Gamma-ray Burst 151021A

(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 21 Oct 15 01:29:44 UT
NOTICE_TYPE:     Swift-BAT GRB Position
TRIGGER_NUM:     660671,   Seg_Num: 0
GRB_RA:          337.611d {+22h 30m 27s} (J2000),
                 337.836d {+22h 31m 21s} (current),
                 336.900d {+22h 27m 36s} (1950)
GRB_DEC:         -33.199d {-33d 11' 55"} (J2000),
                 -33.117d {-33d 07' 02"} (current),
                 -33.456d {-33d 27' 19"} (1950)
GRB_ERROR:       3.00 [arcmin radius, statistical only]
GRB_INTEN:       6187 [cnts]    Image_Peak=139 [image_cnts]
TRIGGER_DUR:     1.024 [sec] 
TRIGGER_INDEX:   155     E_range: 50-350 keV
BKG_INTEN:       30302 [cnts]
BKG_TIME:        5336.69 SOD {01:28:56.69} UT
BKG_DUR:         8 [sec]
GRB_DATE:        17316 TJD;   294 DOY;   15/10/21
GRB_TIME:        5352.57 SOD {01:29:12.57} UT
GRB_PHI:          37.53 [deg]
GRB_THETA:        43.64 [deg]
SOLN_STATUS:     0x83
RATE_SIGNIF:     35.02 [sigma]
IMAGE_SIGNIF:    6.89 [sigma]
MERIT_PARAMS:     +1  +0  +0  +0  +3  -3  +0  +0 +45  +0 
SUN_POSTN:       205.37d {+13h 41m 29s}  -10.52d {-10d 31' 22"}
SUN_DIST:        117.14 [deg]   Sun_angle= -8.8 [hr] (East of Sun)
MOON_POSTN:      301.18d {+20h 04m 43s}  -15.72d {-15d 43' 24"}
MOON_DIST:        37.36 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:       12.94,-59.08 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      326.60,-22.06 [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:        Since the IMAGE_SIGNIF is less than 7 sigma, this is a questionable detection.  
COMMENTS:        This trigger occurred at longitude,latitude = 258.19,-20.59 [deg].

red DSS finding chart ps-file

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 21 Oct 15 01:46:19 UT
NOTICE_TYPE:     Swift-XRT Position
TRIGGER_NUM:     660671,   Seg_Num: 0
GRB_RA:          337.6467d {+22h 30m 35.20s} (J2000),
                 337.8709d {+22h 31m 29.02s} (current),
                 336.9355d {+22h 27m 44.52s} (1950)
GRB_DEC:         -33.2010d {-33d 12' 03.6"} (J2000),
                 -33.1196d {-33d 07' 10.4"} (current),
                 -33.4578d {-33d 27' 28.1"} (1950)
GRB_ERROR:       5.5 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       2.76e-08 [erg/cm2/sec]
GRB_SIGNIF:      6.24 [sigma]
IMG_START_DATE:  17316 TJD;   294 DOY;   15/10/21
IMG_START_TIME:  5443.28 SOD {01:30:43.28} UT,   90.7 [sec] since BAT Trigger Time
TAM[0-3]:        327.66 237.22 261.53 243.43
AMPLIFIER:       2
WAVEFORM:        134
SUN_POSTN:       205.38d {+13h 41m 32s}  -10.53d {-10d 31' 37"}
SUN_DIST:        117.15 [deg]   Sun_angle= -8.8 [hr] (East of Sun)
MOON_POSTN:      301.34d {+20h 05m 21s}  -15.70d {-15d 41' 54"}
MOON_DIST:        37.28 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:       12.93,-59.11 [deg] galactic lon,lat of the burst
ECL_COORDS:      326.63,-22.07 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Coordinates.  
