GRB 160927A

S. L. Gibson (U Leicester), N. Gehrels (NASA/GSFC),
J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU)
report on behalf of the Swift Team:

At 18:04:49 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 160927A (trigger=713782).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 256.264, +17.311 which is 
   RA(J2000) = 17h 05m 03s
   Dec(J2000) = +17d 18' 38"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
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 18:06:09.3 UT, 79.5 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 256.24288, 17.33165 which is equivalent
to:
   RA(J2000)  = 17h 04m 58.29s
   Dec(J2000) = +17d 19' 53.9"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 103 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 7.14
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 84 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 none of
the XRT error circle. 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 0.08. 

Burst Advocate for this burst is S. L. Gibson (slg44 AT le.ac.uk). 
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.)

P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Using  promptly downlinked XRT event data for GRB 160927A, we find an
enhanced XRT position of the afterglow: RA, Dec: 256.2427, 17.3316
which is equivalent to:
   RA (J2000)  = 17 04 58.24
   Dec (J2000) = +17 19 53.7
with an uncertainty of 2.1 arcseconds (radius, 90% confidence).
Analysis of the promptly available data is online at
http://www.swift.ac.uk/sper/713782.

Position enhancement is is described by Goad et al. (2007, A&A, 476,
1401) and Evans et al. (2009, MNRAS, 397, 1177).

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

A. Tkachenko, R. Burenin, M. Pavlinsky, R. Sunyaev (IKI),
I. Bikmaev, E. Irtuganov, N. Sakhibullin (KFU/AST),
I. Khamitov, H. Kirbiyik (TUG), U. Kiziloglu (METU), E. Gogus (Sabanci Uni.)

report:

We observed field of Swift short hard GRB 160927A (Gibson et al., GCN
19952
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 ) with Russian-Turkish 1.5-m telescope (RTT150, Bakirlitepe,
TUBITAK National Observatory, Turkey) using TFOSC. We obtained four
600s exposures in SDSS-r filter at high airmass and very poor seeing
(2.5 arcsec) starting at 18:59:24 UT, i.e. approximately 55 min after
the burst. 

At the edge of improved XRT localization (Evans et al., GCN 19953
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 ) we
find faint optical source, which is not present in SDSS image, at the
position: 

RA,Dec: 17:04:58.21 +17:19:55.5  (J2000)

For this source we estimate SDSS-r mag =~ 22.3. The finding chart can
be found at: 

http://dlc.rsdc.rssi.ru/~rodion/grb/160927a/r.jpg

Martin Jelinek, Jan Strobl, Rene Hudec, Vojtech Simon
and Cyril Polasek (ASU CAS Ondrejov, CZ) report:

The 50cm robotic telescope (D50) of Ondrejov
observatory in Czech republic reacted robotically to
the alert of GRB160927A (Gibson et al, GCNC 19952),
obtaining a series of 10s unfiltered images starting at
18:05:29.9UT, i.e. 40s post trigger.

We do not detect any new source within or near the XRT
error circle (Evans, GCNC 19953) in comparison to
USNO-B1.0 catalog neither in single images (3-sigma
limit R>18.7) nor in a combined 25x10s frame (mean exp
time 124s post trigger, 3-sigma limit R>20.5).

P. D'Avanzo, A. Melandri (INAF-OAB), E. Palazzi (INAF-IASFBo), V. D'Elia (INAF/OAR & ASI/ASDC), 
L. Di Fabrizio, D. Carosati (INAF-TNG) report on behalf of the CIBO collaboration: 

We observed the field of the short GRB 160927A (Gibson et al., GCN 19952
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 )
 with the 3.6m Telescopio Nazionale Galileo (TNG) equipped with DOLoRes. 
Observations were carried out in the r-sdss filter. Observations started on Sept 27 
at 20:12:49 UT (2.13 hours after the burst) and consist in a series of 5 images, 
each one lasting 180 seconds. 

