-33.48 [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 21:12:38 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     755343,   Seg_Num: 0
POINT_RA:        121.269d {+08h 05m 05s} (J2000)
POINT_DEC:       -13.980d {-13d 58' 49"} (J2000)
ROLL:            304.399d
IMG_START_DATE:  17904 TJD;   151 DOY;   17/05/31
IMG_START_TIME:  75974.86 SOD {21:06:14.86} UT
FILTER:          10, White
EXPOSURE_ID:     517957594
X_OFFSET:        486 [pixels]
Y_OFFSET:        669 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       646
Y_GRB_POS:       829
BINNING_INDEX:   1
IM_URL:          sw00755343000msuni517957594.fits
SUN_POSTN:        69.00d {+04h 36m 01s}  +22.03d {+22d 02' 00"}
SUN_DIST:         62.81 [deg]   Sun_angle= -3.5 [hr] (East of Sun)
MOON_POSTN:      154.94d {+10h 19m 46s}  +10.78d {+10d 46' 33"}
MOON_DIST:        41.44 [deg]
MOON_ILLUM:      43 [%]
GAL_COORDS:      234.00,  9.36 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      127.15,-33.48 [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 #21172
M. H. Siegel (PSU) and P. A. Evans (U Leicester) report on behalf of the Swift/UVOT team:
UVOT took a finding chart exposure of 150 seconds with the White filter starting 3522 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 coverage of the XRT error circle by the 8'x8' region for the list of sources generated on-board is uncertain because the large number of sources filled the available telemetry. 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.
GCN Circular #21176
G. Pignata (Univ. Andres Bello), L. Wang (CASSACA), P. D'Avanzo (INAF-OAB), D. Malesani (Dark/NBI), L. Izzo (IAA-CSIC), S. Campana (INAF OAB), J. Anderson (ESO), R. Cartier (Southampton), L. Galbany (Univ. de Chile), C. Inserra (QUB), E. Kankare (QUB), K. Maguire (ESO), S. Smartt (QUB), K. W. Smith (QUB), M. Sullivan (Southampton), S. Valenti (UC Davis), O. Yaron (Weizmann), D. Young (QUB), I. Manulis (Weizmann)
We observed GRB 170531A (Evans et al., GCN 21170) under the extended Public ESO Spectroscopic Survey for Transient Objects (ePESSTO; see Smartt et al. 2015, A&A, 579, 40 http://www.pessto.org ). The observations were performed on the ESO New Technology Telescope at La Silla with the EFOSC2 instrument in imaging mode starting from 2017-05-31 at 23:12:07 UT (i.e. 3.1 hours from the burst).
We obtained two consecutive images each one lasting 150s with the r filter. No optical afterglow is detected in the coadded image within the XRT error circle (http://www.swift.ac.uk/xrt_positions/) down to a limiting magnitude of r > 23 (AB; 3sigma c.l.). The photometric calibration was carried out assuming r=17.07 (AB) for the star at RA(J2000), Dec(J2000) = 08:05:00.30, -14:01:21.3, as reported in the PANSTARRS catalogue.
GCN Circular #21180
A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), S. J. LaPorte (PSU), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), B. Mingo (U. Leicester), V. D'Elia (ASDC) and P.A. Evans report on behalf of the Swift-XRT team:
We have analysed 6.2 ks of XRT data for GRB 170531A (Evans et al. GCN Circ. 21170), from 254 s to 44.6 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The best available XRT position (using the promptly downlinked event data, the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue) is RA, Dec = 121.2524, -14.0227 which is equivalent to:
```
RA (J2000): 08 05 00.57
Dec(J2000): -14 01 21.8
```
with an uncertainty of 3.5 arcsec (radius, 90% confidence).
The light curve can be modelled with a power-law decay with a decay index of alpha=0.62 (+0.20, -0.16).
If the light curve continues to decay with a power-law decay index of 0.62, the count rate at T+24 hours will be 1.3 x 10^-3 count s^-1
The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00755343.
This circular is an official product of the Swift-XRT team.