COMMENTS:        The XRT position is 1.77 arcmin from the BAT position.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 21 Oct 15 01:46:29 UT
NOTICE_TYPE:     Swift-XRT Image
TRIGGER_NUM:     660671,   Seg_Num: 0
GRB_RA:          337.6467d {+22h 30m 35.2s} (J2000),
                 337.8709d {+22h 31m 29.0s} (current),
                 336.9355d {+22h 27m 44.5s} (1950)
GRB_DEC:         -33.2010d {-33d 12' 03.6"} (J2000),
                 -33.1196d {-33d 07' 10.4"} (current),
                 -33.4578d {-33d 27' 28.1"} (1950)
GRB_ERROR:       5.4 [arcsec, radius, statistical plus systematic]
GRB_INTEN:       39 [cnts]
IMG_START_DATE:  17316 TJD;   294 DOY;   15/10/21
IMG_START_TIME:  5443.28 SOD {01:30:43.28} UT,   90.7 [sec] since BAT Trigger Time
CENTROID_X:      353.92,  raw= 354  [pixels]
CENTROID_Y:      310.76,  raw= 311  [pixels]
ROLL:            234.71 [deg]
GAIN:            1
MODE:            2, Short Image mode
WAVEFORM:        134
EXPO_TIME:       0.10 [sec]
GRB_POS_XRT_Y:   30.56
GRB_POS_XRT_Z:   114.61
IMAGE_URL:       sw00660671000msxps_rw.img
SUN_POSTN:       205.38d {+13h 41m 32s}  -10.53d {-10d 31' 37"}
SUN_DIST:        117.15 [deg]   Sun_angle= -8.8 [hr] (East of Sun)
MOON_POSTN:      301.34d {+20h 05m 21s}  -15.70d {-15d 41' 54"}
MOON_DIST:        37.28 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:       12.93,-59.11 [deg] galactic lon,lat of the burst
ECL_COORDS:      326.63,-22.07 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Image.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 21 Oct 15 01:47:05 UT
NOTICE_TYPE:     Swift-XRT Processed Image
TRIGGER_NUM:     660671,   Seg_Num: 0
GRB_RA:          337.6467d {+22h 30m 35.2s} (J2000),
                 337.8709d {+22h 31m 29.0s} (current),
                 336.9355d {+22h 27m 44.5s} (1950)
GRB_DEC:         -33.2010d {-33d 12' 03.6"} (J2000),
                 -33.1196d {-33d 07' 10.4"} (current),
                 -33.4578d {-33d 27' 28.1"} (1950)
GRB_ERROR:       5.4 [arcsec, radius, statistical plus systematic]
GRB_INTEN:       39 [cnts]
IMG_START_DATE:  17316 TJD;   294 DOY;   15/10/21
IMG_START_TIME:  5443.28 SOD {01:30:43.28} UT,   90.7 [sec] since BAT Trigger Time
CENTROID_X:      353.92,  raw= 354  [pixels]
CENTROID_Y:      310.76,  raw= 311  [pixels]
ROLL:            234.71 [deg]
GAIN:            1
MODE:            2, Short Image mode
WAVEFORM:        134
EXPO_TIME:       0.10 [sec]
GRB_POS_XRT_Y:   30.56
GRB_POS_XRT_Z:   114.61
IMAGE_URL:       sw00660671000msxps_rw.img
SUN_POSTN:       205.38d {+13h 41m 32s}  -10.53d {-10d 31' 37"}
SUN_DIST:        117.15 [deg]   Sun_angle= -8.8 [hr] (East of Sun)
MOON_POSTN:      301.34d {+20h 05m 23s}  -15.70d {-15d 41' 50"}
MOON_DIST:        37.27 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:       12.93,-59.11 [deg] galactic lon,lat of the burst
ECL_COORDS:      326.63,-22.07 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Processed Image.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 21 Oct 15 01:47:53 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     660671,   Seg_Num: 0
POINT_RA:        337.608d {+22h 30m 26s} (J2000)
POINT_DEC:       -33.210d {-33d 12' 35"} (J2000)
POINT_ROLL:      234.714d
IMG_START_DATE:  17316 TJD;   294 DOY;   15/10/21
IMG_START_TIME:  5451.99 SOD {01:30:51.99} UT,   99.4 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        2.231
N_STARS:         27
X_OFFSET:        448 [pixels]
Y_OFFSET:        648 [pixels]
X_MAX:           1407 [pixels]
Y_MAX:           1607 [pixels]
DET_THRESH:      12
PHOTO_THRESH:    6
SL_URL:          sw00660671000msufc0099.fits
SUN_POSTN:       205.38d {+13h 41m 32s}  -10.53d {-10d 31' 38"}
SUN_DIST:        117.12 [deg]   Sun_angle= -8.8 [hr] (East of Sun)
MOON_POSTN:      301.35d {+20h 05m 25s}  -15.70d {-15d 41' 46"}
MOON_DIST:        37.24 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:       12.92,-59.07 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      326.59,-22.07 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 21 Oct 15 01:48:43 UT
NOTICE_TYPE:     Swift-UVOT Image
TRIGGER_NUM:     660671,   Seg_Num: 0
POINT_RA:        337.608d {+22h 30m 26s} (J2000)
POINT_DEC:       -33.210d {-33d 12' 35"} (J2000)
ROLL:            234.714d
IMG_START_DATE:  17316 TJD;   294 DOY;   15/10/21
IMG_START_TIME:  5451.99 SOD {01:30:51.99} UT,   99.4 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     467083867
X_OFFSET:        564 [pixels]
Y_OFFSET:        932 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       724
Y_GRB_POS:       1092
BINNING_INDEX:   1
IM_URL:          sw00660671000msuni0115.fits
SUN_POSTN:       205.39d {+13h 41m 32s}  -10.53d {-10d 31' 39"}
SUN_DIST:        117.12 [deg]   Sun_angle= -8.8 [hr] (East of Sun)
MOON_POSTN:      301.36d {+20h 05m 26s}  -15.70d {-15d 41' 42"}
MOON_DIST:        37.23 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:       12.92,-59.07 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      326.59,-22.07 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Image.  
