Gamma-ray Burst 160821B

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• none

• GCN/BACODINE POSITION NOTICE
  

  TITLE:           GCN/SWIFT NOTICE
  NOTICE_DATE:     Sun 21 Aug 16 22:29:26 UT
  NOTICE_TYPE:     Swift-BAT GRB Position
  TRIGGER_NUM:     709357,   Seg_Num: 0
  GRB_RA:          279.987d {+18h 39m 57s} (J2000),
                   280.026d {+18h 40m 06s} (current),
                   279.870d {+18h 39m 29s} (1950)
  GRB_DEC:         +62.393d {+62d 23' 34"} (J2000),
                   +62.409d {+62d 24' 32"} (current),
                   +62.345d {+62d 20' 42"} (1950)
  GRB_ERROR:       3.00 [arcmin radius, statistical only]
  GRB_INTEN:       580 [cnts]    Image_Peak=97 [image_cnts]
  TRIGGER_DUR:     0.064 [sec] 
  TRIGGER_INDEX:   10418     E_range: 25-100 keV
  BKG_INTEN:       19276 [cnts]
  BKG_TIME:        80943.25 SOD {22:29:03.25} UT
  BKG_DUR:         8 [sec]
  GRB_DATE:        17621 TJD;   234 DOY;   16/08/21
  GRB_TIME:        80953.17 SOD {22:29:13.17} UT
  GRB_PHI:          28.78 [deg]
  GRB_THETA:        22.55 [deg]
  SOLN_STATUS:     0x3
  RATE_SIGNIF:     34.61 [sigma]
  IMAGE_SIGNIF:    10.14 [sigma]
  MERIT_PARAMS:     +1  +0  +0  -4  +2  +0  +0  +0 +10  +0 
  SUN_POSTN:       151.40d {+10h 05m 36s}  +11.72d {+11d 43' 24"}
  SUN_DIST:         95.91 [deg]   Sun_angle= -8.6 [hr] (East of Sun)
  MOON_POSTN:       16.03d {+01h 04m 08s}   +3.78d {+03d 46' 43"}
  MOON_DIST:        89.42 [deg]
  MOON_ILLUM:      84 [%]
  GAL_COORDS:       92.19, 25.13 [deg] galactic lon,lat of the burst (or transient)
  ECL_COORDS:      320.51, 84.02 [deg] ecliptic lon,lat of the burst (or transient)
  COMMENTS:        SWIFT-BAT GRB Coordinates.  
  COMMENTS:        This is a rate trigger.  
  COMMENTS:        A point_source was found.  
  COMMENTS:        This does not match any source in the on-board catalog.  
  COMMENTS:        This does not match any source in the ground catalog.  
  COMMENTS:        This is a GRB.  
  COMMENTS:        This trigger occurred at longitude,latitude = 272.93,9.08 [deg].  
  

• red DSS finding chart ps-file
  
  
• GCN NOTICE
  

  TITLE:           GCN/SWIFT NOTICE
  NOTICE_DATE:     Sun 21 Aug 16 22:30:49 UT
  NOTICE_TYPE:     Swift-XRT Position
  TRIGGER_NUM:     709357,   Seg_Num: 0
  GRB_RA:          279.9760d {+18h 39m 54.23s} (J2000),
                   280.0147d {+18h 40m 03.52s} (current),
                   279.8593d {+18h 39m 26.23s} (1950)
  GRB_DEC:         +62.3897d {+62d 23' 22.9"} (J2000),
                   +62.4058d {+62d 24' 20.8"} (current),
                   +62.3417d {+62d 20' 30.2"} (1950)
  GRB_ERROR:       5.3 [arcsec radius, statistical plus systematic, 90% containment]
  GRB_INTEN:       1.45e-09 [erg/cm2/sec]
  GRB_SIGNIF:      7.00 [sigma]
  IMG_START_DATE:  17621 TJD;   234 DOY;   16/08/21
  IMG_START_TIME:  81019.14 SOD {22:30:19.14} UT,   66.0 [sec] since BAT Trigger Time
  TAM[0-3]:        327.67 237.17 261.62 243.49
  AMPLIFIER:       2
  WAVEFORM:        134
  SUN_POSTN:       151.40d {+10h 05m 36s}  +11.72d {+11d 43' 23"}
  SUN_DIST:         95.91 [deg]   Sun_angle= -8.6 [hr] (East of Sun)
  MOON_POSTN:       16.05d {+01h 04m 11s}   +3.78d {+03d 46' 59"}
  MOON_DIST:        89.43 [deg]
  MOON_ILLUM:      84 [%]
  GAL_COORDS:       92.18, 25.14 [deg] galactic lon,lat of the burst
  ECL_COORDS:      320.45, 84.02 [deg] ecliptic lon,lat of the burst
  COMMENTS:        SWIFT-XRT Coordinates.  
  COMMENTS:        The XRT position is 0.36 arcmin from the BAT position.  
  