At the enhanced XRT position (Evans, GCN 19953
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 ) in the co-added image we 
detect an object at the following coordinates (J2000):

RA, Dec = 17:04:58.22, +17:19:54.9

+/- 0.5". The object has a magnitude r = 22.6 +/- 0.1 (calibrated against nearby SDSS stars). 
This object is the same reported by Tkachenko et al. (GCN 19954
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 ). Given the mild evidence 
for fading, we propose this object as the optical afterglow of GRB 160927A.

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

Using 3482 s of XRT Photon Counting mode data and 5 UVOT
images for GRB 160927A, 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 = 256.24259, +17.33197 which is equivalent
to:

RA (J2000): 17h 04m 58.22s
Dec (J2000): +17d 19' 55.1"

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.

K.L. Page (U. Leicester), A. D'Ai (INAF-IASFPA), A. Melandri
(INAF-OAB), P. D'Avanzo (INAF-OAB), L.M. McCauley (PSU), J.A. Kennea
(PSU), B. Sbarufatti (INAF-OAB/PSU), B. Mingo (U. Leicester), A.P.
Beardmore (U. Leicester) and S.L. Gibson report on behalf of the
Swift-XRT team:

We have analysed 6.6 ks of XRT data for GRB 160927A (Gibson et al. GCN
Circ. 19952
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 ), from 89 s to 13.2 ks after the  BAT trigger. The data are
entirely in Photon Counting (PC) mode. The enhanced XRT position for
this burst was given by Goad et al. (GCN Circ. 19953
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 ).

The light curve can be modelled with a power-law decay with a decay
index of alpha=0.99 (+0.08, -0.07).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.72 (+0.29, -0.17). The
best-fitting absorption column is  7.6 (+9.0, -0.4) x 10^20 cm^-2,
consistent with the Galactic value of 7.1 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 3.9 x 10^-11 (4.3 x
10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     7.6 (+9.0, -0.4) x 10^20 cm^-2
Galactic foreground: 7.1 x 10^20 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.72 (+0.29, -0.17)

If the light curve continues to decay with a power-law decay index of
0.99, the count rate at T+24 hours will be 9.0 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.5 x
10^-14 (3.9 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00713782.

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

P Wiseman (MPE Garching), J Bolmer (ESO Vitacura), and J Greiner (MPE
Garching) report on behalf of the GROND team:

We observed the field of GRB 160927A (Swift trigger 713782; Gibson et al.,
GCN #19952
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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 23:26 UT on 27/09/2016, 5h 21m after the GRB
trigger, and continued for ~1 hour, mostly during astronomical twilight.
They were performed at an average seeing of 1.8" and at an average airmass
of 2.

Based on 22.4 min of total exposures around a mid-time of 5.58 hours after
the trigger we marginally detect the source reported by Tkachenko et al.
(GCN #19954
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 ) and D'Avanzo et al. (GCN#19956
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 ) in the GROND r�-band with an
AB magnitude of:

r' = 23.9 +/- 0.4 mag.

The apparent fading since the RTT150 and TNG observations is convincing
evidence for this source being the afterglow, and further suggests a
steepening of the afterglow light-curve since the TNG observation.

The given magnitude is calibrated against SDSS zeropoints and is not
corrected for the expected Galactic foreground extinction corresponding to
a reddening of E_(B-V)=0.07 mag in the direction of the burst (Schlafly &
Finkbeiner, 2011).

D. Xu (NAOC/CAS), D. Malesani (DARK/NBI and DTU Space), H. Dahle, B. 
Racine (Oslo U.), T. Pursimo (NOT), J. P. U. Fynbo (DARK/NBI), report on 
behalf of a larger collaboration:

We observed the field of GRB 160927A (Gibson et al., GCN 19952
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 ) using 
the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC 
camera. Observations started at 21:24:39 UT on 2016-09-27 (i.e., 3.33 hr 
after the trigger) and 5x300s SDSS r-band frames were obtained.

The previously reported optical afterglow of the burst (Tkachenko et 
al., GCN 19954
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 ; D'Avanzo et al., GCN 19956
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 ; Wiseman et al., GCN 19959
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 ) 
is clearly detected in our stacked r-band image. It has m(r)=23.07 +/- 
0.10 at a median time of 3.56 hr post-burst, calibrated with nearby SDSS 
stars.

