Gamma-ray Burst 150831B

(All information courtesy of the instrument teams.)

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Results of Observations

GCN/BACODINE POSITION NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 31 Aug 15 22:19:49 UT
NOTICE_TYPE:     Swift-BAT GRB Position
TRIGGER_NUM:     653978,   Seg_Num: 0
GRB_RA:          271.021d {+18h 04m 05s} (J2000),
                 271.267d {+18h 05m 04s} (current),
                 270.237d {+18h 00m 57s} (1950)
GRB_DEC:         -27.252d {-27d 15' 07"} (J2000),
                 -27.250d {-27d 15' 00"} (current),
                 -27.255d {-27d 15' 18"} (1950)
GRB_ERROR:       3.00 [arcmin radius, statistical only]
GRB_INTEN:       3820 [cnts]    Image_Peak=116 [image_cnts]
TRIGGER_DUR:     1.024 [sec] 
TRIGGER_INDEX:   149     E_range: 25-100 keV
BKG_INTEN:       23269 [cnts]
BKG_TIME:        80352.92 SOD {22:19:12.92} UT
BKG_DUR:         8 [sec]
GRB_DATE:        17265 TJD;   243 DOY;   15/08/31
GRB_TIME:        80367.26 SOD {22:19:27.26} UT
GRB_PHI:          -4.95 [deg]
GRB_THETA:        57.10 [deg]
SOLN_STATUS:     0x20000003
RATE_SIGNIF:     16.18 [sigma]
IMAGE_SIGNIF:    8.54 [sigma]
MERIT_PARAMS:     +1  +0  +0  +0  +2  -1  +0  +1 +38  +0 
SUN_POSTN:       159.84d {+10h 39m 22s}   +8.50d {+08d 29' 48"}
SUN_DIST:        112.88 [deg]   Sun_angle= -7.4 [hr] (East of Sun)
MOON_POSTN:        7.92d {+00h 31m 40s}   +2.82d {+02d 49' 19"}
MOON_DIST:        97.20 [deg]
MOON_ILLUM:      93 [%]
GAL_COORDS:        3.51, -2.66 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      270.91, -3.82 [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 = 50.70,-9.32 [deg].  
COMMENTS:           
COMMENTS:        NOTE: This BAT event is temporally(0.0<100sec) coincident with the FERMI_GBM event (trignum=462752371).

red DSS finding chart ps-file

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Mon 31 Aug 15 22:20:02 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      48
TRIGGER_NUM:     462752371
GRB_RA:          278.433d {+18h 33m 44s} (J2000),
                 278.669d {+18h 34m 41s} (current),
                 277.681d {+18h 30m 43s} (1950)
GRB_DEC:         -22.133d {-22d 07' 59"} (J2000),
                 -22.120d {-22d 07' 12"} (current),
                 -22.172d {-22d 10' 19"} (1950)
GRB_ERROR:       8.55 [deg radius, statistical plus systematic]
GRB_INTEN:       255 [cnts/sec]
DATA_SIGNIF:     13.60 [sigma]
INTEG_TIME:      2.048 [sec]
GRB_DATE:        17265 TJD;   243 DOY;   15/08/31
GRB_TIME:        80367.16 SOD {22:19:27.16} UT
GRB_PHI:         120.00 [deg]
GRB_THETA:        90.00 [deg]
DATA_TIME_SCALE: 2.0480 [sec]
HARD_RATIO:      1.33
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      89%  GRB
2nd_MOST_LIKELY:   9%  SGR_1806n20
DETECTORS:       0,0,1, 0,0,0, 0,0,0, 1,1,0, 0,0,
SUN_POSTN:       159.84d {+10h 39m 22s}   +8.50d {+08d 29' 48"}
SUN_DIST:        119.83 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:        7.92d {+00h 31m 41s}   +2.82d {+02d 49' 21"}
MOON_DIST:        90.37 [deg]
MOON_ILLUM:      93 [%]
GAL_COORDS:       11.21, -6.21 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      277.81,  1.08 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2015/bn150831930/quicklook/glg_lc_medres34_bn150831930.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 314.22,20.60 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.  
