Gamma-ray Burst 180821A

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• GCN/BACODINE POSITION NOTICE
  

  TITLE:           GCN/SWIFT NOTICE
  NOTICE_DATE:     Tue 21 Aug 18 16:42:48 UT
  NOTICE_TYPE:     Swift-BAT GRB Position
  TRIGGER_NUM:     854578,   Seg_Num: 0
  GRB_RA:            7.763d {+00h 31m 03s} (J2000),
                     7.990d {+00h 31m 58s} (current),
                     7.151d {+00h 28m 36s} (1950)
  GRB_DEC:         -38.619d {-38d 37' 07"} (J2000),
                   -38.516d {-38d 30' 57"} (current),
                   -38.895d {-38d 53' 40"} (1950)
  GRB_ERROR:       3.00 [arcmin radius, statistical only]
  GRB_INTEN:       0 [cnts]    Image_Peak=1050 [image_cnts]
  TRIGGER_DUR:     120.000 [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:        18351 TJD;   233 DOY;   18/08/21
  GRB_TIME:        60034.53 SOD {16:40:34.53} UT
  GRB_PHI:          32.55 [deg]
  GRB_THETA:        44.83 [deg]
  SOLN_STATUS:     0x13
  RATE_SIGNIF:     0.00 [sigma]
  IMAGE_SIGNIF:    7.81 [sigma]
  MERIT_PARAMS:     +1  +0  +0  +7  +1  -2  +0  +0 +74  +0 
  SUN_POSTN:       150.73d {+10h 02m 56s}  +11.97d {+11d 57' 57"}
  SUN_DIST:        137.59 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
  MOON_POSTN:      276.86d {+18h 27m 27s}  -20.64d {-20d 38' 16"}
  MOON_DIST:        78.17 [deg]
  MOON_ILLUM:      81 [%]
  GAL_COORDS:      322.02,-77.74 [deg] galactic lon,lat of the burst (or transient)
  ECL_COORDS:      348.93,-37.92 [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 = 182.41,13.27 [deg].  
  

• red DSS finding chart ps-file
  
  
• GCN NOTICE
  

  TITLE:           GCN/SWIFT NOTICE
  NOTICE_DATE:     Tue 21 Aug 18 16:44:48 UT
  NOTICE_TYPE:     Swift-XRT Image
  TRIGGER_NUM:     854578,   Seg_Num: 0
  GRB_RA:            7.7893d {+00h 31m 09.4s} (J2000),
                     8.0167d {+00h 32m 04.0s} (current),
                     7.1779d {+00h 28m 42.6s} (1950)
  GRB_DEC:         -38.6248d {-38d 37' 29.2"} (J2000),
                   -38.5220d {-38d 31' 19.2"} (current),
                   -38.9008d {-38d 54' 02.9"} (1950)
  GRB_ERROR:       6.8 [arcsec, radius, statistical plus systematic]
  GRB_INTEN:       22 [cnts]
  IMG_START_DATE:  18351 TJD;   233 DOY;   18/08/21
  IMG_START_TIME:  60244.83 SOD {16:44:04.83} UT,   210.3 [sec] since BAT Trigger Time
  CENTROID_X:      283.04,  raw= 283  [pixels]
  CENTROID_Y:      292.35,  raw= 292  [pixels]
  ROLL:            118.92 [deg]
  GAIN:            1
  MODE:            3, Long Image mode
  WAVEFORM:        134
  EXPO_TIME:       2.50 [sec]
  GRB_POS_XRT_Y:   -14.25
  GRB_POS_XRT_Z:   -55.04
  IMAGE_URL:       sw00854578000msxps_rw.img
  SUN_POSTN:       150.73d {+10h 02m 56s}  +11.97d {+11d 57' 56"}
  SUN_DIST:        137.57 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
  MOON_POSTN:      276.88d {+18h 27m 31s}  -20.64d {-20d 38' 18"}
  MOON_DIST:        78.17 [deg]
  MOON_ILLUM:      81 [%]
  GAL_COORDS:      321.92,-77.75 [deg] galactic lon,lat of the burst
  ECL_COORDS:      348.95,-37.94 [deg] ecliptic lon,lat of the burst
  COMMENTS:        SWIFT-XRT Image.  
  