Combined with the RTT150, TNG, and GROND data points reported in the 
above GCN Circulars, we confirm the steepening of the afterglow 
light-curve (Wiseman et al., GCN 19959
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 ). A rough estimate gives rise to 
alpha_1~0.6, a break time of ~3.1 hr post-burst, and alpha_2~1.7 (but 
poorly constrained).

A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC),
N. Gehrels (GSFC), S. L. Gibson (U Leicester), H. A. Krimm (GSFC/USRA),
A. Y. Lien (GSFC/UMBC), C. B. Markwardt (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-61 to T+242 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 160927A (trigger #713782)
(Gibson, et al., GCN Circ. 19952
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 ).  The BAT ground-calculated position is
RA, Dec = 256.268, 17.334 deg which is
    RA(J2000)  =  17h 05m 04.2s
    Dec(J2000) = +17d 20' 01.8"
with an uncertainty of 1.8 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 50%.
  
The mask-weighted light curve in 16 msec binning shows at least two
peaks.  The first peak starts at T-0.2 sec, peaks at T+0.2 sec and
ends at T+0.3 sec.  The second peak starts at T+0.3 sec, peaks at
T+0.4 sec and ends at T+0.5 sec.  T90 (15-350 keV) is 0.48 +- 0.10 sec
(estimated error including systematics).

The time-averaged spectrum from T+0 to T+0.54 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.12 +- 0.26.  The fluence in the 15-150 keV band is 1.4 +- 0.2 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T-0.23 sec in the 15-150 keV band
is 1.9 +- 0.3 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/713782/BA/

U.Quadri, L.Strabla and R.Girelli report:

We imaged the field of GRB 160927A detected by SWIFT(trigger 713782)
with the robotic telescope of (IAU station 565) Bassano Bresciano 
Observatory, Italy. Member of: 
AAVSO - American Association of Variable Star Observers.
ISSP - Italian Supernovae Search Project.
UAI/SSV - Unione Astrofili Italiani/sezione stelle variabili.

The observations started 11.72 min after the GRB trigger,  
with our Newton telescope D=250 mm F/D=4.8.

Weather conditions were good.

We co-added 52 exposures of 60 sec each.

Start T0+      End T0+      CV lim
11.72 min     65.71 min     19.5

We did not found any optical counterpart in the error box of the XRTcandidate.
S. L. Gibson (U Leicester), N. Gehrels (NASA/GSFC), et al.

Magnitudes were estimated with the UCAC4 cat. and 
are not corrected for galactic dust extinction.

Reference:
http://www.osservatoriobassano.org/GRB.asp

The message may be cited.

Paul Kuin(MSSL-UCL) and S. L. Gibson (U. Leicester)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 160927A
85 s after the BAT trigger (Gibson et al., GCN Circ. 19952
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 ).
No optical afterglow consistent with the optical position
(D'Avanzo et al. GCN Circ. 19956
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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            85          235          147         >21.0
u_FC               297          547          246         >20.4
white               85        13184         1426         >21.8
v                  627         6999          510         >19.9
b                  553        12500         1179         >21.5
u                  297        11587         1433         >21.1
w1                 850         7411          491         >20.2
m2                 826         7204          294         >20.5
w2                 602         6794          510         >21.0

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.08 in the direction of the burst
(Schlegel et al. 1998).

W. Fong (Univ. of Arizona), S. Sheppard (Carnegie DTM) and E. Berger
(Harvard) report:

We imaged the location of the short-duration GRB 160927A (Gibson et al.,
GCN 19952
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 ) with the Inamori-Magellan Areal Camera and Spectrograph (IMACS)
mounted on the Magellan/Baade 6.5-m telescope at a mid-time of 2016 Sep
28.00 UT (5.94 hr post-burst). We obtained 4x200-sec of r-band exposures in
0.75" seeing at an airmass of 1.9. We clearly detect the optical afterglow
(Tkachenko et al., GCN 19954
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 ; D'Avanzo et al., GCN 19956
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 ; Wiseman et al.,
GCN 19959
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 ; Xu et al., GCN 19960
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 ). Tied to 10 SDSS stars in the field, we
calculate a magnitude of r_AB=23.84 +/- 0.21, not corrected for Galactic
extinction in the direction of the burst.