COMMENTS:        The GRB Position came from the XRT Position Command.  
COMMENTS:        The image has 2x2 binning (compression).

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Wed 21 Oct 15 01:50:30 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     660671,   Seg_Num: 0
POINT_RA:        337.608d {+22h 30m 26s} (J2000)
POINT_DEC:       -33.210d {-33d 12' 35"} (J2000)
POINT_ROLL:      234.714d
IMG_START_DATE:  17316 TJD;   294 DOY;   15/10/21
IMG_START_TIME:  5451.99 SOD {01:30:51.99} UT,   99.4 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        2.231
N_STARS:         27
X_OFFSET:        448 [pixels]
Y_OFFSET:        648 [pixels]
X_MAX:           1407 [pixels]
Y_MAX:           1607 [pixels]
DET_THRESH:      12
PHOTO_THRESH:    6
SL_URL:          sw00660671000msufc0099.fits
SUN_POSTN:       205.39d {+13h 41m 33s}  -10.53d {-10d 31' 40"}
SUN_DIST:        117.12 [deg]   Sun_angle= -8.8 [hr] (East of Sun)
MOON_POSTN:      301.38d {+20h 05m 31s}  -15.69d {-15d 41' 32"}
MOON_DIST:        37.22 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:       12.92,-59.07 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      326.59,-22.07 [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 21 Oct 15 01:50:16 UT
NOTICE_TYPE:     Swift-UVOT Processed Image
TRIGGER_NUM:     660671,   Seg_Num: 0
POINT_RA:        337.608d {+22h 30m 26s} (J2000)
POINT_DEC:       -33.210d {-33d 12' 35"} (J2000)
ROLL:            234.714d
IMG_START_DATE:  17316 TJD;   294 DOY;   15/10/21
IMG_START_TIME:  5451.99 SOD {01:30:51.99} UT,   99.4 [sec] since BAT Trigger Time
FILTER:          10, White
EXPOSURE_ID:     467083867
X_OFFSET:        564 [pixels]
Y_OFFSET:        932 [pixels]
WIDTH:           160 [pixels]
HEIGHT:          160 [pixels]
X_GRB_POS:       724
Y_GRB_POS:       1092
BINNING_INDEX:   1
IM_URL:          sw00660671000msuni0115.fits
SUN_POSTN:       205.39d {+13h 41m 33s}  -10.53d {-10d 31' 40"}
SUN_DIST:        117.12 [deg]   Sun_angle= -8.8 [hr] (East of Sun)
MOON_POSTN:      301.38d {+20h 05m 30s}  -15.69d {-15d 41' 33"}
MOON_DIST:        37.22 [deg]
MOON_ILLUM:      52 [%]
GAL_COORDS:       12.92,-59.07 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      326.59,-22.07 [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 #18425
A. Melandri (INAF-OAB), S. D. Barthelmy (GSFC), P. D'Avanzo (INAF-OAB), H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (UCL-MSSL), D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team:
At 01:29:12 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 151021A (trigger=660671). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 337.611, -33.199 which is
```
   RA(J2000) = 22h 30m 27s
   Dec(J2000) = -33d 11' 55"
```
with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex multi-peaked structure with a duration of at least 100 sec. The peak count rate was 2500 counts/sec (15-350 keV), at 10 sec after the trigger, although the lightcurve around the time of trigger is not immediately available.