• GCN NOTICE
  

  TITLE:           GCN/SWIFT NOTICE
  NOTICE_DATE:     Sun 21 Aug 16 22:30:58 UT
  NOTICE_TYPE:     Swift-XRT Image
  TRIGGER_NUM:     709357,   Seg_Num: 0
  GRB_RA:          279.9760d {+18h 39m 54.2s} (J2000),
                   280.0147d {+18h 40m 03.5s} (current),
                   279.8593d {+18h 39m 26.2s} (1950)
  GRB_DEC:         +62.3897d {+62d 23' 22.9"} (J2000),
                   +62.4058d {+62d 24' 20.8"} (current),
                   +62.3417d {+62d 20' 30.2"} (1950)
  GRB_ERROR:       5.0 [arcsec, radius, statistical plus systematic]
  GRB_INTEN:       49 [cnts]
  IMG_START_DATE:  17621 TJD;   234 DOY;   16/08/21
  IMG_START_TIME:  81019.14 SOD {22:30:19.14} UT,   66.0 [sec] since BAT Trigger Time
  CENTROID_X:      324.33,  raw= 324  [pixels]
  CENTROID_Y:      307.00,  raw= 307  [pixels]
  ROLL:            309.71 [deg]
  GAIN:            1
  MODE:            3, Long Image mode
  WAVEFORM:        134
  EXPO_TIME:       2.50 [sec]
  GRB_POS_XRT_Y:   21.68
  GRB_POS_XRT_Z:   44.87
  IMAGE_URL:       sw00709357000msxps_rw.img
  SUN_POSTN:       151.40d {+10h 05m 36s}  +11.72d {+11d 43' 23"}
  SUN_DIST:         95.91 [deg]   Sun_angle= -8.6 [hr] (East of Sun)
  MOON_POSTN:       16.05d {+01h 04m 12s}   +3.78d {+03d 47' 00"}
  MOON_DIST:        89.43 [deg]
  MOON_ILLUM:      84 [%]
  GAL_COORDS:       92.18, 25.14 [deg] galactic lon,lat of the burst
  ECL_COORDS:      320.45, 84.02 [deg] ecliptic lon,lat of the burst
  COMMENTS:        SWIFT-XRT Image.  
  

  
• GCN NOTICE
  

  TITLE:           GCN/SWIFT NOTICE
  NOTICE_DATE:     Sun 21 Aug 16 22:31:10 UT
  NOTICE_TYPE:     Swift-XRT Processed Image
  TRIGGER_NUM:     709357,   Seg_Num: 0
  GRB_RA:          279.9760d {+18h 39m 54.2s} (J2000),
                   280.0147d {+18h 40m 03.5s} (current),
                   279.8593d {+18h 39m 26.2s} (1950)
  GRB_DEC:         +62.3897d {+62d 23' 22.9"} (J2000),
                   +62.4058d {+62d 24' 20.8"} (current),
                   +62.3417d {+62d 20' 30.2"} (1950)
  GRB_ERROR:       5.0 [arcsec, radius, statistical plus systematic]
  GRB_INTEN:       49 [cnts]
  IMG_START_DATE:  17621 TJD;   234 DOY;   16/08/21
  IMG_START_TIME:  81019.14 SOD {22:30:19.14} UT,   66.0 [sec] since BAT Trigger Time
  CENTROID_X:      324.33,  raw= 324  [pixels]
  CENTROID_Y:      307.00,  raw= 307  [pixels]
  ROLL:            309.71 [deg]
  GAIN:            1
  MODE:            3, Long Image mode
  WAVEFORM:        134
  EXPO_TIME:       2.50 [sec]
  GRB_POS_XRT_Y:   21.68
  GRB_POS_XRT_Z:   44.87
  IMAGE_URL:       sw00709357000msxps_rw.img
  SUN_POSTN:       151.40d {+10h 05m 36s}  +11.72d {+11d 43' 22"}
  SUN_DIST:         95.91 [deg]   Sun_angle= -8.6 [hr] (East of Sun)
  MOON_POSTN:       16.05d {+01h 04m 12s}   +3.78d {+03d 47' 03"}
  MOON_DIST:        89.43 [deg]
  MOON_ILLUM:      84 [%]
  GAL_COORDS:       92.18, 25.14 [deg] galactic lon,lat of the burst
  ECL_COORDS:      320.45, 84.02 [deg] ecliptic lon,lat of the burst
  COMMENTS:        SWIFT-XRT Processed Image.  
  

  
• GCN NOTICE
  

  TITLE:           GCN/SWIFT NOTICE
  NOTICE_DATE:     Sun 21 Aug 16 22:32:46 UT
  NOTICE_TYPE:     Swift-BAT GRB Lightcurve
  TRIGGER_NUM:     709357,   Seg_Num: 0
  GRB_RA:          279.987d {+18h 39m 57s} (J2000),
                   280.026d {+18h 40m 06s} (current),
                   279.870d {+18h 39m 29s} (1950)
  GRB_DEC:         +62.393d {+62d 23' 34"} (J2000),
                   +62.409d {+62d 24' 32"} (current),
                   +62.345d {+62d 20' 42"} (1950)
  GRB_DATE:        17621 TJD;   234 DOY;   16/08/21
  GRB_TIME:        80953.30 SOD {22:29:13.30} UT
  TRIGGER_INDEX:   10418
  GRB_PHI:          28.78 [deg]
  GRB_THETA:        22.55 [deg]
  DELTA_TIME:      -44.00 [sec]
  TRIGGER_DUR:     0.064 [sec] 
  SOLN_STATUS:     0x3
  RATE_SIGNIF:     34.61 [sigma]
  IMAGE_SIGNIF:    10.14 [sigma]
  LC_URL:          sw00709357000msb.lc
  SUN_POSTN:       151.40d {+10h 05m 36s}  +11.72d {+11d 43' 21"}
  SUN_DIST:         95.91 [deg]   Sun_angle= -8.6 [hr] (East of Sun)
  MOON_POSTN:       16.07d {+01h 04m 16s}   +3.79d {+03d 47' 21"}
  MOON_DIST:        89.43 [deg]
  MOON_ILLUM:      84 [%]
  GAL_COORDS:       92.19, 25.13 [deg] galactic lon,lat of the burst (or transient)
  ECL_COORDS:      320.51, 84.02 [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 = 272.93,9.08 [deg].  
  

