GRB 160504A

D. Malesani (DARK/NBI), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC),
D. M. Palmer (LANL), T. G. R. Roegiers (PSU),
B. Sbarufatti (INAF-OAB/PSU) and T. N. Ukwatta (LANL) report on behalf
of the Swift Team:

At 19:30:37 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 160504A (trigger=685124).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 170.043, +55.997 which is 
   RA(J2000) = 11h 20m 10s
   Dec(J2000) = +55d 59' 48"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 10 sec.  The peak count rate
was ~1300 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 19:32:46.0 UT, 128.7 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 170.07309, 56.00016 which is equivalent
to:
   RA(J2000)  = 11h 20m 17.54s
   Dec(J2000) = +56d 00' 00.6"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 61 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 8.12
x 10^19 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 98 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 0.01. 

Burst Advocate for this burst is D. Malesani (malesani AT dark-cosmology.dk). 
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.)

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 160504A detected by SWIFT
(trigger 685124) with the TAROT robotic telescope (D=25cm)
located at the Calern observatory, France.

The observations started 57.1s after the GRB trigger
(16.5s after the notice). The elevation of the field increased from
77 degrees above horizon and weather conditions
were good but during twilight.

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+57.1s to t0+117.1s : R > 15.3

The second image is 30.0s exposure in tracking mode:
t0+132.5s to t0+162.5s : R > 15.7

We co-added a series of exposures:
t0+132.5s to t0+426.0s : R > 17.2

Magnitudes were estimated with the nearby USNO-B1 stars
and are not corrected for galactic dust extinction.

T. Kruehler (MPE Garching), A. de Ugarte Postigo (IAA-CSIC and
DARK/NBI), and T. Kuutma (NOT) report on behalf of a larger
collaboration:

We observed the field of GRB 160504A (Malesani et al., GCN #19382
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 )
with the NOT equipped with ALFOSC. Imaging in the r, i and z-band
filter started at 20:54:10 UT on 2016-05-04, 1.4 hr after the GRB
trigger.

We detect no sources in the enhanced XRT error circle given at
http://www.swift.ac.uk/sper/685124/.

Calibrating our images against magnitudes from field stars of the
SDSS catalog, we measure preliminary upper limits of

r > 23.3 mag
i > 22.8 mag
z > 22.0 mag

These limits are not corrected for the expected foreground
extinction corresponding to a reddening of E(B-V) = 0.01 mag
(Schlafly & Finkbeiner 2011).

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

Using 559 s of XRT Photon Counting mode data and 1 UVOT
images for GRB 160504A, 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 = 170.07224, +56.00158 which is equivalent
to:

RA (J2000): 11h 20m 17.34s
Dec (J2000): +56d 00' 05.7"

with an uncertainty of 1.9 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.M. McCauley (PSU), J.A. Kennea (PSU), A.P. Beardmore (U. Leicester),
P.A. Evans (U. Leicester), S.L. Gibson (U. Leicester), V. D'Elia
(ASDC), A. D'Ai (INAF-IASFPA), A. Maselli  (INAF-IASFPA), B. Sbarufatti
(INAF-OAB/PSU) and D. Malesani report on behalf of the Swift-XRT team:

We have analysed 7.8 ks of XRT data for GRB 160504A (Malesani et al.
GCN Circ. 19382
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 ), from 80 s to 18.1 ks after the  BAT trigger. The data
comprise 31 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 Goad et al.
(GCN Circ. 19386
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 ).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=2.4 (+1.2, -3.4). At T+100 s  the decay
steepens to an alpha of 5.5 (+0.9, -0.7). The light curve breaks again
at T+209 s to a decay with alpha=0.15 (+0.08, -0.65),  before a final
break at T+4742 s s after which the decay index is 0.68 (+0.15, -0.14).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.97 (+/-0.15). The
best-fitting absorption column is  1.1 (+/-0.4) x 10^21 cm^-2, in
excess of the Galactic value of 8.1 x 10^19 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.4 x 10^-11 (4.2 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.1 (+/-0.4) x 10^21 cm^-2
Galactic foreground: 8.1 x 10^19 cm^-2
Excess significance: 4.4 sigma
Photon index:	     1.97 (+/-0.15)

If the light curve continues to decay with a power-law decay index of
0.68, the count rate at T+24 hours will be 0.022 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.5 x
10^-13 (9.3 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/00685124.

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 160504A detected by SWIFT(trigger 685124)
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 22.45 min after the GRB trigger,  
with our Schmidt telescope D=320/400 mm F/D=3.1.

Weather conditions were good.

We co-added 2 series of 20 exposures of 120 sec each.

Start T0+      End T0+      Vlim
22.45 min    111.33 min     20

We did not found any optical counterpart in the error 
box of the XRTcandidate.
(D. Malesani (DARK/NBI), J. A. Kennea (PSU)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.