GCN Circular #21181
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
R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres The Instituto de Astrofisica de Canarias
D.Buckley South African Astronomical Observatory
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
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-SAAO robotic telescope (MASTER-Net: http://observ.pereplet.ru) located in SAAO was pointed to the GRB170531.84 25 sec after notice time and 99 sec after trigger time at 2017-05-31 20:09:15 UT. On our first (20s exposure) set we haven`t found optical transient within SWIFT error-box (ra=121.258 dec=-14.0328 r=0.05). The 5-sigma upper limit has been about 15.5mag
The observations made on zenit distance = 84 degrees, galaxy latitude b = 10 degree. The moon (43 % bright part) is 19 degrees above the horizon. The distance between moon and object is 41 The sun altitude is -56.5 degree.
MASTER-IAC robotic telescope (2x4 square degrees, MASTER-Net: http://observ.pereplet.ru) was pointed to the GRB170531A (Evans et al., GCN #21170; Siegel et al. GCN #21172) 895 sec after notice time and 2115 sec after trigger time on the 2017-05-31 20:42:48 UT.
The observations were made on zenit distance = 74 degrees during the twilight. The sun altitude was -9.6 degree. The moon (43 % bright part) is 58 degrees above the horizon. The distance between moon and object is 41deg.
On our 5-th (180s exposure) set we haven`t found optical transient within SWIFT error-box (ra=121.258 dec=-14.0328 r=0.1). The 5-sigma upper limit m~13.9mag (unfiltered) on the first image and
MASTER-OAFA robotic telescope (MASTER-Net: http://observ.pereplet.ru) located in OAFA was pointed to the GRB170531.84 15674 sec after notice time and 15751 sec after trigger time at 2017-06-01 00:30:03 UT. On our first (180s exposure) set we haven`t found optical transient within Master error-box (ra=121.25 dec=-14.0225 r=0.1). The 5-sigma upper limit has been about 18mag The message may be cited.
The observations made on zenit distance = 63 degrees, galaxy latitude b = 10 degree. The moon (45 % bright part) is 38 degrees above the horizon. The distance between moon and object is 43 The sun altitude is -35.4 degree. The object can be observed till 2017-06-01 02:41:37.
GCN Circular #21184
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 170531A 3523 s after the BAT trigger (Evans et al., GCN Circ. 21170). No optical afterglow consistent with the XRT position (Melandri et al., GCN Circ. 21180) is detected in the initial UVOT exposu= res. We note that the XRT position is 3.5=94 away from a known USNO-B1 source,= which we detect. There is no indication of a second source within the error cir= cle. 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          3523         3673          147         >18.1
white             3523         4292          344         >18.2
v                 4502         4702          197         >17.5
b                 3887         4087          197         >18.2
u                 3682         3881          197         >18.0
w2                4298         4497          197         >19.4
```
The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) =3D 0.09 in the direction of the burst (Schlegel et al. 1998).
GCN Circular #21185
D. M. Palmer (LANL), 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), T. Sakamoto (AGU), M. Stamatikos (OSU), T. N. Ukwatta (LANL) (i.e. the Swift-BAT team):
Using the data set from T-65 to T+963 sec from the recent telemetry downlink, we report further analysis of BAT GRB 170531A (trigger #755343) (Evans et al., GCN Circ. 21170). The BAT ground-calculated position is RA, Dec = 121.218, -14.017 deg which is
```
  RA(J2000)  =  08h 04m 52.4s
  Dec(J2000) = -14d 00' 59.7"
```
with an uncertainty of 2.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 92%.
The mask-weighted light curve shows a weak pulse that starts ~T-10 s and ends at ~T+25 s. T90 (15-350 keV) is 26.74 +- 7.94 sec (estimated error including systematics).
The time-averaged spectrum from T-13.24 to T+25.78 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.22 +- 0.28. The fluence in the 15-150 keV band is 7.5 +- 1.2 x 10^-7 erg/cm2. 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/755343/BA/

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Jochen Greiner, last update: 01-Jun-2017 [Disclaimer]