  
• GCN NOTICE
  

  TITLE:           GCN/SWIFT NOTICE
  NOTICE_DATE:     Tue 21 Aug 18 16:45:36 UT
  NOTICE_TYPE:     Swift-BAT GRB Lightcurve
  TRIGGER_NUM:     854578,   Seg_Num: 0
  GRB_RA:            7.763d {+00h 31m 03s} (J2000),
                     7.990d {+00h 31m 58s} (current),
                     7.151d {+00h 28m 36s} (1950)
  GRB_DEC:         -38.619d {-38d 37' 07"} (J2000),
                   -38.516d {-38d 30' 57"} (current),
                   -38.895d {-38d 53' 40"} (1950)
  GRB_DATE:        18351 TJD;   233 DOY;   18/08/21
  GRB_TIME:        60034.53 SOD {16:40:34.53} UT
  TRIGGER_INDEX:   20000
  GRB_PHI:          32.55 [deg]
  GRB_THETA:        44.83 [deg]
  DELTA_TIME:      0.00 [sec]
  TRIGGER_DUR:     120.000 [sec] 
  SOLN_STATUS:     0x13
  RATE_SIGNIF:     0.00 [sigma]
  IMAGE_SIGNIF:    7.81 [sigma]
  LC_URL:          sw00854578000msb.lc
  SUN_POSTN:       150.73d {+10h 02m 56s}  +11.97d {+11d 57' 55"}
  SUN_DIST:        137.59 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
  MOON_POSTN:      276.89d {+18h 27m 32s}  -20.64d {-20d 38' 19"}
  MOON_DIST:        78.15 [deg]
  MOON_ILLUM:      81 [%]
  GAL_COORDS:      322.02,-77.74 [deg] galactic lon,lat of the burst (or transient)
  ECL_COORDS:      348.93,-37.92 [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 = 182.41,13.27 [deg].  
  

  
• GCN NOTICE
  

  TITLE:           GCN/SWIFT NOTICE
  NOTICE_DATE:     Tue 21 Aug 18 16:44:50 UT
  NOTICE_TYPE:     Swift-XRT Processed Image
  TRIGGER_NUM:     854578,   Seg_Num: 0
  GRB_RA:            7.7893d {+00h 31m 09.4s} (J2000),
                     8.0167d {+00h 32m 04.0s} (current),
                     7.1779d {+00h 28m 42.6s} (1950)
  GRB_DEC:         -38.6248d {-38d 37' 29.2"} (J2000),
                   -38.5220d {-38d 31' 19.2"} (current),
                   -38.9008d {-38d 54' 02.9"} (1950)
  GRB_ERROR:       6.8 [arcsec, radius, statistical plus systematic]
  GRB_INTEN:       22 [cnts]
  IMG_START_DATE:  18351 TJD;   233 DOY;   18/08/21
  IMG_START_TIME:  60244.83 SOD {16:44:04.83} UT,   210.3 [sec] since BAT Trigger Time
  CENTROID_X:      283.04,  raw= 283  [pixels]
  CENTROID_Y:      292.35,  raw= 292  [pixels]
  ROLL:            118.92 [deg]
  GAIN:            1
  MODE:            3, Long Image mode
  WAVEFORM:        134
  EXPO_TIME:       2.50 [sec]
  GRB_POS_XRT_Y:   -14.25
  GRB_POS_XRT_Z:   -55.04
  IMAGE_URL:       sw00854578000msxps_rw.img
  SUN_POSTN:       150.73d {+10h 02m 56s}  +11.97d {+11d 57' 56"}
  SUN_DIST:        137.57 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
  MOON_POSTN:      276.88d {+18h 27m 31s}  -20.64d {-20d 38' 18"}
  MOON_DIST:        78.17 [deg]
  MOON_ILLUM:      81 [%]
  GAL_COORDS:      321.92,-77.75 [deg] galactic lon,lat of the burst
  ECL_COORDS:      348.95,-37.94 [deg] ecliptic lon,lat of the burst
  COMMENTS:        SWIFT-XRT Processed Image.  
  