COMMENTS:           
COMMENTS:        NOTE: This GBM event is temporally(0.0<100sec) coincident with the SWIFT_BAT event (trignum=653978).

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Mon 31 Aug 15 22:20:21 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      58
TRIGGER_NUM:     462752371
GRB_RA:          280.920d {+18h 43m 41s} (J2000),
                 281.165d {+18h 44m 40s} (current),
                 280.137d {+18h 40m 33s} (1950)
GRB_DEC:         -27.530d {-27d 31' 47"} (J2000),
                 -27.513d {-27d 30' 47"} (current),
                 -27.581d {-27d 34' 50"} (1950)
GRB_ERROR:       5.11 [deg radius, statistical only]
DATA_SIGNIF:     23.80 [sigma]
DATA_INTERVAL:   2.048 [sec]
GRB_DATE:        17265 TJD;   243 DOY;   15/08/31
GRB_TIME:        80367.16 SOD {22:19:27.16} UT
GRB_PHI:         123.00 [deg]
GRB_THETA:        95.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   4153 (Gnd S/W Version number)
SUN_POSTN:       159.84d {+10h 39m 22s}   +8.50d {+08d 29' 47"}
SUN_DIST:        121.62 [deg]   Sun_angle= -8.1 [hr] (East of Sun)
MOON_POSTN:        7.92d {+00h 31m 41s}   +2.82d {+02d 49' 25"}
MOON_DIST:        88.43 [deg]
MOON_ILLUM:      93 [%]
GAL_COORDS:        7.31,-10.61 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      279.70, -4.46 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2015/bn150831930/quicklook/glg_lc_medres34_bn150831930.gif
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file will not be created/available until ~15 min after the trigger.  
COMMENTS:           
COMMENTS:        NOTE: This GBM event is temporally(0.0<100sec) coincident with the SWIFT_BAT event (trignum=653978).

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Mon 31 Aug 15 22:21:08 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      0
TRIGGER_NUM:     462752371
GRB_RA:          277.940d {+18h 31m 46s} (J2000),
                 278.181d {+18h 32m 43s} (current),
                 277.170d {+18h 28m 41s} (1950)
GRB_DEC:         -25.120d {-25d 07' 11"} (J2000),
                 -25.108d {-25d 06' 27"} (current),
                 -25.157d {-25d 09' 23"} (1950)
GRB_ERROR:       4.13 [deg radius, statistical only]
DATA_SIGNIF:     26.00 [sigma]
DATA_INTERVAL:   5.120 [sec]
GRB_DATE:        17265 TJD;   243 DOY;   15/08/31
GRB_TIME:        80367.16 SOD {22:19:27.16} UT
GRB_PHI:         120.00 [deg]
GRB_THETA:        93.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41531 (Gnd S/W Version number)
SUN_POSTN:       159.84d {+10h 39m 22s}   +8.50d {+08d 29' 47"}
SUN_DIST:        119.20 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:        7.93d {+00h 31m 43s}   +2.83d {+02d 49' 34"}
MOON_DIST:        90.97 [deg]
MOON_ILLUM:      93 [%]
GAL_COORDS:        8.32, -7.15 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      277.19, -1.88 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2015/bn150831930/quicklook/glg_lc_medres34_bn150831930.gif
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        This is likely a Long GRB.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file will not be created/available until ~15 min after the trigger.  
COMMENTS:           
COMMENTS:        NOTE: This GBM event is temporally(0.0<100sec) coincident with the SWIFT_BAT event (trignum=653978).

