GRB 150831A

W. Fong (U. Arizona) and B. A. Zauderer (Harvard) report:

"We observed the field of the short-duration GRB 150831A (Lien et al., GCN
18209
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 ) with the Karl G. Jansky Very Large Array (VLA) beginning on 2015 Aug
31.934 UT (11.9 hr post-burst) at a mean frequency of 9.8 GHz. In 1 hour of
observations, we do not detect any radio source within or around the
enhanced XRT position (Osborne et al., GCN 18212
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 ) to a 3-sigma limit of 25
microJy. Furthermore, we obtained a second set of observations beginning on
2015 Sep 2.954 UT (2.5 days post-burst) at a mean frequency of 9.8 GHz and
do not detect any radio source in or around the XRT position to a 3-sigma
limit of 21 microJy.

We therefore place 3-sigma limits of 25 microJy and 21 microJy on the radio
afterglow of GRB 150831A at 11.9 hr and 2.5 days after the burst,
respectively.

We thank the VLA staff for quickly executing these observations."

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 short-duration GRB 150831A (Swift trigger 648437:
Lien, et al., GCN Circ. 18209
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 ; Ukwatta, et al., GCN Circ. 18214
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 ;
INTEGRAL detection: Mereghetti et al., GCN Circ. 18210
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 )
triggered Konus-Wind at T0=38049.519 s UT (10:34:09.519).

The burst light curve shows a short pulse with
a total duration of ~1 s.

As observed by Konus-Wind, the burst had a fluence of
(2.4 +/- 0.4)x10^-6 erg/cm2, and a 16-ms peak flux,
measured from T0-0.082 s, of (9.1 +/- 2.5)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

Since the most intense part of the burst was detected before
the trigger, the spectral analysis was performed using
the KW 3-channel light curve data.

Modeling the 3-channel time-integrated spectrum
(from T0-0.108 s to T0+0.912 s) by a power law
with exponential cutoff (CPL) model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
yields alpha = -0.5 +/- 0.3, and Ep = 564 +/- 122 keV.

The K-W light curve of this burst is available at
http://www.ioffe.rssi.ru/LEA/GRBs/GRB150831_T38049/

All the quoted errors are estimated at the 1 sigma confidence level.
All the presented results are preliminary.

F. Knust (MPE Garching), S. Klose (TLS Tautenburg), and J. Greiner (MPE Garching) report on behalf of the GROND team: 
We observed the field of GRB 150831A (Swift trigger 653838; Lien et al., GCN #18209
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 ) simultaneously in g'r'i'z'JHK 
with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla Observatory (Chile). 
Observations started at 23:21UT on 2015-08-31, 13hrs after the GRB trigger. They were performed at
an average seeing of 2.4" and at an average airmass of 1.5. 

We do not detect a source within the Swift-XRT error circle reported by Osborne et al. (GCN #18212
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 ) down to 
g'> 24.9 mag, 
r'> 24.5 mag, 
i'> 23.8 mag, 
z'> 23.8 mag, 
J> 21.4 mag, 
H> 20.9 mag, and 
K> 18.6 mag. 

The given limits are derived based on calibrating the images against GROND zeropoints as well as 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V)=0.1 mag in the direction of the burst (Schlegel et al. 1998).

M. H. Siegel (PSU) and A. Y. Lien (GSFC/UMBC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 150831A
88 s after the BAT trigger (Lien et al., GCN Circ. 18209
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 ).
No optical afterglow consistent with the XRT position
(Osborne et al. GCN Circ. 18212
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 )
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            88          238          147         >20.5
u_FC               300          550          246         >19.9
white               88         6622          746         >21.2
v                  631        17885         1356         >19.8
b                  557         6417          452         >20.4
u                  300        12784         1326         >21.0
w1                 680        12112         1153         >21.3
m2                 655        18529         1750         >21.3
w2                 607        16972         1357         >21.1

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.11 in the direction of the burst
(Schlegel et al. 1998).

A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), D.N. Burrows (PSU),
T.G.R. Roegiers (PSU), L.M. McCauley (PSU), K.L. Page (U. Leicester),
C. Pagani (U. Leicester), A.P. Beardmore (U. Leicester), V. D'Elia
(ASDC) and A.Y. Lien report on behalf of the Swift-XRT team:

We have analysed 8.6 ks of XRT data for GRB 150831A (Lien et al. GCN
Circ. 18209
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 ), from 72 s to 22.5 ks after the  BAT trigger. The data
comprise 8 s in Windowed Timing (WT) mode (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.
18212
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 ).

The light curve can be modelled with a power-law decay with a decay
index of alpha=2.52 (+0.21, -0.22).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.25 (+0.17, -0.12). The
best-fitting absorption column is  1.27 (+0.89, -0.13) x 10^21 cm^-2,
consistent with the Galactic value of 1.1 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 5.0 x 10^-11 (5.4 x
10^-11) erg cm^-2 count^-1. 