  
• GCN NOTICE
  

  TITLE:           GCN/SWIFT NOTICE
  NOTICE_DATE:     Tue 21 Aug 18 16:44:38 UT
  NOTICE_TYPE:     Swift-XRT Position
  TRIGGER_NUM:     854578,   Seg_Num: 0
  GRB_RA:            7.7893d {+00h 31m 09.43s} (J2000),
                     8.0167d {+00h 32m 04.01s} (current),
                     7.1779d {+00h 28m 42.69s} (1950)
  GRB_DEC:         -38.6248d {-38d 37' 29.2"} (J2000),
                   -38.5220d {-38d 31' 19.2"} (current),
                   -38.9008d {-38d 54' 02.9"} (1950)
  GRB_ERROR:       6.9 [arcsec radius, statistical plus systematic, 90% containment]
  GRB_INTEN:       3.67e-10 [erg/cm2/sec]
  GRB_SIGNIF:      4.69 [sigma]
  IMG_START_DATE:  18351 TJD;   233 DOY;   18/08/21
  IMG_START_TIME:  60244.83 SOD {16:44:04.83} UT,   210.3 [sec] since BAT Trigger Time
  TAM[0-3]:        327.69 237.18 261.70 243.71
  AMPLIFIER:       2
  WAVEFORM:        134
  SUN_POSTN:       150.73d {+10h 02m 56s}  +11.97d {+11d 57' 56"}
  SUN_DIST:        137.57 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
  MOON_POSTN:      276.88d {+18h 27m 30s}  -20.64d {-20d 38' 18"}
  MOON_DIST:        78.17 [deg]
  MOON_ILLUM:      81 [%]
  GAL_COORDS:      321.92,-77.75 [deg] galactic lon,lat of the burst
  ECL_COORDS:      348.95,-37.94 [deg] ecliptic lon,lat of the burst
  COMMENTS:        SWIFT-XRT Coordinates.  
  COMMENTS:        The XRT position is 1.28 arcmin from the BAT position.  
  

• GCN NOTICE
  

  TITLE:           GCN/SWIFT NOTICE
  NOTICE_DATE:     Tue 21 Aug 18 16:46:57 UT
  NOTICE_TYPE:     Swift-UVOT Source List
  TRIGGER_NUM:     854578,   Seg_Num: 0
  POINT_RA:          7.774d {+00h 31m 06s} (J2000)
  POINT_DEC:       -38.615d {-38d 36' 53"} (J2000)
  POINT_ROLL:      118.924d
  IMG_START_DATE:  18351 TJD;   233 DOY;   18/08/21
  IMG_START_TIME:  60253.20 SOD {16:44:13.20} UT,   218.7 [sec] since BAT Trigger Time
  FILTER:          10, White
  BKG_MEAN:        1.732
  N_STARS:         22
  X_OFFSET:        480 [pixels]
  Y_OFFSET:        632 [pixels]
  X_MAX:           1439 [pixels]
  Y_MAX:           1591 [pixels]
  DET_THRESH:      11
  PHOTO_THRESH:    5
  SL_URL:          sw00854578000msufc0218.fits
  SUN_POSTN:       150.73d {+10h 02m 56s}  +11.97d {+11d 57' 54"}
  SUN_DIST:        137.58 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
  MOON_POSTN:      276.90d {+18h 27m 35s}  -20.64d {-20d 38' 20"}
  MOON_DIST:        78.15 [deg]
  MOON_ILLUM:      81 [%]
  GAL_COORDS:      321.98,-77.75 [deg] galactic lon,lat of the pointing direction
  ECL_COORDS:      348.94,-37.92 [deg] ecliptic lon,lat of the pointing direction
  COMMENTS:        SWIFT-UVOT Source List.  
  