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Mon 31 Aug 15 22:21:43 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      1
TRIGGER_NUM:     462752371
GRB_RA:          278.280d {+18h 33m 07s} (J2000),
                 278.518d {+18h 34m 04s} (current),
                 277.521d {+18h 30m 05s} (1950)
GRB_DEC:         -23.150d {-23d 08' 59"} (J2000),
                 -23.137d {-23d 08' 13"} (current),
                 -23.188d {-23d 11' 17"} (1950)
GRB_ERROR:       2.89 [deg radius, statistical only]
DATA_SIGNIF:     24.70 [sigma]
DATA_INTERVAL:   6.144 [sec]
GRB_DATE:        17265 TJD;   243 DOY;   15/08/31
GRB_TIME:        80367.16 SOD {22:19:27.16} UT
GRB_PHI:         120.00 [deg]
GRB_THETA:        91.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41531 (Gnd S/W Version number)
SUN_POSTN:       159.84d {+10h 39m 22s}   +8.50d {+08d 29' 46"}
SUN_DIST:        119.63 [deg]   Sun_angle= -7.9 [hr] (East of Sun)
MOON_POSTN:        7.94d {+00h 31m 45s}   +2.83d {+02d 49' 41"}
MOON_DIST:        90.58 [deg]
MOON_ILLUM:      93 [%]
GAL_COORDS:       10.23, -6.54 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      277.61,  0.07 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2015/bn150831930/quicklook/glg_lc_medres34_bn150831930.gif
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        This is likely a Long GRB.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file will not be created/available until ~15 min after the trigger.  
COMMENTS:           
COMMENTS:        NOTE: This GBM event is temporally(0.0<100sec) coincident with the SWIFT_BAT event (trignum=653978).

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 31 Aug 15 22:23:17 UT
NOTICE_TYPE:     Swift-BAT GRB Lightcurve
TRIGGER_NUM:     653978,   Seg_Num: 0
GRB_RA:          271.021d {+18h 04m 05s} (J2000),
                 271.267d {+18h 05m 04s} (current),
                 270.237d {+18h 00m 57s} (1950)
GRB_DEC:         -27.252d {-27d 15' 07"} (J2000),
                 -27.250d {-27d 15' 00"} (current),
                 -27.255d {-27d 15' 18"} (1950)
GRB_DATE:        17265 TJD;   243 DOY;   15/08/31
GRB_TIME:        80367.26 SOD {22:19:27.26} UT
TRIGGER_INDEX:   149
GRB_PHI:          -4.95 [deg]
GRB_THETA:        57.10 [deg]
DELTA_TIME:      6.00 [sec]
TRIGGER_DUR:     1.024 [sec] 
SOLN_STATUS:     0x3
RATE_SIGNIF:     16.18 [sigma]
IMAGE_SIGNIF:    8.54 [sigma]
LC_URL:          sw00653978000msb.lc
SUN_POSTN:       159.84d {+10h 39m 22s}   +8.50d {+08d 29' 45"}
SUN_DIST:        112.88 [deg]   Sun_angle= -7.4 [hr] (East of Sun)
MOON_POSTN:        7.95d {+00h 31m 48s}   +2.83d {+02d 49' 59"}
MOON_DIST:        97.24 [deg]
MOON_ILLUM:      93 [%]
GAL_COORDS:        3.51, -2.66 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      270.91, -3.82 [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 = 50.70,-9.32 [deg].

GCN Circular #18215
F. E. Marshall (NASA/GSFC), S. D. Barthelmy (GSFC), A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL) and T. Sakamoto (AGU) report on behalf of the Swift Team:
At 22:19:27 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 150831B (trigger=653978). Swift did not slew due to observing constraint. The BAT on-board calculated location is RA, Dec 271.021, -27.252, which is
```
   RA(J2000)  =  18h 04m 05s
   Dec(J2000) = -27d 15' 07"
```
with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a couple of overlapping peaks structure with a total duration of about 3 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger.
Due to an observing constraint, Swift will not slew until T0+49.6 minutes. There will be no XRT or UVOT data until this time.