  
• GCN NOTICE
  

  TITLE:           GCN/SWIFT NOTICE
  NOTICE_DATE:     Sun 21 Aug 16 22:33:26 UT
  NOTICE_TYPE:     Swift-UVOT Source List
  TRIGGER_NUM:     709357,   Seg_Num: 0
  POINT_RA:        279.964d {+18h 39m 51s} (J2000)
  POINT_DEC:       +62.402d {+62d 24' 08"} (J2000)
  POINT_ROLL:      309.713d
  IMG_START_DATE:  17621 TJD;   234 DOY;   16/08/21
  IMG_START_TIME:  81029.31 SOD {22:30:29.31} UT,   76.1 [sec] since BAT Trigger Time
  FILTER:          10, White
  BKG_MEAN:        2.173
  N_STARS:         105
  X_OFFSET:        375 [pixels]
  Y_OFFSET:        572 [pixels]
  X_MAX:           1334 [pixels]
  Y_MAX:           1531 [pixels]
  DET_THRESH:      12
  PHOTO_THRESH:    6
  SL_URL:          sw00709357000msufc0076.fits
  SUN_POSTN:       151.40d {+10h 05m 37s}  +11.72d {+11d 43' 21"}
  SUN_DIST:         95.90 [deg]   Sun_angle= -8.6 [hr] (East of Sun)
  MOON_POSTN:       16.07d {+01h 04m 17s}   +3.79d {+03d 47' 28"}
  MOON_DIST:        89.44 [deg]
  MOON_ILLUM:      84 [%]
  GAL_COORDS:       92.19, 25.15 [deg] galactic lon,lat of the pointing direction
  ECL_COORDS:      320.49, 84.04 [deg] ecliptic lon,lat of the pointing direction
  COMMENTS:        SWIFT-UVOT Source List.  
  

• GCN NOTICE
  

  TITLE:           GCN/SWIFT NOTICE
  NOTICE_DATE:     Sun 21 Aug 16 22:33:51 UT
  NOTICE_TYPE:     Swift-UVOT Processed Source List
  TRIGGER_NUM:     709357,   Seg_Num: 0
  POINT_RA:        279.964d {+18h 39m 51s} (J2000)
  POINT_DEC:       +62.402d {+62d 24' 08"} (J2000)
  POINT_ROLL:      309.713d
  IMG_START_DATE:  17621 TJD;   234 DOY;   16/08/21
  IMG_START_TIME:  81029.31 SOD {22:30:29.31} UT,   76.1 [sec] since BAT Trigger Time
  FILTER:          10, White
  BKG_MEAN:        2.173
  N_STARS:         105
  X_OFFSET:        375 [pixels]
  Y_OFFSET:        572 [pixels]
  X_MAX:           1334 [pixels]
  Y_MAX:           1531 [pixels]
  DET_THRESH:      12
  PHOTO_THRESH:    6
  SL_URL:          sw00709357000msufc0076.fits
  SUN_POSTN:       151.40d {+10h 05m 37s}  +11.72d {+11d 43' 20"}
  SUN_DIST:         95.90 [deg]   Sun_angle= -8.6 [hr] (East of Sun)
  MOON_POSTN:       16.08d {+01h 04m 18s}   +3.79d {+03d 47' 33"}
  MOON_DIST:        89.44 [deg]
  MOON_ILLUM:      84 [%]
  GAL_COORDS:       92.19, 25.15 [deg] galactic lon,lat of the pointing direction
  ECL_COORDS:      320.49, 84.04 [deg] ecliptic lon,lat of the pointing direction
  COMMENTS:        SWIFT-UVOT Processed Source List.  
  COMMENTS:        If you have elected to receive attachments:  
  COMMENTS:        The uvot_catalog_srclist.fits.gz file does not exist; skipping the attachment.  
  

  
• GCN NOTICE
  

  TITLE:           GCN/SWIFT NOTICE
  NOTICE_DATE:     Sun 21 Aug 16 22:34:59 UT
  NOTICE_TYPE:     Swift-UVOT Image
  TRIGGER_NUM:     709357,   Seg_Num: 0
  POINT_RA:        279.964d {+18h 39m 51s} (J2000)
  POINT_DEC:       +62.402d {+62d 24' 08"} (J2000)
  ROLL:            309.713d
  IMG_START_DATE:  17621 TJD;   234 DOY;   16/08/21
  IMG_START_TIME:  81029.31 SOD {22:30:29.31} UT,   76.1 [sec] since BAT Trigger Time
  FILTER:          10, White
  EXPOSURE_ID:     493511446
  X_OFFSET:        695 [pixels]
  Y_OFFSET:        892 [pixels]
  WIDTH:           160 [pixels]
  HEIGHT:          160 [pixels]
  X_GRB_POS:       855
  Y_GRB_POS:       1052
  BINNING_INDEX:   1
  IM_URL:          sw00709357000msuni0093.fits
  SUN_POSTN:       151.40d {+10h 05m 37s}  +11.72d {+11d 43' 19"}
  SUN_DIST:         95.90 [deg]   Sun_angle= -8.6 [hr] (East of Sun)
  MOON_POSTN:       16.09d {+01h 04m 21s}   +3.80d {+03d 47' 45"}
  MOON_DIST:        89.44 [deg]
  MOON_ILLUM:      84 [%]
  GAL_COORDS:       92.19, 25.15 [deg] galactic lon,lat of the pointing direction
  ECL_COORDS:      320.49, 84.04 [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:     Sun 21 Aug 16 22:35:16 UT
  NOTICE_TYPE:     Swift-UVOT Processed Image
  TRIGGER_NUM:     709357,   Seg_Num: 0
  POINT_RA:        279.964d {+18h 39m 51s} (J2000)
  POINT_DEC:       +62.402d {+62d 24' 08"} (J2000)
  ROLL:            309.713d
  IMG_START_DATE:  17621 TJD;   234 DOY;   16/08/21
  IMG_START_TIME:  81029.31 SOD {22:30:29.31} UT,   76.1 [sec] since BAT Trigger Time
  FILTER:          10, White
  EXPOSURE_ID:     493511446
  X_OFFSET:        695 [pixels]
  Y_OFFSET:        892 [pixels]
  WIDTH:           160 [pixels]
  HEIGHT:          160 [pixels]
  X_GRB_POS:       855
  Y_GRB_POS:       1052
  BINNING_INDEX:   1
  IM_URL:          sw00709357000msuni0093.fits
  SUN_POSTN:       151.40d {+10h 05m 37s}  +11.72d {+11d 43' 19"}
  SUN_DIST:         95.90 [deg]   Sun_angle= -8.6 [hr] (East of Sun)
  MOON_POSTN:       16.09d {+01h 04m 21s}   +3.80d {+03d 47' 48"}
  MOON_DIST:        89.44 [deg]
  MOON_ILLUM:      84 [%]
  GAL_COORDS:       92.19, 25.15 [deg] galactic lon,lat of the pointing direction
  ECL_COORDS:      320.49, 84.04 [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:        If you have elected to receive attachments:  
  COMMENTS:        The uvot_catalog_image.fits.gz file does not exist; skipping the attachment.  
  