• GCN NOTICE
  

  TITLE:           GCN/SWIFT NOTICE
  NOTICE_DATE:     Tue 21 Aug 18 16:47:10 UT
  NOTICE_TYPE:     Swift-UVOT Processed Source List
  TRIGGER_NUM:     854578,   Seg_Num: 0
  POINT_RA:          7.774d {+00h 31m 06s} (J2000)
  POINT_DEC:       -38.615d {-38d 36' 53"} (J2000)
  POINT_ROLL:      118.924d
  IMG_START_DATE:  18351 TJD;   233 DOY;   18/08/21
  IMG_START_TIME:  60253.20 SOD {16:44:13.20} UT,   218.7 [sec] since BAT Trigger Time
  FILTER:          10, White
  BKG_MEAN:        1.732
  N_STARS:         22
  X_OFFSET:        480 [pixels]
  Y_OFFSET:        632 [pixels]
  X_MAX:           1439 [pixels]
  Y_MAX:           1591 [pixels]
  DET_THRESH:      11
  PHOTO_THRESH:    5
  SL_URL:          sw00854578000msufc0218.fits
  SUN_POSTN:       150.73d {+10h 02m 56s}  +11.96d {+11d 57' 54"}
  SUN_DIST:        137.58 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
  MOON_POSTN:      276.90d {+18h 27m 36s}  -20.64d {-20d 38' 21"}
  MOON_DIST:        78.15 [deg]
  MOON_ILLUM:      81 [%]
  GAL_COORDS:      321.98,-77.75 [deg] galactic lon,lat of the pointing direction
  ECL_COORDS:      348.94,-37.92 [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:     Tue 21 Aug 18 16:48:22 UT
  NOTICE_TYPE:     Swift-UVOT Image
  TRIGGER_NUM:     854578,   Seg_Num: 0
  POINT_RA:          7.774d {+00h 31m 06s} (J2000)
  POINT_DEC:       -38.615d {-38d 36' 53"} (J2000)
  ROLL:            118.924d
  IMG_START_DATE:  18351 TJD;   233 DOY;   18/08/21
  IMG_START_TIME:  60253.20 SOD {16:44:13.20} UT,   218.7 [sec] since BAT Trigger Time
  FILTER:          10, White
  EXPOSURE_ID:     556562674
  X_OFFSET:        912 [pixels]
  Y_OFFSET:        857 [pixels]
  WIDTH:           160 [pixels]
  HEIGHT:          160 [pixels]
  X_GRB_POS:       1072
  Y_GRB_POS:       1017
  BINNING_INDEX:   1
  IM_URL:          sw00854578000msuni0240.fits
  SUN_POSTN:       150.74d {+10h 02m 56s}  +11.96d {+11d 57' 53"}
  SUN_DIST:        137.58 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
  MOON_POSTN:      276.91d {+18h 27m 38s}  -20.64d {-20d 38' 22"}
  MOON_DIST:        78.14 [deg]
  MOON_ILLUM:      81 [%]
  GAL_COORDS:      321.98,-77.75 [deg] galactic lon,lat of the pointing direction
  ECL_COORDS:      348.94,-37.92 [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:     Tue 21 Aug 18 16:48:42 UT
  NOTICE_TYPE:     Swift-UVOT Processed Image
  TRIGGER_NUM:     854578,   Seg_Num: 0
  POINT_RA:          7.774d {+00h 31m 06s} (J2000)
  POINT_DEC:       -38.615d {-38d 36' 53"} (J2000)
  ROLL:            118.924d
  IMG_START_DATE:  18351 TJD;   233 DOY;   18/08/21
  IMG_START_TIME:  60253.20 SOD {16:44:13.20} UT,   218.7 [sec] since BAT Trigger Time
  FILTER:          10, White
  EXPOSURE_ID:     556562674
  X_OFFSET:        912 [pixels]
  Y_OFFSET:        857 [pixels]
  WIDTH:           160 [pixels]
  HEIGHT:          160 [pixels]
  X_GRB_POS:       1072
  Y_GRB_POS:       1017
  BINNING_INDEX:   1
  IM_URL:          sw00854578000msuni0240.fits
  SUN_POSTN:       150.74d {+10h 02m 56s}  +11.96d {+11d 57' 53"}
  SUN_DIST:        137.58 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
  MOON_POSTN:      276.91d {+18h 27m 39s}  -20.64d {-20d 38' 22"}
  MOON_DIST:        78.14 [deg]
  MOON_ILLUM:      81 [%]
  GAL_COORDS:      321.98,-77.75 [deg] galactic lon,lat of the pointing direction
  ECL_COORDS:      348.94,-37.92 [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 #23158
  A. Tohuvavohu (PSU), J.D. Gropp (PSU), J. A. Kennea (PSU), H. A. Krimm (NSF/USRA), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL), B. Sbarufatti (PSU), M. H. Siegel (PSU) and K. K. Simpson (PSU) report on behalf of the Neil Gehrels Swift Observatory Team:
  At 16:40:34 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180821A (trigger=854578). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 7.763, -38.619 which is