We note that this source is in the projected direction of the Galactic bulge (lon,lat = 3.51,-2.66) and for that reason there is a strong possibility that this is a Galactic source. However, its light curve and the presence of flux above 100 keV are more consistent with a cosmological GRB.
Determination of the nature of this event will require XRT observations and the full downlinked dataset.
Burst Advocate for this burst is F. E. Marshall (marshall AT milkyway.gsfc.nasa.gov). 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:     Mon 31 Aug 15 23:10:59 UT
NOTICE_TYPE:     Swift-XRT Nack-Position
TRIGGER_NUM:     653978,   Seg_Num: 0
POINT_RA:        271.007d {+18h 04m 02s} (J2000)
POINT_DEC:       -27.254d {-27d 15' 12"} (J2000)
IMG_START_DATE:  17265 TJD;   243 DOY;   15/08/31
IMG_START_TIME:  83452.80 SOD {23:10:52.80} UT,   3085.5 [sec] since BAT Trigger Time
COUNTS:          7    Min_needed= 20
STD_DEV:         0.00   Max_StdDev_for_Good=28.44 [arcsec]
PH2_ITER:        1    Max_iter_allowed= 4
ERROR_CODE:      1
COMMENTS:        SWIFT-XRT Nack Position.  
COMMENTS:        No source found in the image.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 31 Aug 15 23:14:35 UT
NOTICE_TYPE:     Swift-XRT Position
TRIGGER_NUM:     653978,   Seg_Num: 0
GRB_RA:          271.0389d {+18h 04m 09.33s} (J2000),
                 271.2845d {+18h 05m 08.28s} (current),
                 270.2549d {+18h 01m 01.18s} (1950)
GRB_DEC:         -27.2589d {-27d 15' 32.0"} (J2000),
                 -27.2571d {-27d 15' 25.6"} (current),
                 -27.2620d {-27d 15' 43.3"} (1950)
GRB_ERROR:       4.6 [arcsec radius, statistical plus systematic, 90% containment]
GRB_INTEN:       1.00e-10 [erg/cm2/sec]
GRB_SIGNIF:      10.00 [sigma]
IMG_START_DATE:  17265 TJD;   243 DOY;   15/08/31
IMG_START_TIME:  83471.00 SOD {23:11:11.00} UT,   3103.7 [sec] since BAT Trigger Time
TAM[0-3]:        100.00 100.00 100.00 100.00
AMPLIFIER:       1
WAVEFORM:        31
SUN_POSTN:       159.87d {+10h 39m 30s}   +8.48d {+08d 28' 58"}
SUN_DIST:        112.86 [deg]   Sun_angle= -7.4 [hr] (East of Sun)
MOON_POSTN:        8.46d {+00h 33m 51s}   +3.00d {+02d 59' 59"}
MOON_DIST:        97.75 [deg]
MOON_ILLUM:      93 [%]
GAL_COORDS:        3.51, -2.68 [deg] galactic lon,lat of the burst
ECL_COORDS:      270.93, -3.82 [deg] ecliptic lon,lat of the burst
COMMENTS:        SWIFT-XRT Coordinates.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
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 NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 31 Aug 15 23:16:47 UT
NOTICE_TYPE:     Swift-UVOT Source List
TRIGGER_NUM:     653978,   Seg_Num: 0
POINT_RA:        271.003d {+18h 04m 01s} (J2000)
POINT_DEC:       -27.252d {-27d 15' 06"} (J2000)
POINT_ROLL:      267.995d
IMG_START_DATE:  17265 TJD;   243 DOY;   15/08/31
IMG_START_TIME:  83456.39 SOD {23:10:56.39} UT,   3089.1 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        0.459
N_STARS:         190
X_OFFSET:        378 [pixels]
Y_OFFSET:        606 [pixels]
X_MAX:           1337 [pixels]
Y_MAX:           1565 [pixels]
DET_THRESH:      6
PHOTO_THRESH:    3
SL_URL:          sw00653978000msufc3089.fits
SUN_POSTN:       159.88d {+10h 39m 30s}   +8.48d {+08d 28' 56"}
SUN_DIST:        112.83 [deg]   Sun_angle= -7.4 [hr] (East of Sun)
MOON_POSTN:        8.48d {+00h 33m 56s}   +3.01d {+03d 00' 25"}
MOON_DIST:        97.81 [deg]
MOON_ILLUM:      93 [%]
GAL_COORDS:        3.50, -2.65 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      270.89, -3.82 [deg] ecliptic lon,lat of the pointing direction
COMMENTS:        SWIFT-UVOT Source List.