The XRT began observing the field at 01:30:43.2 UT, 90.7 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 337.64619, -33.19907 which is equivalent to:
```
   RA(J2000)  = 22h 30m 35.09s
   Dec(J2000) = -33d 11' 56.7"
```
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This location is 106 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 destiny in excess of the Galactic value (1.13 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.9 (+2.55/-2.23) x 10^21 cm^-2 (90% confidence).
The initial flux in the 0.1 s image was 2.76e-08 erg cm^-2 s^-1 (0.2-10 keV).
UVOT took a finding chart exposure of 150 seconds with the White filter starting 99 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. 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 0.01.
Burst Advocate for this burst is A. Melandri (andrea.melandri AT brera.inaf.it). 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 21 Oct 15 02:11:43 UT
NOTICE_TYPE:     Swift-XRT Position UPDATE
TRIGGER_NUM:     660671,   Seg_Num: 0
GRB_RA:          337.6461d {+22h 30m 35.06s} (J2000),
                 337.8703d {+22h 31m 28.87s} (current),
                 336.9349d {+22h 27m 44.37s} (1950)
GRB_DEC:         -33.1990d {-33d 11' 56.3"} (J2000),
                 -33.1176d {-33d 07' 03.2"} (current),
                 -33.4558d {-33d 27' 20.9"} (1950)
GRB_ERROR:       3.6 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       1.00e-10 [erg/cm2/sec]
GRB_SIGNIF:      10.00 [sigma]
IMG_START_DATE:  17316 TJD;   294 DOY;   15/10/21
IMG_START_TIME:  6848.00 SOD {01:54:08.00} UT,   1495.4 [sec] since BAT Trigger Time
TAM[0-3]:        100.00 100.00 100.00 100.00
AMPLIFIER:       1
WAVEFORM:        31
SUN_POSTN:       205.40d {+13h 41m 36s}  -10.53d {-10d 32' 00"}
SUN_DIST:        117.14 [deg]   Sun_angle= -8.8 [hr] (East of Sun)
MOON_POSTN:      301.58d {+20h 06m 20s}  -15.66d {-15d 39' 37"}
MOON_DIST:        37.11 [deg]
MOON_ILLUM:      53 [%]
GAL_COORDS:       12.94,-59.11 [deg] galactic lon,lat of the burst
ECL_COORDS:      326.63,-22.07 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Coordinates.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        This is an Update Notice -- the RA,Dec values herein supersede the previous XRT_POS Notice.  
COMMENTS:        TAM values, flux and significance fields are not valid.  
COMMENTS:        This position was automatically generated on the ground using  
COMMENTS:        Photon Counting data telemetered via TDRSS (SPER data).  
COMMENTS:        See http://www.swift.ac.uk/sper/docs.php for details.  
COMMENTS:        The probability that this is a serendipitous source in the  
COMMENTS:        SPER window is 0.58% < P(seren) < 1.1%.

GCN Circular #18426
A. de Ugarte Postigo, (IAA-CSIC, DARK/NBI), D. Malesani (DARK/NBI), D. Xu (NAOC/CAS) report on behalf of a larger collaboration:
We have observed the field of GRB 151021A (Melandri et al., GCN 18425) with X-shooter at the VLT (Paranal Observatory, Chile). Observation started at 02:13 UT (44 min after the burst) and cover the range between 3000 and 24800 AA.
In the close proximity of the XRT error circle, we detect a bright, uncatalogued object in the r'-band acquisition image, at coordinates (J2000, 0.5" error):
```
RA = 22:30:34.369
Dec = -33:11:49.45
```
We suggest that this is the optical afterglow of GRB 151021A and we measure for it R = 18.2 (Vega), derived from comparison with R-band magnitudes for the USNO-B1.0 catalogue. An object at consistent coordinates is also visible in the UVOT white-band image.
On a preliminary reduction, the spectrum shows a bright continuum with multiple absorption features, including SII, OI, SiII, SiIV, CIV, AlII, ZnII, CrII, FeII, NiII, MnII, MgII, MgI, as well as fine-structure lines from FeII* and NiII* and Ly-alpha absorption at a common redshift of z = 2.330, which we identify as the redshift of the GRB. We also identify an intervening system at z = 1.490 with features of FeII, MgII, and MgI.