     RA(J2000) = 00h 31m 03s
     Dec(J2000) = -38d 37' 07"
  

  with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is usual for an image trigger, the immediately-available BAT lightcurve shows no obvious variation.
  The XRT began observing the field at 16:44:04.8 UT, 210.3 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source located at RA, Dec 7.78833, -38.62660 which is equivalent to:

     RA(J2000)  = 00h 31m 09.20s
     Dec(J2000) = -38d 37' 35.8"
  

  with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 76 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. 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 3.67e-10 erg cm^-2 s^-1 (0.2-10 keV).
  UVOT took a finding chart exposure of 150 seconds with the White filter starting 218 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 0.02.
  Burst Advocate for this burst is A. Tohuvavohu (aaronb AT swift.psu.edu). 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:     Tue 21 Aug 18 17:04:04 UT
  NOTICE_TYPE:     Swift-XRT Position UPDATE
  TRIGGER_NUM:     854578,   Seg_Num: 0
  GRB_RA:            7.7883d {+00h 31m 09.19s} (J2000),
                     8.0157d {+00h 32m 03.77s} (current),
                     7.1769d {+00h 28m 42.45s} (1950)
  GRB_DEC:         -38.6266d {-38d 37' 35.7"} (J2000),
                   -38.5238d {-38d 31' 25.7"} (current),
                   -38.9026d {-38d 54' 09.4"} (1950)
  GRB_ERROR:       3.7 [arcsec radius, statistical plus systematic, 90% containment]
  GRB_INTEN:       1.00e-10 [erg/cm2/sec]
  GRB_SIGNIF:      10.00 [sigma]
  IMG_START_DATE:  18351 TJD;   233 DOY;   18/08/21
  IMG_START_TIME:  60424.00 SOD {16:47:04.00} UT,   389.5 [sec] since BAT Trigger Time
  TAM[0-3]:        100.00 100.00 100.00 100.00
  AMPLIFIER:       1
  WAVEFORM:        31
  SUN_POSTN:       150.75d {+10h 02m 59s}  +11.96d {+11d 57' 40"}
  SUN_DIST:        137.57 [deg]   Sun_angle= 9.5 [hr] (West of Sun)
  MOON_POSTN:      277.05d {+18h 28m 11s}  -20.64d {-20d 38' 37"}
  MOON_DIST:        78.04 [deg]
  MOON_ILLUM:      81 [%]
  GAL_COORDS:      321.92,-77.74 [deg] galactic lon,lat of the burst
  ECL_COORDS:      348.95,-37.94 [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:        The probability that this is a serendipitous source in the  
  COMMENTS:        SPER window is 0.58% < P(seren) < 1.1%.  
  

• GCN Circular #23159
  E. Fernandez-Garcia, Y.-D. Hu, J. C. Tello, A. Ayala and A. J. Castro-Tirado (IAA-CSIC), I. Carrasco and C. Perez del Pulgar (Univ. de Malaga), M. D. Caballero-Garcia and M. Jelinek (ASU-CAS, CZ), R. Cunniffe (Inst. of Physics, CZ) and R. Querel (NIWA), on behalf of a larger collaboration, report:
  The 60cm BOOTES-3/YA robotic telescope at NIWA Lauder in Otago (New Zealand) automatically responded in 59s (and 193s after the GRB onset) to the Swift trigger of GRB 180821A (Tohuvavohu et al., GCNC 23158). The first images (clear filter) were obtained starting at 16:43:47 UT. At the position of the Swift/XR X-ray position, no optical afterglow is detected down to 19 mag on a 100s co-added image at 16:44:45 (mid-exposure), in agreement with the non-detection by Swift/UVOT. Further analysis is ongoing.
  We thank the staff at NIWA for its excellent support.
• GCN Circular #23160
  A.K.H. Kong (NTHU, Taiwan) reports
  We observed the field of GRB 180821A (Tohuvavohu et al. GCN 23158) with the T30 0.51m telescope of iTelescope.Net in Siding Spring, Australia. The observation was done with a luminance filter (400-700 nm) beginning at 2018-08-21 17:26:22 UT (46 min after the BAT trigger). At the position of the Swift X-ray afterglow, an optical source is marginally detected with a magnitude of 20.4 +/- 0.2. We propose this source is the optical afterglow to GRB 180821A. Follow-up observations are encouraged.
• GCN Circular #23162
  P.A. Evans, M.R. Goad, J.P. Osborne and A.P. Beardmore (U. Leicester) report on behalf of the Swift-XRT team.
  Using 2528 s of XRT Photon Counting mode data and 6 UVOT images for GRB 180821A, 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 = 7.79011, -38.62713 which is equivalent to:

  RA (J2000): 00h 31m 9.63s
  Dec (J2000): -38d 37' 37.7"
  

  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 #23163
  C. Guidorzi, R. Martone (U. Ferrara), S. Kobayashi (LJMU), C.G. Mundell (U. Bath), A. Gomboc (U. Nova Gorica), I.A. Steele (LJMU), A. Cucchiara, D. Morris (U. of Virgin Islands) on behalf of a large collaboration report:
  We automatically observed Swift GRB 180821A (Tohuvavohu et al. GCN 23158) with the LCO 2-m unit at Siding Springs on August 21, from 16:54 to 17:07 UT (corresponding to 14 to 27 minutes from the GRB trigger time) with the SDSS i' filter. Within the enhanced Swift-XRT error circle (Evans et al. GCN 23162) we do not detect any source down to i'>21.0 mag from a 10x60s exposure at a mid time of 0.33 hours post GRB, as calibrated against nearby USNOB1 objects.
• GCN Circular #23164
  D.N. Burrows (PSU), S. J. LaPorte (PSU), P.A. Evans (U. Leicester), S.L. Gibson (U. Leicester), Z. Liu (NAOC / U. Leicester), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC) and A. Tohuvavohu report on behalf of the Swift-XRT team:
  We have analysed 8.8 ks of XRT data for GRB 180821A (Tohuvavohu et al. GCN Circ. 23158), from 199 s to 46.5 ks after the BAT trigger. The data comprise 171 s in Windowed Timing (WT) mode (the first 9 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 Evans et al. (GCN Circ. 23162).
  The late-time light curve (from T0+3.8 ks) can be modelled with a power-law decay with a decay index of alpha=0.49 (+0.13, -0.12).
  A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 5.1 (+0.4, -0.3). The best-fitting absorption column is 6.8 (+/-0.8) x 10^21 cm^-2, in excess of the Galactic value of 2.1 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 2.80 (+0.31, -0.29) and a best-fitting absorption column of 3.4 (+1.0, -0.8) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 2.6 x 10^-11 (6.8 x 10^-11) erg cm^-2 count^-1.
  A summary of the PC-mode spectrum is thus:

  Total column:	     3.4 (+1.0, -0.8) x 10^21 cm^-2
  Galactic foreground: 2.1 x 10^20 cm^-2
  Excess significance: 6.2 sigma
  Photon index:	     2.80 (+0.31, -0.29)
  

  If the light curve continues to decay with a power-law decay index of 0.49, the count rate at T+24 hours will be 0.014 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.6 x 10^-13 (9.4 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/00854578.
  This circular is an official product of the Swift-XRT team.
• GCN Circular #23165
  A. K. H. Kong (NTHU, Taiwan) reports
  We reanalyse the image obtained with the T30 telescope of iTelescope.Net (Kong GCN 23160) together with the enhanced Swift XRT position (Evans et al. GCN 23162). Within the new error circle, we do not detect any source with a detection limit of ~20.5 mag. The result is consistent with Guidorzi et al. (GCN 23163). We conclude that the previous counterpart candidate (which is outside the enhanced X-ray error circle) is not the optical afterglow.
• GCN Circular #23166
  H. A. Krimm (NSF/USRA), S. D. Barthelmy (GSFC), J. R. Cummings (CPI), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), D. M. Palmer (LANL), T. Sakamoto (AGU), M. Stamatikos (OSU), A. Tohuvavohu (PSU), 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 180821A (trigger #854578) (Tohuvavohu, et al., GCN Circ. 23158). The BAT ground-calculated position is RA, Dec = 7.739, -38.612 deg which is