  
• GCN NOTICE
  

  TITLE:           GCN/SWIFT NOTICE
  NOTICE_DATE:     Sun 21 Aug 16 22:40:36 UT
  NOTICE_TYPE:     Swift-UVOT Source List
  TRIGGER_NUM:     709357,   Seg_Num: 0
  POINT_RA:        279.960d {+18h 39m 50s} (J2000)
  POINT_DEC:       +62.402d {+62d 24' 06"} (J2000)
  POINT_ROLL:      309.711d
  IMG_START_DATE:  17621 TJD;   234 DOY;   16/08/21
  IMG_START_TIME:  81241.37 SOD {22:34:01.37} UT,   288.2 [sec] since BAT Trigger Time
  FILTER:          7, U
  BKG_MEAN:        0.346
  N_STARS:         104
  X_OFFSET:        375 [pixels]
  Y_OFFSET:        572 [pixels]
  X_MAX:           1334 [pixels]
  Y_MAX:           1531 [pixels]
  DET_THRESH:      6
  PHOTO_THRESH:    2
  SL_URL:          sw00709357000msufc0288.fits
  SUN_POSTN:       151.41d {+10h 05m 38s}  +11.72d {+11d 43' 14"}
  SUN_DIST:         95.90 [deg]   Sun_angle= -8.6 [hr] (East of Sun)
  MOON_POSTN:       16.14d {+01h 04m 34s}   +3.81d {+03d 48' 48"}
  MOON_DIST:        89.45 [deg]
  MOON_ILLUM:      84 [%]
  GAL_COORDS:       92.19, 25.15 [deg] galactic lon,lat of the pointing direction
  ECL_COORDS:      320.47, 84.04 [deg] ecliptic lon,lat of the pointing direction
  COMMENTS:        SWIFT-UVOT Source List.  
  

• GCN NOTICE
  

  TITLE:           GCN/SWIFT NOTICE
  NOTICE_DATE:     Sun 21 Aug 16 22:40:53 UT
  NOTICE_TYPE:     Swift-UVOT Processed Source List
  TRIGGER_NUM:     709357,   Seg_Num: 0
  POINT_RA:        279.960d {+18h 39m 50s} (J2000)
  POINT_DEC:       +62.402d {+62d 24' 06"} (J2000)
  POINT_ROLL:      309.711d
  IMG_START_DATE:  17621 TJD;   234 DOY;   16/08/21
  IMG_START_TIME:  81241.37 SOD {22:34:01.37} UT,   288.2 [sec] since BAT Trigger Time
  FILTER:          7, U
  BKG_MEAN:        0.346
  N_STARS:         104
  X_OFFSET:        375 [pixels]
  Y_OFFSET:        572 [pixels]
  X_MAX:           1334 [pixels]
  Y_MAX:           1531 [pixels]
  DET_THRESH:      6
  PHOTO_THRESH:    2
  SL_URL:          sw00709357000msufc0288.fits
  SUN_POSTN:       151.41d {+10h 05m 38s}  +11.72d {+11d 43' 14"}
  SUN_DIST:         95.90 [deg]   Sun_angle= -8.6 [hr] (East of Sun)
  MOON_POSTN:       16.14d {+01h 04m 34s}   +3.81d {+03d 48' 51"}
  MOON_DIST:        89.45 [deg]
  MOON_ILLUM:      84 [%]
  GAL_COORDS:       92.19, 25.15 [deg] galactic lon,lat of the pointing direction
  ECL_COORDS:      320.47, 84.04 [deg] ecliptic lon,lat of the pointing direction
  COMMENTS:        SWIFT-UVOT Processed Source List.  
  COMMENTS:        If you have elected to receive attachments:  
  COMMENTS:        The uvot_catalog_srclist.fits.gz file does not exist; skipping the attachment.  
  

  
• GCN NOTICE
  

  TITLE:           GCN/SWIFT NOTICE
  NOTICE_DATE:     Sun 21 Aug 16 22:41:12 UT
  NOTICE_TYPE:     Swift-XRT Position UPDATE
  TRIGGER_NUM:     709357,   Seg_Num: 0
  GRB_RA:          279.9780d {+18h 39m 54.72s} (J2000),
                   280.0167d {+18h 40m 04.00s} (current),
                   279.8613d {+18h 39m 26.72s} (1950)
  GRB_DEC:         +62.3920d {+62d 23' 31.2"} (J2000),
                   +62.4081d {+62d 24' 29.0"} (current),
                   +62.3440d {+62d 20' 38.5"} (1950)
  GRB_ERROR:       2.5 [arcsec radius, statistical plus systematic, 90% containment]
  GRB_INTEN:       1.00e-10 [erg/cm2/sec]
  GRB_SIGNIF:      10.00 [sigma]
  IMG_START_DATE:  17621 TJD;   234 DOY;   16/08/21
  IMG_START_TIME:  81225.00 SOD {22:33:45.00} UT,   271.8 [sec] since BAT Trigger Time
  TAM[0-3]:        100.00 100.00 100.00 100.00
  AMPLIFIER:       1
  WAVEFORM:        31
  SUN_POSTN:       151.41d {+10h 05m 38s}  +11.72d {+11d 43' 14"}
  SUN_DIST:         95.91 [deg]   Sun_angle= -8.6 [hr] (East of Sun)
  MOON_POSTN:       16.15d {+01h 04m 35s}   +3.82d {+03d 48' 55"}
  MOON_DIST:        89.45 [deg]
  MOON_ILLUM:      84 [%]
  GAL_COORDS:       92.18, 25.14 [deg] galactic lon,lat of the burst
  ECL_COORDS:      320.47, 84.02 [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 0.58% < P(seren) < 1.1%.  
  

