GRB 160824A

P.A. Evans (U Leicester), S. B. Cenko (GSFC),
H. A. Krimm (CRESST/GSFC/USRA), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 23:22:25 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 160824A (trigger=709765).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 80.078, +40.040 which is 
   RA(J2000) = 05h 20m 19s
   Dec(J2000) = +40d 02' 25"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 40 sec.  The peak count rate
was ~2100 counts/sec (15-350 keV), at ~5 sec after the trigger. 

The XRT began observing the field at 23:23:56.5 UT, 91.1 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
80.0877, 40.0286 which is equivalent to:
   RA(J2000)  = 05h 20m 21.05s
   Dec(J2000) = +40d 01' 43.1"
with an uncertainty of 2.1 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 in excess of the Galactic value (6.32 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 4.7
(+6.16/-4.59) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 1.04e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 101 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 large, but uncertain extinction expected. 

Burst Advocate for this burst is P.A. Evans (pae9 AT star.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.)

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

Using 2198 s of XRT Photon Counting mode data and 5 UVOT
images for GRB 160824A, 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 = 80.08758, +40.02826 which is equivalent
to:

RA (J2000): 05h 20m 21.02s
Dec (J2000): +40d 01' 41.7"

with an uncertainty of 1.4 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.

B. Mingo (U. Leicester), A.P. Beardmore (U. Leicester), P. D'Avanzo
(INAF-OAB), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), T.G.R. Roegiers
(PSU), L.M. McCauley (PSU), J.A. Kennea (PSU) and P.A. Evans report on
behalf of the Swift-XRT team:

We have analysed 10 ks of XRT data for GRB 160824A (Evans et al. GCN
Circ. 19855
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 ), from 81 s to 29.6 ks after the  BAT trigger. The data
comprise 105 s in Windowed Timing (WT) mode (the first 8 s were taken
while Swift was slewing) with the remainder in Photon Counting (PC)
mode. The enhanced XRT position for this burst was given by Goad et al.
(GCN Circ. 19857
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 ).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=3.33 (+0.17, -0.18). At T+258 s  the decay
flattens to an alpha of 0.44 (+0.05, -0.06) before breaking again at
T+7762 s to a final decay with index alpha=1.31 (+/-0.14).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 2.07 (+/-0.10). The
best-fitting absorption column is  1.18 (+0.11, -0.10) x 10^22 cm^-2,
in excess of the Galactic value of 6.3 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.96 (+/-0.13) and a
best-fitting absorption column of 1.05 (+0.14, -0.13) x 10^22 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 5.0 x 10^-11 (9.1 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.05 (+0.14, -0.13) x 10^22 cm^-2
Galactic foreground: 6.3 x 10^21 cm^-2
Excess significance: 5.3 sigma
Photon index:	     1.96 (+/-0.13)

If the light curve continues to decay with a power-law decay index of
1.31, the count rate at T+24 hours will be 0.018 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 8.8 x
10^-13 (1.6 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00709765.

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

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

We imaged the field of GRB 160824A detected by SWIFT(trigger 709765)
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 20.77 min after the GRB trigger,  
with our Schmidt telescope D=320/400 mm F/D=3.1.

Weather conditions not good for Moon proximity.

We co-added 4 series of 20 exposures of 120 sec each.

Start T0+      End T0+      CVlim
20.77 min    196.52 min     17.5

We did not found any optical counterpart in the error box of the XRTcandidate.
P.A. Evans (U Leicester), S. B. Cenko (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.

F. E. Marshall (NASA/GSFC) and P. A. Evans (U. Leicester)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 160824A
101 s after the BAT trigger (Evans et al., GCN Circ. 19855
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 ).
UVOT marginally detects a source consistent with the enhanced XRT position
(Goad et al. GCN Circ. 19857
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 )
in the initial UVOT exposure with the white filter.

The preliminary analysis produces the following marginal detection 
and 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373):

Filter         T_start(s)   T_stop(s)      Exp(s)           Mag

white              101          787          186         20.16 +/- 0.27
v                  645          837           39        >17.9
b                  571          763           39        >18.7
u                  313          738          265        >19.7
w1                 694          714           19        >18.5
m2                 669          861           39        >19.2
w2                 620          813           39        >18.5

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.69 in the direction of the burst
(Schlegel et al. 1998).