S. B. Cenko (NASA/GSFC), A. J. Levan (U. Warwick), N. R. Tanvir (U. Leicester), A. Cucchiara (STScI), and A. Fruchter (STScI) report on behalf of a larger collaboration:

We imaged the location of GRB160504A (Malesani et al., GCN 19382
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 ) with the Near Infrared Imager and Spectrometer (NIRI) on the 8 m Gemini North telescope beginning at 5:24 UT on 5 May 2016 (~ 9.9 hr after the Swift trigger).  A total of 10 min of integration was obtained in each of the J, H, and K filters.  Within the enhanced XRT localization (Goad et al., GCN 19386
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 ) we identify a single faint source at J2000.0 coordinates:

  RA: 11:20:17.18
  Dec: +56:00:04.7

Using a nearby point source from 2MASS for calibration, we calculate the following AB magnitudes for this object:

J = 23.6 +/- 0.4
H = 23.8 +/- 0.4
K = 23.3 +/- 0.2

Given the faintness of the object, we cannot determine at this time if it associated with GRB160504A.

M. H. Siegel (PSU) and D. Malesani (DARK/NBI)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 160504A
99 s after the BAT trigger (Malesani et al., GCN Circ. 19382
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 ).
No optical afterglow consistent with the XRT position
(Goad et al. GCN Circ. 19386
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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)             99          249          147         >20.8
white               99        11822         1316         >21.7
v                  587        16542         1317         >20.2
b                  513        10910         1145         >21.5
u (fc)                257          507          246         >20.4
u                  257         6616          678         >21.2
w1                 636         6411          432         >20.4
m2                4571        17443         1275         >21.0
w2                 735        12387          963         >21.7

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.01 in the direction of the burst
(Schlegel et al. 1998).

Nat Butler (ASU), Alan M. Watson (UNAM), Alexander Kutyrev (GSFC), William
H. Lee (UNAM), Michael G. Richer (UNAM), Ori Fox (STScI), J. Xavier
Prochaska (UCSC), Josh Bloom (UCB), Antonino Cucchiara (GSFC/STScI),
Eleonora Troja (GSFC), Owen Littlejohns (ASU), Enrico Ramirez-Ruiz (UCSC),
Jos�� A. de Diego (UNAM), Leonid Georgiev (UNAM), Jes��s Gonz��lez (UNAM),
Carlos Rom��n-Z����iga (UNAM), Neil Gehrels (GSFC), Harvey Moseley (GSFC),
John Capone (UMD), V. Zach Golkhou (ASU), and Vicki Toy (UMD) report:

We observed the field of GRB 160504A (Malesani, et al., GCN 19382
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 ) with the
Reionization and Transients Infrared Camera (RATIR; www.ratir.org) on the
1.5m Harold Johnson Telescope at the Observatorio Astron��mico Nacional on
Sierra San Pedro M��rtir from 2016/05 5.21 to 2016/05 5.36 UTC (19.64 to
23.11 hours after the BAT trigger), obtaining a total of 2.49 hours
exposure in the r and i bands and 1.04 hours exposure in the Z, Y, J, and H
bands.

For a source within the Swift-XRT error circle (Goad, et al., GCN 19386
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 ),
in comparison with the SDSS DR9 and 2MASS catalogs, we obtain the following
detection and upper limits (3-sigma):

 r > 23.70
 i = 23.62 +/- 0.36
 Z > 22.38
 Y > 22.08
 J > 21.73
 H > 21.35

These magnitudes, which appear to be consistent with those reported by
Cenko et al., (GCN 19389
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 ), are in the AB system and are not corrected for
Galactic extinction in the direction of the GRB.

We thank the staff of the Observatorio Astron��mico Nacional in San Pedro
M��rtir.

D. M. Palmer (LANL), S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC),
N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), A. Y. Lien (GSFC/UMBC),
D. Malesani (DARK/NBI), C. B. Markwardt (GSFC), T. Sakamoto (AGU),
M. Stamatikos (OSU), T. N. Ukwatta (LANL) (i.e. the Swift-BAT team):

Using the data set from T-240 to T+963 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 160504A (trigger #685124)
(Malesani, et al., GCN Circ. 19382
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 ).  The BAT ground-calculated position is
RA, Dec = 170.073, 56.002 deg which is
RA(J2000) = 11h 20m 17.5s
Dec(J2000) = +56d 00' 05.7"
with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 88%.

The mask-weighted light curve shows that the main emission starts at T-40 sec, 
has a very broad profile, peaks near T+0 sec and ends by T+30 sec. T90 (15-350 
keV) is 53.9 +- 6.0 sec (estimated error including systematics). We have 
concluded that a soft spike in the light curve of ~10 msec duration at T-135.5 
sec is of instrumental and not astrophysical origin.  Our analysis therefore 
excludes the time before T-135 sec.

The time-averaged spectrum from T-42.3 to T+16.2 sec is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
1.88 +- 0.20. The fluence in the 15-150 keV band is 7.3 +- 0.9 x 10^-7erg/cm2.
The 1-sec peak photon flux measured from T+0.51 sec in the 15-150 keV band
is 0.7 +- 0.1 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/685124/BA/

K. P. Mooley, T. D. Staley, R. P. Fender (Oxford), G. E. Anderson 
(Curtin), T. Cantwell (Manchester), C. Rumsey, D. Titterington, S. 
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 GRB 
160504A (Malesani et al., GCN 19382
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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 May 04.85, May 05.88, May 
07.82, May 11.90, and May 14.90 (UT) do not reveal any radio source at 
the XRT location (Goad et al., GCN 19386
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 ), with 3sigma upper limits of 
93 uJy, 72 uJy, 129 uJy, 123 uJy, and 120 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/.