• GCN NOTICE
  

  TITLE:           GCN/SWIFT NOTICE
  NOTICE_DATE:     Sun 21 Aug 16 22:42:10 UT
  NOTICE_TYPE:     Swift-UVOT Image
  TRIGGER_NUM:     709357,   Seg_Num: 0
  POINT_RA:        279.960d {+18h 39m 50s} (J2000)
  POINT_DEC:       +62.402d {+62d 24' 06"} (J2000)
  ROLL:            309.711d
  IMG_START_DATE:  17621 TJD;   234 DOY;   16/08/21
  IMG_START_TIME:  81241.37 SOD {22:34:01.37} UT,   288.2 [sec] since BAT Trigger Time
  FILTER:          7, U
  EXPOSURE_ID:     493511658
  X_OFFSET:        695 [pixels]
  Y_OFFSET:        892 [pixels]
  WIDTH:           160 [pixels]
  HEIGHT:          160 [pixels]
  X_GRB_POS:       855
  Y_GRB_POS:       1052
  BINNING_INDEX:   1
  IM_URL:          sw00709357000msuni0305.fits
  SUN_POSTN:       151.41d {+10h 05m 38s}  +11.72d {+11d 43' 13"}
  SUN_DIST:         95.90 [deg]   Sun_angle= -8.6 [hr] (East of Sun)
  MOON_POSTN:       16.16d {+01h 04m 37s}   +3.82d {+03d 49' 06"}
  MOON_DIST:        89.46 [deg]
  MOON_ILLUM:      84 [%]
  GAL_COORDS:       92.19, 25.15 [deg] galactic lon,lat of the pointing direction
  ECL_COORDS:      320.47, 84.04 [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:     Sun 21 Aug 16 22:42:25 UT
  NOTICE_TYPE:     Swift-UVOT Processed Image
  TRIGGER_NUM:     709357,   Seg_Num: 0
  POINT_RA:        279.960d {+18h 39m 50s} (J2000)
  POINT_DEC:       +62.402d {+62d 24' 06"} (J2000)
  ROLL:            309.711d
  IMG_START_DATE:  17621 TJD;   234 DOY;   16/08/21
  IMG_START_TIME:  81241.37 SOD {22:34:01.37} UT,   288.2 [sec] since BAT Trigger Time
  FILTER:          7, U
  EXPOSURE_ID:     493511658
  X_OFFSET:        695 [pixels]
  Y_OFFSET:        892 [pixels]
  WIDTH:           160 [pixels]
  HEIGHT:          160 [pixels]
  X_GRB_POS:       855
  Y_GRB_POS:       1052
  BINNING_INDEX:   1
  IM_URL:          sw00709357000msuni0305.fits
  SUN_POSTN:       151.41d {+10h 05m 38s}  +11.72d {+11d 43' 13"}
  SUN_DIST:         95.90 [deg]   Sun_angle= -8.6 [hr] (East of Sun)
  MOON_POSTN:       16.16d {+01h 04m 38s}   +3.82d {+03d 49' 08"}
  MOON_DIST:        89.46 [deg]
  MOON_ILLUM:      84 [%]
  GAL_COORDS:       92.19, 25.15 [deg] galactic lon,lat of the pointing direction
  ECL_COORDS:      320.47, 84.04 [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:        If you have elected to receive attachments:  
  COMMENTS:        The uvot_catalog_image.fits.gz file does not exist; skipping the attachment.  
  

  
• GCN Circular #19833
  M. H. Siegel (PSU), S. D. Barthelmy (GSFC), D. N. Burrows (PSU), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team:
  At 22:29:13 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 160821B (trigger=709357). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 279.987, +62.393, which is

     RA(J2000)  =  18h 39m 57s
     Dec(J2000) = +62d 23' 34"
  

  with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a single spike structure with a duration of about 0.5 sec. The peak count rate was ~5000 counts/sec (15-350 keV), at ~0 sec after the trigger.
  The XRT began observing the field at 22:30:19.1 UT, 66.0 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 279.9780, 62.3920 which is equivalent to:

     RA(J2000)  = 18h 39m 54.71s
     Dec(J2000) = +62d 23' 31.3"
  

  with an uncertainty of 2.5 arcseconds (radius, 90% containment). This location is 15 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 5.75 x 10^20 cm^-2 (Willingale et al. 2013).
  The initial flux in the 2.5 s image was 1.45e-09 erg cm^-2 s^-1 (0.2-10 keV).
  UVOT took a finding chart exposure of 150 seconds with the White filter starting 76 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' covers 100% of the XRT error region. The typical 3-sigma upper limit has been about 19.6 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.05.
  Burst Advocate for this burst is M. H. Siegel (siegel 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 Circular #19834
  D. Xu (NAOC/CAS), D. Malesani (DARK/NBI and DTU Space), A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), E. Gafton, I. Rivero Losada (NOT) report on behalf of a larger collaboration:
  We observed the field of the short GRB 160821B (Siegel et al., GCN 19833) using the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations started at 23:02:07 UT on 2016-08-21 (i.e., 0.548 hr after the burst), and 12 x 300s SDSS r-band frames were obtained under variable cirrus.
  We detected an uncatalogued point source at coordinates:

  R.A. = 18:39:54.56
  Dec. = +62:23:30.5
  

  with an uncertainty radius of ~0.2 arcsec, which is within the XRT error circle reported in Siegel et al. (GCN 19833). This source had R=22.6+/-0.1 mag at 0.95 hr after the burst, calibrated against the star of R = 17.6 at RA = 18:39:57.59, Dec = +62:22:31.7 in the USNO B1 catalogue.
  We also note the presence of a bright, extended, nearby galaxy, 5.5" away, with a magnitude R ~ 19.2, which is a candidate host galaxy of GRB 160821B.
  Lacking variability, we cannot confirm whether the source we identified is the afterglow of GRB 160821B. Further observations are encouraged.
• GCN Circular #19837
  P.A. Evans, M.R. Goad, J.P. Osborne and A.P. Beardmore (U. Leicester) report on behalf of the Swift-XRT team.
  Using 1754 s of XRT Photon Counting mode data and 2 UVOT images for GRB 160821B, 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 = 279.97653, +62.39129 which is equivalent to:

  RA (J2000): 18h 39m 54.37s
  Dec (J2000): +62d 23' 28.6"
  

  with an uncertainty of 2.5 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 #19838
  R. Rebolo, M. Serra-Ricart, G. Israelian, N. Lodieu The Instituto de Astrofisica de Canarias
  V. Lipunov, N.Tyurina, E. Gorbovskoy, V.Kornilov, P.Balanutsa, A.Kuznetsov, D.Kuvshinov Lomonosov Moscow State University, Sternberg Astronomical Institute
  K.Ivanov, O.Gres, N.M.Budnev, S.Yazev, Irkutsk State University
  D.Buckley, S. Potter, A.Kniazev, M.Kotze South African Astronomical Observatory
  A. Tlatov, V.Senik, D. Dormidontov, A.Parkhomenko Kislovodsk Solar Station of the Pulkovo Observatory
  V.Yurkov, Yu.Sergienko, A.Gabovich Blagoveschensk Educational State University, Blagoveschensk
  Hugo Levato, Carlos Saffe Instituto de Ciencias Astronomicas, de la Tierra y del Espacio (ICATE)
  R.Podesta, C.Mallamaci, C.Lopez, F. Podesta Observatorio Astronomico Felix Aguilar (OAFA)
  MASTER-IAC twin robotic telescope (MASTER-Net: http://observ.pereplet.ru, online view http://161.72.133.2/) located in IAC (Tenerife, Spain) was pointed to the GRB160821B (Siegel et al., GCN 19833) 34 sec after notice time and 53 sec after trigger time at 2016-08-21 22:30:06 UT. The alert coordinates altitude was 55 deg. at MASTER-IAC, the Sun altitude was -35d, the Moon distance is 90 degrees.
  On our first (10s exposure - two images in perpendicular polarizations) set we not found optical transient within SWIFT error-box (Evans et al., GCN 19837) and at NOT OT position (Xu et al., GCN 19834) brighter then 16.6.
  The resuts of the first 5 minutes observations are:

  Id 	Pro.type	UT	Exp.time   Limit	Filt.	  Tube.
                         (Start)     (s)     (5sigma)  (polarizator)
  
  285342   Alert       22:30:06     10       16.6           P-      WEST
  285343   Alert       22:30:06     10       16.6           P|      EAST
  285342   Alert       22:30:06     10       16.6           P-      WEST
  285345   Alert       22:30:43     20       17.1           P|      EAST
  285346   Alert       22:31:24     30       17.1           P-      WEST
  285347   Alert       22:31:24     30       16.6           P|      EAST
  285348   Alert       22:32:08     30       16.6           P-      WEST
  285349   Alert       22:32:08     30       16.6           P|      EAST
  285350   Alert       22:32:50     40       17.2           P-      WEST
  285351   Alert       22:32:50     40       17.0           P|      EAST
  285352   Alert       22:33:43     50       17.3           P-      WEST
  285353   Alert       22:33:43     50       17.2           P|      EAST
  285355	 Alert 	     22:34:46	  70	   17.5	          P-	  WEST
  285354	 Alert 	     22:34:46	  70	    0.0	          P|	  EAST
  

  The limit of the coadded west tube images is 18.7 with start time 22:30:06, finish time = 22:35:56 and mean time = 22:33:01 UT.
  The message may be cited.
• GCN Circular #19839
  A. A. Breeveld (UCL-MSSL) and M. H. Siegel (PSU) report on behalf of the Swift/UVOT team:
  The Swift/UVOT began settled observations of the field of GRB 160821B 76 s after the BAT trigger (Siegel et al., GCN Circ. 19833). No optical afterglow consistent with the XRT position (Evans et al. GCN Circ. 19837) or the NOT position (Xu et al. GCN Circ. 19834) 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            76          226          147         >21.1
  u_FC               288          501          209         >20.4
  white               76         6306          344         >21.7
  v                 5080         5280          197         >19.1
  b                 5901         6101          197         >20.2
  u                  288         5895          406         >20.6
  w1                5491         5691          197         >19.6
  m2                5285         5485          197         >19.5
  w2                4875         6439          322         >20.1
  