GCN NOTICE

TITLE:           GCN/SWIFT NOTICE
NOTICE_DATE:     Mon 31 Aug 15 23:17:29 UT
NOTICE_TYPE:     Swift-UVOT Processed Source List
TRIGGER_NUM:     653978,   Seg_Num: 0
POINT_RA:        271.003d {+18h 04m 01s} (J2000)
POINT_DEC:       -27.252d {-27d 15' 06"} (J2000)
POINT_ROLL:      267.995d
IMG_START_DATE:  17265 TJD;   243 DOY;   15/08/31
IMG_START_TIME:  83456.39 SOD {23:10:56.39} UT,   3089.1 [sec] since BAT Trigger Time
FILTER:          10, White
BKG_MEAN:        0.459
N_STARS:         190
X_OFFSET:        378 [pixels]
Y_OFFSET:        606 [pixels]
X_MAX:           1337 [pixels]
Y_MAX:           1565 [pixels]
DET_THRESH:      6
PHOTO_THRESH:    3
SL_URL:          sw00653978000msufc3089.fits
SUN_POSTN:       159.88d {+10h 39m 30s}   +8.48d {+08d 28' 56"}
SUN_DIST:        112.83 [deg]   Sun_angle= -7.4 [hr] (East of Sun)
MOON_POSTN:        8.49d {+00h 33m 58s}   +3.01d {+03d 00' 33"}
MOON_DIST:        97.81 [deg]
MOON_ILLUM:      93 [%]
GAL_COORDS:        3.50, -2.65 [deg] galactic lon,lat of the pointing direction
ECL_COORDS:      270.89, -3.82 [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 Circular #18217
P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester), D.N. Burrows (PSU), J.A. Kennea (PSU), S. Campana (INAF-OAB) and G. Cusumano (INAF-IASF PA) report on behalf of the Swift-XRT team:
The XRT began observing the field of GRB 150831B at 23:10:52.8 UT, 3085.5 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 271.03936, -27.25933 which is equivalent to:
```
   RA(J2000)  = 18h 04m 09.45s
   Dec(J2000) = -27d 15' 33.6"
```
with an uncertainty of 3.9 arcseconds (radius, 90% containment). This location is 64 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at http://www.swift.ac.uk/sper. We cannot determine whether the source is fading at the present time.
A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 5.13 x 10^21 cm^-2 (Willingale et al. 2013).
GCN Circular #18220
P. Veres (UAH) and C. Meegan (UAH) report on behalf of the Fermi GBM Team:
"At 22:19:27.158 UT on 31 August 2015, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 150831B (trigger 462752371 / 150831930) which was also detected by the Swift/BAT and XRT (Marshall et al. 2015, GCN 18215 and Evans et al. 2015, GCN 18217) The GBM on-ground location is consistent with the Swift position.
The angle from the Fermi LAT boresight at the GBM trigger time is 96 degrees.
The GBM light curve has a duration (T90) of about 13.8 s (50-300 keV). The time-averaged spectrum from T0-4.0 s to T0+6.3 s is adequately fit by a power law function with an exponential high-energy cutoff. The power law index is -0.93 +/- 0.12 and the cutoff energy, parameterized as Epeak, is 82.3 +/- 5.5 keV.