T. Sakamoto (AGU), S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC),
P.A. Evans (U Leicester), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
A. Y. Lien (GSFC/UMBC), C. B. Markwardt (GSFC), D. M. Palmer (LANL),
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 160824A (trigger #709765)
(Evans, et al., GCN Circ. 19855
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 ).  The BAT ground-calculated position is
RA, Dec = 80.087, 40.039 deg which is
    RA(J2000)  =  05h 20m 20.8s
    Dec(J2000) = +40d 02' 21.6"
with an uncertainty of 1.2 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 55%.

The mask-weighted light curve shows two episodes of emission.  The first runs
from T-5 sec to roughly T+45 sec and contains multiple peaks.  The second runs
from T+85 sec to T+110 sec.  T90 (15-350 keV) is 99.3 +- 7.8 sec (estimated 
error including systematics).

The time-averaged spectrum from T-0.66 to T+111.58 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.64 +- 0.11.  The fluence in the 15-150 keV band is 2.6 +- 0.2 x 10^-6 erg/cm2. 
The 1-sec peak photon flux measured from T+4.88 sec in the 15-150 keV band is 
1.3 +- 0.2 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/709765/BA/

[GCN OPS NOTE(25aug16):  Per author's request, the TBDs for the time range
of the spectral average was filled in with the actual values.]

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

We observed the field of GRB 160824A (Evans   et al., GCN 19855
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 )  with K-800 
(0.8m f/2.9)   telescope  of ISON/Terskol   observatory. We obtained several 
unfiltered images starting on  August 24 (UT) 23:58:36.  We detected 
optical source in coordinates
(J2000) 05:20:21.18 +40:01:43.5 with uncertainty of 1 arcsec in both 
coordinates. This is 2.6 arcsec apart of the center of the enhanced XRT 
error circle (Goad et al., GCN 19857
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 ).  At this time we cannot say nothing 
about variability of the object.  It is not clear whether our source is the 
same as the source reported by Marshall et al. (GCN 19860
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 ).  Preliminary 
photometry of a combined image is following

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

2016-08-24 23:58:36 0.02758 CR      400    20.6   0.3  20.9

Photometry is based on nearby USNO-B1.0 stars
USNO-B1.0_id    R2
1300-0122742    18.17
1300-0122665    18.04
1300-0122653    18.66
1300-0122680    17.00
1300-0122864    17.60
1299-0128308    18.00
1300-0122784    17.56

Y. Hu, A. Sota and A.J. Castro-Tirado (IAA-CSIC Granada), on behalf of a 
larger collaboration, report:

Following the detection of GRB 160824A by Swift (Evans et al. GCNC 
19855), optical images in the I-band  were obtained at the 1.5m OSN 
telescope in Granada (Spain) starting at 01:25 (Aug 25), i.e. 2.05 h 
postburst. No optical afterglow is detected in the co-added image (13 x 
300s, mid time 02:00 UT, 2.6 h after the trigger) within the enhanced 
Swift/XRT error box (Goad et al. GCNC 19857) down to I = 20.0.

K. P. Mooley, T. D. Staley, R. P. Fender (Oxford), G. E. Anderson 
(Curtin), T. Cantwell (Manchester), D. Titterington, S. H. Carey, J. 
Hickish, Y. C. Perrott, N. Razavi-Ghods, C. Rumsey, P. Scott 
(Cambridge), K. Grainge, A. Scaife (Manchester)

The AMI Large Array robotically triggered on the Swift alert for GRB 
160824A (Evans et al., GCN 19855
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 ) as part of the 4pisky program, and 
subsequent follow up observations were obtained up to 10 days 
post-burst. Our observations at 15 GHz on 2016 Aug 25.11, Aug 26.27, Aug 
28.29, and Sep 01.29 (UT) do not reveal any radio source at the XRT 
location (Goad et al., GCN 19857
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 ), with 3sigma upper limits of 444 uJy, 
126 uJy, 117 uJy, and 117 uJy respectively.

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/.