We acknowledge the excellent support given by the Paranal staff, and in particular Andrea Mehner, Julien Mili and Jose Velasquez.
GCN Circular #18427
V. Lipunov, E. Gorbovskoy, N.Tyurina, V.Kornilov, P.Balanutsa, A.Kuznetsov, D.Kuvshinov, D. Vlasenko Lomonosov Moscow State University, Sternberg Astronomical Institute
D.Buckley, S. Potter, A.Kniazev, M.Kotze South African Astronomical Observatory
K.Ivanov, S.Yazev, N.M.Budnev, O.Gres, O.Chuvalaev, V.A.Poleshchuk Irkutsk State University
A. Tlatov, A.V. Parhomenko, D. Dormidontov, V.Sennik Kislovodsk Solar Station of the Pulkovo Observatory
V.Yurkov, Yu.Sergienko, D.Varda, E.Sinyakov Blagoveschensk Educational State University, Blagoveschensk
V.Krushinski, I.Zalozhnih, A. Popov Ural Federal University, Kourovka
Hugo Levato and Carlos Saffe Instituto de Ciencias Astronomicas, de la Tierra y del Espacio (ICATE)
Claudio Mallamaci, Carlos Lopez and Federico Podest Observatorio Astronomico Felix Aguilar (OAFA)
MASTER II robotic telescope (MASTER-Net: http://observ.pereplet.ru) located in SAAO was pointed to the GRB151021A 25 sec after notice time and 50 sec after trigger time at 2015-10-21 01:30:06 UT in two polarizations. On our first (10s exposure) set we found 1 optical transient within SWIFT error-box (Melandri et. al GCN 18425) brighter then 16.
```
date       time      Tmid-T0      Coord2000                       Mag
2015-10-21 01:30:06   55.0   (22h 30m 34.42s , -33d 11m 50.3s)    14.86
```
The 5-sigma upper limit has been about 16.0 mag The message may be cited.
GCN Circular #18428
J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester) report on behalf of the Swift-XRT team.
Using 2547 s of XRT Photon Counting mode data and 3 UVOT images for GRB 151021A, 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 = 337.64347, -33.19731 which is equivalent to:
```
RA (J2000): 22h 30m 34.43s
Dec (J2000): -33d 11' 50.3"
```
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 #18429
H. A. Krimm (GSFC/USRA), S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC), N. Gehrels (GSFC), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), A. Melandri (INAF-OAB), D. M. Palmer (LANL), T. Sakamoto (AGU), M. Stamatikos (OSU), T. N. Ukwatta (LANL) (i.e. the Swift-BAT team):
Using the data set from T-240 to T+352 sec from the recent telemetry downlinks, we report further analysis of BAT GRB 151021A (trigger #660671) (Melandri, et al., GCN Circ. 18425). The BAT ground-calculated position is RA, Dec = 337.647, -33.189 deg, which is
```
   RA(J2000)  =  22h 30m 35.3s 
   Dec(J2000) = -33d 11' 21.8" 
```
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 10%.
The mask-weighted light curve shows many overlapping peaks starting at ~T-50sec, with a maxiximum peak at ~T+1 sec, and ending at ~T+130 sec. T90 (15-350 keV) is 110.2 +- 3.7 sec (estimated error including systematics).
The time-averaged spectrum from T-21.6 to T+127.4 sec (with a 1.5 sec data gap in the middle) is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.57 +- 0.04. The fluence in the 15-150 keV band is 2.8 +- 0.1 x 10^-5 erg/cm2. The 1-sec peak photon flux measured from T-0.13 sec in the 15-150 keV band is 9.7 +- 0.8 ph/cm2/sec. All the quoted errors are at the 90% confidence level.
The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/660671/BA/
GCN Circular #18430
J.A. Kennea (PSU), P.A. Evans (U. Leicester), S.L. Gibson (U. Leicester), J.P. Osborne (U. Leicester), A. D'ai (INAF-IASFPA), A. Maselli (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU) and A. Melandri report on behalf of the Swift-XRT team:
We have analysed 6.7 ks of XRT data for GRB 151021A (Melandri et al. GCN Circ. 18425), from 80 s to 29.2 ks after the BAT trigger. The data comprise 184 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 Osborne et al. (GCN Circ. 18428).