A summary of the WT-mode spectrum is thus:
Total column:	     1.27 (+0.89, -0.13) x 10^21 cm^-2
Galactic foreground: 1.1 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.25 (+0.17, -0.12)

If the light curve continues to decay with a power-law decay index of
2.52, the count rate at T+24 hours will be 2.1 x 10^-7 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-17 (1.2 x 10^-17) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00653838.

This circular is an official product of the Swift-XRT team.

T. N. Ukwatta (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. Norris (BSU), D. M. Palmer (LANL),
T. Sakamoto (AGU), M. Stamatikos (OSU), (i.e. the Swift-BAT team):

Using the data set from T-240 to T+938 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 150831A (trigger #653838)
(Lien, et al., GCN Circ. 18209
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 ).  The BAT ground-calculated position is
RA, Dec = 221.034, -25.619 deg which is
  RA(J2000)  =  14h 44m 08.1s
  Dec(J2000) = -25d 37' 08.4"
with an uncertainty of 1.3 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 52%.

The mask-weighted light curve shows a short pulse that starts and peaks
at ~ T0, and ends at ~ T+1.4 s. T90 (15-350 keV) is 1.15 +- 0.23 sec
(estimated error including systematics).

The time-averaged spectrum from T-0.03 to T+1.40 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
0.81 +- 0.16.  The fluence in the 15-150 keV band is 3.8 +- 0.3 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+0.0 sec in the 15-150 keV band
is 3.4 +- 0.3 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.

The lag analysis using 8-ms binned light curve gives a spectral lag
of -2 ms +-  5 ms for the 100-350 keV and 25-50 keV band,
and 13 ms +- 20 ms for the 50-100 keV and 15-25 keV band.
These values are consistent with short GRBs.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/653838/BA/

J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 1089 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 150831A, 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 = 221.02422, -25.63499 which is equivalent
to:

RA (J2000): 14h 44m 5.81s
Dec (J2000): -25d 38' 06.0"

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.

L. Elenin (KIAM), A. Volnova (IKI), E. Mazaeva (IKI), I. Molotov (KIAM), 
A. Pozanenko (IKI) report on behalf of larger GRB follow-up collaboration:

We observed the field of Swift and INTEGRAL short GRB 150309A (Lien et 
al., GCN 18209
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 ; Mereghetti et al., GCN 18210
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 ) with with 0.4-m f/2.4 
telescope of ISON-SSO (Siding Spring Observatory) starting on Aug. 31 
(UT) 10:44:10 (i.e. 10 minutes after burst trigger). We obtained 30 
unfiltered images of 30 s exposure. We do not detect any optical source 
within XRT error circle (Lien et al., GCN 18209
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 ). Preliminary photometry 
of the filed is following


Date       UT start   t-T0     Filter   Exp.   UL (3 sigma)
                       (mid, days)       (s)

2015-08-31 10:44:10   0.01299  none    30*30   18.8


Photometry is based on nearby USNO-B1.0 stars (R2)

S.Mereghetti (IASF-Milano), D.Gotz (CEA, Saclay), C.Ferrigno, E.Bozzo, 
L.Ducci (ISDC, Versoix), and J.Borkowski (CAMK, Torun) on behalf of the 
IBAS Localization Team report:

GRB 150831A (Lien et al. GCN 18209
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 ) has been detected by IBAS in the 
IBIS/ISGRI data with a significance slightly below the threshold for 
automatic alert delivery (IBAS Weak Alert n.7228 was distributed at 
10:34:24 UT).

The burst consists of a single peak with duration of 2 s and starting at 
10:34:12 UT.

The IBIS/ISGRI refined coordinates (J2000) are:

R.A.= 221.0207 deg
DEC.= -25.6474 deg

with an uncertainty of 3   arcmin (90% c.l.).

The burst had a fluence of about 3x10e-7 erg/cm2 in the the 20-200 keV 
energy range.

A plot of the light curve will  be posted at
http://ibas.iasf-milano.inaf.it/IBAS_Results.html

A. Y. Lien (GSFC/UMBC), D. N. Burrows (PSU) and J. A. Kennea (PSU)
report on behalf of the Swift Team:

At 10:34:12 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150831A (trigger=653838).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 221.023, -25.649 which is 
   RA(J2000) = 14h 44m 05s
   Dec(J2000) = -25d 38' 57"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a short pulse
with a duration of about 1.2 sec.  The peak count rate
was ~10000 counts/sec (15-350 keV), at ~ 0.1 sec after the trigger. 

The XRT began observing the field at 10:35:36.3 UT, 83.4 seconds after
the BAT trigger. Using promptly downlinked data we find a bright, fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 221.0245,
-25.6356 which is equivalent to:
   RA(J2000)  = 14h 44m 05.87s
   Dec(J2000) = -25d 38' 08.2"
with an uncertainty of 2.6 arcseconds (radius, 90% containment). This
location is 48 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 1.14
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 8.91e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 88 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.11. 

Burst Advocate for this burst is A. Y. Lien (amy.y.lien AT 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.)