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, P. Scott (Cambridge), K. 
Grainge, A. Scaife (Manchester)

The AMI Large Array robotically triggered on the Swift alert for the 
short/hard GRB 160927A (Gibson et al., GCN 19952
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 ) as part of the 4pisky 
program, and subsequent follow up observations were obtained up to 3 
days post-burst. Our observations at 15 GHz on 2016 Sep 27.79, Sep 
28.70, and Sep 29.70 (UT; 1 hour, 23 hours, and 47 hours post-burst 
respectively) do not reveal any radio source at the XRT location (Page 
et al., GCN 19958
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 ), with 3sigma upper limits of 132 uJy, 119 uJy, and 
110 uJy respectively. The 3sigma upper limit from the first two epochs 
combined is 80 uJy.

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/.

M. Moriyama, A. Yoshida, T. Sakamoto, Y. Kawakubo, Y. Yamada (AGU),
K. Yamaoka (Nagoya U), S. Nakahira (JAXA), I. Takahashi (IPMU), Y. Asaoka,
S. Ozawa, S. Torii (Waseda U), Y. Shimizu, T. Tamura (Kanagawa U), W. Ishizaki (ICRR),
M. L. Cherry (LSU), S. Ricciarini (U of Florence), P. S. Marrocchesi (U of Siena)
and the CALET collaboration:

The short-duration GRB 160927A (Gibson et al., GCN Circ. 19952
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 ) was
detected by the ground analysis of the CALET Gamma-ray Burst Monitor
(CGBM) data at 18:04:49.88 on 27 September 2016.  The highest signal-to-noise
based on the light curve data is 5.3 sigma.  The burst signal was seen
by the SGM instrument.

The light curve of the SGM shows a single peak starting at T0, peaking
at T+0.4 sec and ending at T+0.6 sec.  The T90 duration measured by
the SGM data is 0.35 +- 0.11 sec (40-1000 keV).

The light curve is available at

http://cgbm.calet.jp/cgbm_trigger/flight/1159034645/

The CALET data used in this analysis are provided by the Waseda
CALET Operation Center located at the Waseda University.

A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), L. Izzo (IAA-CSIC), 
D.A. Kann (TLS Tautenburg), C.C. Thoene (IAA-CSIC), Z. Cano 
(IAA-CSIC), D. Reverte-Paya (GRANTECAN, IAC, ULL) report on 
behalf of a larger collaboration:

We observed the afterglow of the short-hard GRB 160927A (Gibson 
et al., GCN 19952
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 ) with OSIRIS on the 10.4m GTC telescope. 
Observations consisted in 10x190s exposures in r-band, with a mean 
epoch on 28 September 2016 at 20:26 UT (26.52 hr after the burst). 
The optical counterpart (Tkachenko et al., GCN 19954
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 ; D���Avanzo et 
al., GCN 19956
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 ; Wiseman et al. GCN 1959
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 ; Xu et al., GCN 19960
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 ; 
Fong et al., GCN 19964
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 ) is detected at a magnitude of r_AB = 
25.3+/-0.2, as compared to several SDSS field stars.

Combining this observation with the rest of the available GCN data, 
the light curve indicates an early evolution with a shallow decay, 
followed by a faster decay (as already mentioned by Wiseman et al. 
GCN 1959
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  and Xu et al., GCN 19960
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 ), with the break at around the 
TNG epoch, after which we estimate a decay rate of with alpha = 
-1.03+/-0.08 (where F~t^alpha).This decay is significantly shallower 
than the one reported by (Xu et al., GCN 19960
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 ), indicating that the 
light curve may be flattening at the time of the GTC observation. This 
could be due to the contribution of an additional component (either 
the host galaxy, or more unlikely the contribution of a kilonova) or just 
due to the uncertainties of the different photometric measurements.