The light curve can be modelled with a series of power-law decays. The initial decay index is alpha=1.3 (+1.8, -1.6). At T+102 s the decay steepens to an alpha of 3.90 (+0.33, -0.29). The light curve breaks again at T+230 s to a decay with alpha=0.91 (+0.07, -0.06), before a final break at T+2618 s s after which the decay index is 1.40 (+/-0.08).
A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.68 (+/-0.04). The best-fitting absorption column is 1.47 (+0.20, -0.19) x 10^22 cm^-2, at a redshift of 2.33, in addition to the Galactic value of 1.1 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has a photon index of 1.96 (+0.10, -0.09) and a best-fitting absorption column of 2.3 (+0.5, -0.4) x 10^22 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x 10^-11 (4.4 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
```
Galactic foreground: 1.1 x 10^20 cm^-2
Intrinsic column:    2.3 (+0.5, -0.4) x 10^22 cm^-2 at z=2.33
Photon index:	     1.96 (+0.10, -0.09)
```
If the light curve continues to decay with a power-law decay index of 1.40, the count rate at T+24 hours will be 0.017 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.9 x 10^-13 (7.5 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/00660671.
This circular is an official product of the Swift-XRT team.
GCN Circular #18431
D. Kuroda, K. Yanagisawa, Y. Shimizu, H. Toda (OAO, NAOJ), S. Nagayama (NAOJ), M. Yoshida (Hiroshima), K. Ohta (Kyoto) and N. Kawai(Tokyo Tech) report on behalf of MITSuME and OISTER collaboration:
We observed the field of GRB 151021A (Melandri et al., GCNC 18425) with the optical three color (g', Rc and Ic) CCD camera attached to the MITSuME 50cm telescope of Okayama Astrophysical Observatory.
The observation started on 2015-10-21 09:35:46 UT (8.1 h after the burst). We could not detect the previously reported afterglow (de Ugarte Postigo et al., GCNC 18426; Lipunov et al., GCNC 18427) in all the three bands.
Three sigma upper limits of the OT are listed below. We used GSC2.3 catalog for flux calibration.
```
#T0+[day]  MID-UT    T-EXP[sec]   g'    Rc     Ic
-----------------------------------------------------
0.37964    10:35:52    6180.0   >19.5  >19.7  >19.1
-----------------------------------------------------
T0+ : Elapsed time after the burst [day]
T-EXP: Total Exposure time [sec]
```
GCN Circular #18432
L. M. McCauley (PSU), and A. Melandri (INAF-OAB) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 151021A 99 s after the BAT trigger (Melandri et al., GCN Circ. 18425). A source consistent with the XRT position (Osborne et al. GCN Circ. 18428) is detected in the initial UVOT exposures.
The preliminary UVOT position is:
```
    RA  (J2000) =  22:30:34.43 = 337.64335 (deg.)
    Dec (J2000) = -33:11:50.3  = -33.19715 (deg.)
```
with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence).
Preliminary detections and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are:
```
Filter         T_start(s)   T_stop(s)   Exp(s)     Mag

white                  99         249      294   16.89
white                1668        5949      216  >20.86
v                      81          90        9   15.82
v                    1545        6359      235  >19.28
b                    1644        5744      216  >20.19
u                    1619        5538      216  >19.77
uvw1                 5133        5333      196  >19.53
uvm2                 4928        7484      316  >19.64
uvw2                 5955        6155      196  >19.97
```
The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.01 in the direction of the burst (Schlegel et al. 1998).
GCN Circular #18433
S. Golenetskii, R. Aptekar, D. Frederiks, V. Pal'shin, P. Oleynik, M. Ulanov, D. Svinkin, A. Tsvetkova, A. Lysenko, A.Kozlova and T. Cline on behalf of the Konus-Wind team, report:
The long-duration, intense GRB 151021A (Swift-BAT trigger #660671: Melandri et al., GCN 18425; Krimm et al., GCN 18429) triggered Konus-Wind at T0=5336.535 s UT (01:28:56.535).
The burst light curve shows a multi-peaked structure started at ~T0-6.1 s with a total duration of ~100 s. The emission is seen up to ~10 MeV.