  The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.05 in the direction of the burst (Schlegel et al. 1998).
• GCN Circular #19841
  B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), B. Mingo (U. Leicester), A. Maselli (INAF-IASFPA), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), T.G.R. Roegiers (PSU) and M.H. Siegel report on behalf of the Swift-XRT team:
  We have analysed 9.5 ks of XRT data for GRB 160821B (Siegel et al. GCN Circ. 19833), from 56 s to 35.9 ks after the BAT trigger. The data comprise 198 s in Windowed Timing (WT) mode (the first 8 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. 19837).
  The late-time light curve (from T0+4.9 ks) can be modelled with a power-law decay with a decay index of alpha=0.5 (+/-0.3).
  A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.95 (+0.08, -0.07). The best-fitting absorption column is 7.5 (+2.0, -1.7) x 10^20 cm^-2, consistent with the Galactic value of 5.8 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.3 x 10^-11 (3.9 x 10^-11) erg cm^-2 count^-1.
  A summary of the WT-mode spectrum is thus:

  Total column:	     7.5 (+2.0, -1.7) x 10^20 cm^-2
  Galactic foreground: 5.8 x 10^20 cm^-2
  Excess significance: <1.6 sigma
  Photon index:	     1.95 (+0.08, -0.07)
  

  If the light curve continues to decay with a power-law decay index of 0.5, the count rate at T+24 hours will be 3.8 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x 10^-13 (1.4 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/00709357.
  This circular is an official product of the Swift-XRT team.
• GCN Circular #19843
  M. Stanbro (UAH) and C. Meegan (UAH) report on behalf of the Fermi GBM Team:
  "At 22:29:13.33 UT on 21 August 2016, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 160821B (trigger 493511357 / 160821937), which was also detected by the Swift/BAT (Siegel et al. 2016, GCN 19833) The GBM on-ground location is consistent with the Swift position.
  The angle from the Fermi LAT boresight at the GBM trigger time is 61 degrees.
  The GBM light curve consists of a few episodes with a duration (T90) of about 1 s (50-300 keV). The time-averaged spectrum from T0-0.13 s to T0+0.32 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.37 +/- 0.22 and the cutoff energy, parameterized as Epeak, is 84 +/- 19 keV
  The event fluence (10-1000 keV) in this time interval is (1.68 +/- 0.19)E-06 erg/cm^2. The 64-millisec peak photon flux measured starting from T0+0.00 s in the 10-1000 keV band is 9.16 +/- 1.19 ph/s/cm^2.
  The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."
• GCN Circular #19844
  D. M. Palmer (LANL), S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), J. P. Norris (BSU), T. Sakamoto (AGU), M. H. Siegel (PSU), M. Stamatikos (OSU), T. N. Ukwatta (LANL), G. Younes (GWU) (i.e. the Swift-BAT team):
  Using the data set from T-239 to T+690 sec from the recent telemetry downlink, we report further analysis of BAT GRB 160821B (trigger #709357) (Siegel, et al., GCN Circ. 19833). The BAT ground-calculated position is RA, Dec = 280.005, 62.387 deg which is

    RA(J2000)  =  18h 40m 01.2s
    Dec(J2000) = +62d 23' 13.2"
  

  with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment). The partial coding was 85%.
  The mask-weighted light curve shows a FRED-like structure that starts and peaks at ~T0, and ends at ~T+0.5 s. T90 (15-350 keV) is 0.48 +- 0.07 sec (estimated error including systematics).
  The time-averaged spectrum from T+0.00 to T+0.54 sec is best fit by a power law with an exponential cutoff. This fit gives a photon index of 0.11 +/- 0.88 and an Epeak of 46.3 +/- 6.4 keV (chi squared 58.76 for 56 d.o.f.). For this model the total fluence in the 15-150 keV band is 1.0 +/- 0.1 x 10^-7 erg/cm2, and the 1-sec peak flux measured from T-0.23 sec in the 15-150 keV band is 1.7 +/- 0.2 ph/cm2/sec. A fit to a simple power law gives a photon index of 1.88 +- 0.15 (chi squared 75.26 for 57 d.o.f.). All the quoted errors are at the 90% confidence level.
  The burst spectrum is on the softer end of the BAT short GRB distribution. Using a 4-ms binned light curve, the lag analysis finds a lag of 8 +/- 4 ms for the 50-100 keV to 15-25 keV band, which is consistent with those of a short GRB.
  In addition to a short GRB, the BAT light curve and spectrum are also consistent with those expected from an SGR. In particular, the double blackbody model also produces an acceptable fit to the time-averaged spectrum (chi squared 56.65 for 55 d.o.f.), with the blackbody temperatures (kT1=4.05 and kT2=13.22 keV) consistent with those from an SGR. However, the location of this burst (Galactic latitude of 25.13 deg and not in SMC or LMC), and the fast decay of the XRT light curve makes it unlikely to be an SGR.
  We therefore conclude that this burst is likely to be a short GRB.
  The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/709357/BA/
• GCN Circular #19846
  A.J. Levan (U. Warwick), K. Wiersema (U. Leicester), N.R. Tanvir (U. Leicester), D. Malesani (DARK/NBI, DTU Space), D. Xu (NAOC/CAS), A. de Ugarte Postigo (IAA-CSIC, DARK/NBI) report for a larger collaboration:
  We observed the location of GRB 160821B (Siegel et al. GCN 19833) with the William Herschel Telescope on La Palma, beginning at 22:57UT on 22 Aug 2016. Observations were obtained in the r and z bands. The candidate afterglow (Xu et al. GCN 19834) is present in our images, but has faded by approximately a factor of two between the two epochs taken approximately on e day apart. This variability confirms the source as the afterglow of GRB 160821B.
  In addition we obtained a spectrum of the candidate host galaxy (Xu et al. GCN 19834). Several prominent emission lines are visible, including Hbeta, [OIII] (4959/5007) and H-alpha, at a provisional redshift of z=0.16.
  The offset of the afterglow from the candidate host galaxy is approximately 5.5=E2=80=9D, corresponding to a physical offset of 15 kpc. While we cannot rule out a chance alignment of the afterglow with the host galaxy, we also note that the galaxy light does apparently extend to approximately the afterglow position.
  We thank the staff of WHT, in particular Cecelia Farina, for their help with these observations.
• GCN Circular #19847
  S. Jeong (SKKU/IAA-CSIC), I. H. Park (SKKU), Y. Hu (IAA-CSIC), R. Scarpa (GTC) and A. J. Castro-Tirado (IAA-CSIC), on behalf of a larger collaboration report:
  We observed the field of the short GRB 160821B detected by Swift (Siegel et al. GCNC 19833) and Fermi (Stanbro et al. GCNC 19843) using the 10.4m Gran Telescopio Canarias (GTC) equipped with OSIRIS. Observations started at 00:16:17.480 UT on Aug 22 (i.e., 1.78 hr after the burst), in the riz-band filters.
  We also detect the point-like source proposed by Xu et al. (GCNC 19834) as a potential afterglow candidate) within the XRT error circle reported by Siegel et al. (GCNC 19833), for which we measure a preliminary magnitude (pending final calibration) of R =3D 22.40+/-0.06 at 1.83 hr after the burst.
  Lacking variability, unless there is an initial plateau phase lasting several hour in the afterglow brightness, this would be consistent with a host galaxy detection. Further GTC observations are planned.
  We thanks D. Xu and D. Malesani for providing secondary reference stars in the field.
• GCN Circular #19854
  W. Fong (University of Arizona), K. D. Alexander (Harvard), and T. Laskar (NRAO/UC Berkeley) report:
  "We observed the position of the short-duration GRB 160821B (Siegel et al., GCN 19833) with the Karl G. Jansky Very Large Array (VLA) beginning on 2016 Aug 22.088 UT (3.6 hr post-burst) at a mean frequency of 5.0 GHz. In 1 hour of observations, we detect a faint radio source at the position:

  RA(J2000) = 18:39:54.56
  Dec(J2000) = 62:23:30.3
  

  with an uncertainty of 0.3" in each coordinate. This position is consistent with the position of the X-ray afterglow (Evans et al., GCN 19837), and coincident with the position of the optical afterglow (Xu et al., GCN 19834; Levan et al., GCN 19846).
  We measure a preliminary 5.0 GHz flux density of ~35 microJy. To assess variability, we obtained an additional set of VLA observations starting at 26.5 hr after the burst. The source is no longer detected in these observations to a 3-sigma limit of 18 microJy.
  Due to the fading of the radio source, as well as the positional coincidence with the X-ray and optical afterglows, we suggest this source as the radio afterglow of GRB 160821B.
  We thank the VLA staff for quickly executing these observations."
• GCN Circular #19898
  K. P. Mooley, T. D. Staley, R. P. Fender (Oxford), G. E. Anderson (Curtin), T. Cantwell (Manchester), D. Titterington, S. H. Carey, J. Hickish, Y. C. Perrott, N. Razavi-Ghods, C. Rumsey, P. Scott (Cambridge), K. Grainge, A. Scaife (Manchester)
  We observed the short GRB 160821B (Siegel et al., GCN 19833) with the AMI Large Array as part of the 4pisky program. The observations at 15 GHz on 2016 Aug 22.85, Aug 23.82, Aug 25.83, and Sep 03.83 (UT) do not reveal any radio source at the VLA location (Fong et al., GCN 19854), with 3sigma upper limits of 138 uJy, 227 uJy, 129 uJy, and 171 uJy respectively.
  We note that, due to power outage, the AMI-LA did not trigger robotically, and could only obtain the first observation 22 hours post-burst. This is several hours after the VLA 6 GHz detection (~3.6 hours post-burst).
  We thank the AMI staff for scheduling these observations. The AMI-GRB database is a log of all GRB follow up observations with the AMI, and is available at http://4pisky.org/ami-grb/.
• GCN Circular #20222
  E. Troja (UMD/GSFC), N. Tanvir (U. Leicester), S. B. Cenko (NASA/GSFC), A. Levan (U. Warwick), J. Barnes (U. Berkeley), A. Castro-Tirado (IAA- CSIC), A. S. Fruchter (STScI), N. Gehrels (NASA/GSFC), J. Greiner (MPE), N. Kawai (Tokyo Tech), R. Hounsell (UCSC), J. Hjorth (DARK/NBI), A. Lien (NASA/GSFC), B. Metzger (Columbia), D. Perley (DARK/NBI), S. Rosswog (U. Stockholm), T. Sakamoto (AGU), C. Thoene (IAA-CSIC), A. de Ugarte Postigo (IAA-CSIC), and D. Watson (DARK/NBI) report:
  We monitored the location of the short GRB 160821B (Siegel et al. GCN 19833; Xu et al. GCN 19834) with the Hubble Space Telescope under our approved guest observer programs (GO14237 PI: Tanvir; GO14087 PI: Troja). Observations were carried out with the Wide Field Camera (WFC3) in three filters, F606W, F110W and F160W, at epochs 3.6, 10.4 and 23.2 days post- burst. The GRB counterpart is clearly detected in all filters during the first two epochs, and fades from a magnitude of F606W~25.8 (AB) in the first epoch to become undetectable in the third epoch.
  Assuming a redshift of z=0.162 from the nearby galaxy identified as the likely host (Levan et al. GCN 19846), our observations rule out the presence of an emerging supernova comparable to SN1998bw or to other SNe associated to long GRBs. The observed fluxes constrain the contribution of any r-process kilonova/macronova component to be at least a factor ~5 fainter in the IR than that seen in GRB 130603B. The lack of a bright supernova and the moderate-to-low ejecta mass implied by our observations are consistent with this event being produced by the merger of two neutron stars.
  However, the current dataset cannot firmly exclude the presence of an underlying, higher redshift host galaxy. Deeper HST observations aimed at placing better constraints on the GRB redshift are on-going.
  We thank the STScI staff, in particular Tricia Royle, for assistance with rapidly scheduling our observations.