GRB 160506A

L. M. Z. Hagen (PSU), S. D. Barthelmy (GSFC),
H. A. Krimm (CRESST/GSFC/USRA), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the
Swift Team:

At 03:29:15 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 160506A (trigger=685245).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 265.774, -46.117 which is 
   RA(J2000)  =  17h 43m 06s
   Dec(J2000) = -46d 07' 00"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows broad peak
with a total duration of about 50 sec.  The peak count rate
was ~1200 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 03:30:50.6 UT, 94.8 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
265.8484, -46.1340 which is equivalent to:
   RA(J2000)  = 17h 43m 23.60s
   Dec(J2000) = -46d 08' 02.3"
with an uncertainty of 2.0 arcseconds (radius, 90% containment). This
location is 195 arcseconds from the BAT onboard position, outside 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 (2.67 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 4.8
(+2.85/-2.47) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 2.68e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the U 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 upper limit has been 19.6 mag. The
8'x8' region for the list of sources generated on-board covers 100% of the XRT
error circle. The typical upper limit has been 18.0 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.26. 

Burst Advocate for this burst is L. M. Z. Hagen (lea.zernow.hagen AT gmail.com). 
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.)

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

Using 1815 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 160506A, 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 = 265.84841, -46.13410 which is equivalent
to:

RA (J2000): 17h 43m 23.62s
Dec (J2000): -46d 08' 02.8"

with an uncertainty of 2.2 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.

F. Knust, C. Delvaux, J. Greiner (all MPE Garching) report on behalf of
the GROND team:

We observed the field of GRB 160506A (Swift trigger 685245; Hagen et al.,
GCN #19394
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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  06:45 UT on 2016-05-06, 3.25 hrs after the GRB
trigger. They were performed through substantial cirrus at an average
seeing of 2.1" and at an average airmass of 1.1.

We do not detect any source within the Swift-XRT error circle reported by
Osborne et al. (GCN #19395
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 ) down to

g' > 20.9 mag,
r' > 22.1 mag,
i' > 21.5 mag,
z' > 21.7 mag,
J > 19.4 mag,
H > 19.1 mag, and
K > 17.2 mag.
(all in AB),

but we see two faint sources north-east and sout-west of the XRT error
circle, each about 4 arcsec away from the XRT centroid.

Given magnitudes are calibrated against USNO as well as 2MASS field stars
and are not corrected for the expected Galactic foreground extinction
corresponding to a reddening of E_(B-V)=0.26 mag in the direction of the
burst (Schlegel et al. 1998).

T. Sakamoto (AGU), S. D. Barthelmy (GSFC),  J. R. Cummings (GSFC/UMBC),
N. Gehrels (GSFC), L. M. Z. Hagen (PSU), 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 160506A (trigger #685245)
(Hagen, et al., GCN Circ. 19394
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 ).  The BAT ground-calculated position is
RA, Dec = 265.857, -46.127 deg which is
    RA(J2000)  =  17h 43m 25.6s
    Dec(J2000) = -46d 07' 36.1"
with an uncertainty of 1.6 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 100%.

The mask-weighted light curve shows a long and complex structure.  The emission 
starts at T-10 sec, and continues with multiple peaks out to about T+275 sec.
The highest count rate is at T+100 sec.  The peaks after the slew ended at 
T+94.8 sec are also seen as flares in the XRT.  T90 (15-350 keV) is 254.5 +- 
24.9 sec (estimated error including systematics).

The time-averaged spectrum from T-8.33 to T+267.03 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.09 +- 0.21.  The fluence in the 15-150 keV band is 1.5 +- 0.2 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+98.80 sec in the 15-150 keV band
is 0.4 +- 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/685245/BA/

L. M. Z. Hagen (PSU) reports on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 160506A
102 s after the BAT trigger (Hagen et al., GCN Circ. 19394
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 ). No optical
afterglow consistent with the XRT position (Osborne et al. GCN Circ.
19395
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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

v                  407         2279          233        >19.36
b                  357         2229          252        >20.37
u_FC               101          351          245        >20.13
u                  101         2353          626        >20.58
uvw1               457         2329          233        >19.75
uvm2               432         2304          233        >19.52
uvw2               383         2255          252        >19.86


The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.26 in the direction of the burst
(Schlegel et al. 1998).