The event fluence (10-1000 keV) in this time interval is (2.51 +/- 0.11)E-6 erg/cm^2. The 1-sec peak photon flux measured starting from T0+0.1 s in the 10-1000 keV band is 9.1 +/- 0.4 ph/s/cm^2.
The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."
GCN Circular #18221
R. Rebolo, M. Serra, N. Lodieu, G. Israelian, L. Suarez-Andres The Instituto de Astrofisica de Canarias
V. Lipunov, E. Gorbovskoy, N.Tyurina, V.Kornilov, P.Balanutsa, A.Kuznetsov, D.Kuvshinov, E.Popova 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, A.V. Parhomenko, D. Dormidontov, V.Sennik Kislovodsk Solar Station of the Pulkovo Observatory
V.Yurkov, Yu.Sergienko, D.Varda, E.Sinyakov, A.Gabovich Blagoveschensk Educational State University, Blagoveschensk
V.Krushinski, I.Zalozhnih 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; http://161.72.133.2/) located in IAC was pointed to the GRB150831B 24 sec after notice time and 56 sec after trigger time at 2015-08-31 22:20:23 UT (Swift: Marshall et al., GCN 18215; and Fermi: Veres and C. Meegan, GCN 18220) . On our first (10s exposure) set (two polarisations) we did not found optical transient within SWIFT error-box (Marshall et al., GCN 18215; Evans et al., GCN 18217) brighter then 16.3. The observations were stoped by beautifull Canarian sun rise ( see http://observ.pereplet.ru/images/webcam01spt2015.jpg ) at 2015-09-01 06:37:36 UT.
```
Pro.type	Date time + -	     Exp.time	Limit	Filt.	Tube.
                    Start
Alert      2015-08-31 22:20:23   	10	16.3	P	WEST
Alert 	   2015-08-31 22:20:23	        10	15.6	P	EAST
Alert 	   2015-08-31 22:21:07	        20	16.4	P	WEST
Alert 	   2015-08-31 22:21:07	        20	15.9	P	EAST
Alert 	   2015-08-31 22:21:37	        30	16.8	P	WEST
Alert 	   2015-08-31 22:21:37	        30	16.2	P	EAST
Alert 	   2015-08-31 22:22:18	        30	0.0*	P	WEST 
Alert 	   2015-08-31 22:22:18	        30	16.2	P	EAST
Alert 	   2015-08-31 22:22:57	        40	16.9	P	WEST
Alert 	   2015-08-31 22:22:57	        40	16.3	P	EAST
Alert 	   2015-08-31 22:23:47	        50	17.0	P	WEST
Alert 	   2015-08-31 22:23:47	        50	16.5	P	EAST
Alert 	   2015-08-31 22:24:47	        60	17.0	P	WEST
Alert 	   2015-08-31 22:24:47	        60	16.5	P	EAST
Alert 	   2015-08-31 22:25:56	        80	17.2	P	WEST
Alert 	   2015-08-31 22:25:56	        80	16.6	P	EAST
Alert 	   2015-08-31 22:27:26	        100	17.2	P	WEST
Alert 	   2015-08-31 22:27:26	        100	16.6	P	EAST
Alert 	   2015-08-31 22:29:17	        120	17.3	P	WEST
Alert 	   2015-08-31 22:29:17 	        120	16.6	P	EAST
Alert 	   2015-08-31 22:31:27	        140	17.3	P	WEST
Alert 	   2015-08-31 22:31:27	        140	16.7	P	EAST
Alert 	   2015-08-31 22:33:57	        170	17.3	P	WEST
Alert 	   2015-08-31 22:33:57	        170	16.7	P	EAST

* - no stars 
Our server hase a lot of job in Milky Way star dust now.
```
This message may be cited.