   RA(J2000)  =  00h 30m 57.4s 
   Dec(J2000) = -38d 36' 41.9" 
  

  with an uncertainty of 2.3 arcmin, (radius, sys+stat, 90% containment). The partial coding was 17%.
  The mask-weighted light curve showed a broad peak with some structure, running from ~T+0 to T+70 sec. T90 (15-350 keV) is 64.00 +- 22.6 sec (estimated error including systematics).
  The time-averaged spectrum from T+1.00 to T+65.00 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.79 +- 0.31. The fluence in the 15-150 keV band is 1.4 +- 0.3 x 10^-6 erg/cm2. Since the peak was so broad, the automated analysis software was unable to derive a significant peak photon flux at the highest point in the light curve, T+56.5 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/854578/BA/
• GCN Circular #23167
  F. E. Marshall (NASA/GSFC) and A. Tohuvavohu (PSU) report on behalf of the Swift/UVOT team:
  The Swift/UVOT began settled observations of the field of GRB 180821A 219 s after the BAT trigger (Tohuvavohu et al., GCN Circ. 23158). No optical afterglow consistent with the enhanced XRT position (Evans et al. GCN Circ. 23162) 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           219          369          147         >21.0
  white              219          495          167         >21.1
  v                  526          546           19         >17.7
  v                 4604         6039          393         >20.0
  b                  451          471           19         >19.3
  b                 3988         5624          393         >21.3
  u                  426          446           19         >18.6
  u                 3783         5418          393         >20.5
  w1                 402          421           19         >19.3
  w1                5015        15906          241         >20.5
  m2                4808        10398          812         >20.5
  w2                4399         6034          393         >20.3
  

  The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.02 in the direction of the burst (Schlegel et al. 1998).
• GCN Circular #23168
  Paul Vreeswijk (Radboud U. Nijmegen), Daniele Malesani, Jonatan Selsing (DAWN/NBI, DARK/NBI), Nial Tanvir (U. Leicester), Giovanna Pugliese (API/U. Amsterdam), Andrea Rossi (INAF-OAS) and Andrew Levan (U. Warwick) report on behalf of the Stargate collaboration:
  The Swift XRT error circle (Tohuvavohu et al., GCN 23158; Burrows et al., GCN 23164) of GRB 180821A (Krimm et al., GCN 23166) was observed with the Very Large Telescope on the night of August 21/22, 2018, using the telescope/instrument combinations Kueyen/X-shooter and Antu/FORS2.
  The FORS2 imaging observations started at 5:58 UT on August 22 (13.3 hours after the GRB trigger) and consisted of 8 exposures of 120 seconds in the z_Gunn filter. No object is detected in the enhanced XRT error circle (Evans et al., GCN 23162), down to a 3-sigma limiting magnitude of z(AB)=24.9 (see also Fernandez-Garcia, GCN 23159; Guidorzi et al., GCN 23163; Kong, GCNs 23160 and 23165; Marshall et al., GCN 23167). The z-band coverage of this field by the SkyMapper Southern Sky Survey (see http://skymapper.anu.edu.au) was used in the determination of the magnitude limit.
  The X-shooter spectroscopic observations started at 6:55 UT on August 22 (14.3 hours after the GRB). A sequence of 4 spectra of 10 minutes each was taken of the object at the Southwestern edge of the preliminary XRT error circle, at coordinates R.A.: 00:31:09.0, decl.: -38:37:39.5 (J2000). The reduced spectra show emission lines typical of a star-forming galaxy, such as [OII] 3727, Hbeta, [OIII] 4959, 5007 and Halpha, at a redshift of z=0.58. Although the location of this galaxy is consistent with the initial XRT afterglow localization, it is about 6 arcseconds to the West and well outside of the enhanced XRT error circle, and is therefore unrelated to the GRB.
  We acknowledge the excellent assistance of the staff at Paranal observatory, and in particular that of S. Brillant, C. Ledoux, F. Gaete, E. Sedaghati and J. Velasquez.