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), A. Melandri (INAF-OAB), B. Sbarufatti
(INAF-OAB/PSU), D.N. Burrows (PSU) and L.M.Z. Hagen report on behalf of
the Swift-XRT team:

We have analysed 7.4 ks of XRT data for GRB 160506A (Hagen et al. GCN
Circ. 19394
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 ), from 86 s to 36.2 ks after the  BAT trigger. The data
comprise 422 s in Windowed Timing (WT) mode (the first 7 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. 19395
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 ).

The late-time light curve (from T0+5.6 ks) can be modelled with a
power-law decay with a decay index of alpha=0.54 (+0.22, -0.21).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.65 (+/-0.05). The
best-fitting absorption column is  7.1 (+/-0.5) x 10^21 cm^-2, in
excess of the Galactic value of 2.7 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 2.11 (+0.21, -0.20)
and a best-fitting absorption column of 7.9 (+1.7, -1.5) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 4.3 x 10^-11 (8.2 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     7.9 (+1.7, -1.5) x 10^21 cm^-2
Galactic foreground: 2.7 x 10^21 cm^-2
Excess significance: 5.9 sigma
Photon index:	     2.11 (+0.21, -0.20)

If the light curve continues to decay with a power-law decay index of
0.54, the count rate at T+24 hours will be 0.013 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.4 x
10^-13 (1.0 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/00685245.

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

K. L. Murata (Nagoya U.), Y.-H. Lee, B.-C. Koo (Seoul National U.), and T.
Nagayama (Kagoshima U.)

We observed the field of GRB 160506A ( Hagen et al., GCN Circular #19394
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 )
with the near-infrared (J, H, Ks) simultaneous imaging camera SIRIUS
attached to 1.4 m telescope IRSF ( InfraRed Survey Facility) in Sutherland
observatory, South Africa. We observed with a narrow band filter with
double transmissions at 1.189 um and 1.644 um.

The observations started on 2016-05-06 04:18:47 UT (~ 49 min. after the
burst). We could not detect the afterglow within the XRT error circle
reported in the GCN circular. We have obtained the following preliminary
upper limits (Vega magnitude system):
 M_1.189um > 17.3 mag
 M_1.644um > 16.6 mag

Given magnitudes of M_1.189um and M_1.644um were calibrated against J band
and H band of 2MASS point sources in this field, respectively. The upper
limits were determined as the magnitudes of the faintest star within 1
arcmin from the XRT position.

This observation was carried out by IRSF and OISTER collaboration.

Klotz A., Turpin D., Atteia J.L. (CNRS-OMP-IRAP),
Boer, M., Laugier, R. (CNRS-ARTEMIS),
Gendre B. (UVI - Etelman Obs.) report:

We imaged the field of GRB 160506A detected by SWIFT
(trigger 685245) with the TAROT robotic telescope (D=25cm)
located at the European Southern Observatory,
La Silla observatory, Chile.

The observations started 62.1s after the GRB trigger
(14.0s after the notice). The elevation of the field increased from
41 degrees above horizon but clouds were presents in the sky.

The first image is trailed with a duration of 60.0s
(see the description in Klotz et al., 2006, A&A 451, L39).
We do not detect any OT with a limiting magnitude of:
t0+62.1s to t0+122.1s : Rlim = 12.7

Then the observations were stopped due to the bad weather
and start again 39 minutes later. We co-added a series
of exposures:
t0+2366s to t0+3314s : Rlim = 16.7

Magnitudes were estimated with the nearby NOMAD1 stars
and are not corrected for galactic dust extinction.