The Konus-Wind light curve of this GRB is available at http://www.ioffe.ru/LEA/GRBs/GRB151021_T05336/
As observed by Konus-Wind, the burst had a fluence of 7.66(-0.87,+0.97)x10^-5 erg/cm2, and a 64-ms peak flux, measured from T0+22.256 s, of 4.92(-1.44,+1.41)x10^-6 erg/cm2/s (both in the 20 keV - 10 MeV energy range).
The time-averaged spectrum of the burst (measured from T0 to T0+131.328 s) is best fit in the 20 keV - 10 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -1.14 (-0.14,+0.15), the high energy photon index beta = -2.46 (-0.32,+0.18), the peak energy 170 (-18,+23) keV (chi2 = 94/96 dof)
The spectrum near the maximum count rate (measured from T0+16.640 to T0+33.024 s) is best fit in the 20 keV - 10 MeV range by the GRB (Band) model with the following parameters: the low-energy photon index alpha = -0.89 (-0.19,+0.19), the high energy photon index beta = -2.29 (-0.32,+0.14), the peak energy 200 (-27,+46) keV (chi2 = 73/96 dof)
Assuming the redshift z=2.330 (de Ugarte Postigo et al., GCN 18426) and a standard cosmology model with H_0 = 70 km/s/Mpc, Omega_M = 0.27, and Omega_Lambda = 0.73, we estimate the following rest-frame parameters: the isotropic energy release E_iso is ~1.0x10^54 erg, the peak luminosity L_iso is ~2.2x10^53 erg/s, and the rest-frame peak energy of the time-integrated spectrum, Ep,i, is ~666 keV.
All the quoted errors are at the 90% confidence level. All the quoted values are preliminary.
GCN Circular #18435
K. Yanagisawa, D. Kuroda, Y. Shimizu, H. Izumiura(OAO/NAOJ), M. Yoshida (Hiroshima), K. Ohta(Kyoto) and N. Kawai(Tokyo Tech.) report on behalf of the MITSuME collaboration:
We observed the field of GRB 151021A (Melandri et al., GCN 18425) in Ks-band with a wide-field near infrared imager at Okayama Astrophysical Observatory (Japan). The imager has effective aperture of 0.91 m.
Observations started from 10:42 UT on 21th October, 9.2h after the BAT trigger, to 11:11 UT. The total exposure of 11.7 min was successfully obtained.
In our co-add image, we did not find any new point source within the enhanced XRT error circle (Osborne et al., GCN 18428) down to limiting magnitude of Ks=14.5 (Vega, S/N=10). The photometric calibration was made against 2MASS field stars.
```
T0+[sec]    MID-UT     T-EXP[sec]      Ks
---------------------------------------------------
 +34,049    10:56      700.0           >14.5
---------------------------------------------------
T0+ : Elapsed time after the burt [sec]
T-EXP: Total Exposure time [sec]
```
GCN Circular #18472
A. Trotter, D. Reichart, J. Haislip, A. Smith, D. Caton, L. Hawkins, J. Moore, N. Frank, M. Maples, E. Johnson, R. Joyner, J. Martin, C. Salemi, J. A. Crain, K. Ivarsen, A. LaCluyze, and M. Nysewander report:
Skynet observed the Swift BAT/XRT localization of GRB 151021A (Melandri et al., GCN 18425, Swift trigger=660671) with with one 16" telescope (PROMPT 5; I band) of the PROMPT array at CTIO, Chile, and with the 17" telescope at the Appalachian State University Dark Sky Observatory (DSO17; B band) in NC, USA. Starting at 2015-10-21 01:31:14 UT and continuing until 03:00 UT (t=2m-91m post-trigger), Skynet took a total of 38 exposures ranging from 10-160s.
In the PROMPT I-band images, we clearly detect a fading optical afterglow at the position reported by de Ugarte Postigo et al. (GCN 18426), with I~14 at t=2m at I~19 at t=75m. A DSO17 B-band image taken at t=2.2m shows no source to a 3-sigma limit of B>15.7.
A preliminary light curve is at: http://www.skynet.unc.edu/grb/grb151021a.png
Limiting magnitudes are in the Vega System, calibrated to 4 APASS DR9 stars in the field. Magnitudes have not been corrected for the small line-of-sight Milky Way dust extinction, with expected E(B-V)=0.009 (Schlafly & Finkbeiner 2011).
No further Skynet observations are scheduled.

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Jochen Greiner, last update: 26-Oct-2015 [Disclaimer]