GCN Circular #18222
P. D'Avanzo (INAF-OAB), T.G.R. Roegiers (PSU), L.M. McCauley (PSU), J.A. Kennea (PSU), C. Pagani (U. Leicester), A.P. Beardmore (U. Leicester), V. D'Elia (ASDC), A. D'ai (INAF-IASFPA) and F. E. Marshall (NASA/GSFC) report on behalf of the Swift-XRT team:
We have analysed 3.8 ks of XRT data for GRB 150831B (Marshall et al. GCN Circ. 18215), from 3.1 ks to 28.3 ks after the BAT trigger. The data are entirely in Photon Counting (PC) mode. The refined XRT position is RA, Dec = 271.0396, -27.2590 which is equivalent to:
```
RA (J2000): 18 04 09.49
Dec(J2000): -27 15 32.4
```
with an uncertainty of 3.7 arcsec (radius, 90% confidence).
The light curve can be modelled with a power-law decay with a decay index of alpha=1.32 (+0.26, -0.21).
A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.5 (+0.7, -0.6). The best-fitting absorption column is 1.7 (+1.0, -0.7) x 10^22 cm^-2, in excess of the Galactic value of 5.1 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.4 x 10^-11 (1.6 x 10^-10) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
```
Total column:	     1.7 (+1.0, -0.7) x 10^22 cm^-2
Galactic foreground: 5.1 x 10^21 cm^-2
Excess significance: 2.9 sigma
Photon index:	     2.5 (+0.7, -0.6)
```
If the light curve continues to decay with a power-law decay index of 1.32, the count rate at T+24 hours will be 1.7 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.6 x 10^-14 (2.7 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/00653978.
This circular is an official product of the Swift-XRT team.
GCN Circular #18223
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), F. E. Marshall (GSFC), 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-239 to T+963 sec from the recent telemetry downlink, we report further analysis of BAT GRB 150831B (trigger #653978) (Marshall et al., GCN Circ. 18215). The BAT ground-calculated position is RA, Dec = 271.030, -27.243 deg, which is RA(J2000) = 18h 04m 07.2s Dec(J2000) = -27d 14' 34.4" with an uncertainty of 2.0 arcmin, (radius, sys+stat, 90% containment). The partial coding was 7%.
The mask-weighted light curve shows two peaks starting at ~T-0.5 sec, peaking at ~T+1 sec, and ending at ~T+8 sec. T90 (15-350 keV) is 6.2 +- 1.0 sec (estimated error including systematics).
The time-averaged spectrum from T-0.58 to T+6.84 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 1.87 +- 0.19. The fluence in the 15-150 keV band is 1.4 +- 0.2 x 10^-06 erg/cm2. The 1-sec peak photon flux measured from T-0.15 sec in the 15-150 keV band is 6.1 +- 1.1 ph/cm2/sec. All the quoted errors are at the 90% confidence level.
In the BAT energy band, the hardness ratio vs T90 is typical of a GRB origin, but not inconsistant with a galactic origin. The T90 is too long for a magnetar origin. We also note that the fading in the X-ray band is tyical for a GRB (D'Avanzo, GCN 18222).
The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/653978/BA/
GCN Circular #18227
F. E. Marshall (NASA/GSFC) reports on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 150831B 3089 s after the BAT trigger (Marshall et al., GCN Circ. 18215). No optical afterglow consistent with the refined XRT position (D'Avanzo et al., GCN Circ. 18222) is detected in the initial UVOT exposures. We note that this is a very crowded field. 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          3089         3239          147         >20.6
v                 3246         3445          197         >18.2
b                 8838         9745          885         >20.3
u                 3861         5659          158         >19.4
w1                3656         3856          197         >19.2
m2                3451         3651          197         >20.2
```
The magnitudes in the table are not corrected for the large Galactic extinction in the direction of the burst (Schlegel et al. 1998).

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Jochen Greiner, last update: 01-Sep-2015 [Disclaimer]