GRB 130504A

P. D'Avanzo (INAF-OAB), A. Melandri (INAF-OAB), K.L. Page (U.
Leicester), P.A. Evans (U. Leicester)  report on behalf of the
Swift-XRT team:

We have analysed 654 s of XRT data for GRB 130504A,  from 99 s to 754 s
after the  BAT trigger. The data are entirely in Photon Counting (PC)
mode.

No X-ray afterglow is detected within the BAT error circle (Cummings et
al. GCN 14551
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 ). We note however the presence of a bright, uncatalogued,
fading X-ray source at the following refined position RA, Dec =
272.45484, -16.31423 which is equivalent to:

RA (J2000): 18 09 49.16
Dec(J2000): -16 18 51.2

with an uncertainty of 3.5 arcsec (radius, 90% confidence). This
position is about 5.5 arc min away from the BAT position. Given its
fading nature, we propose it as the X-ray afterglow of GRB 130504A.

The light curve can be modelled with  a power-law decay with a decay
index of alpha=0.79 (+/-0.19).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.7 (+/-0.4). The
best-fitting absorption column is  1.6 (+0.5, -0.4) x 10^22 cm^-2, in
excess of the Galactic value of 8.1 x 10^21 cm^-2 (Kalberla et al.
2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 7.4 x 10^-11 (1.3 x 10^-10) erg
cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column:		  1.6 (+0.5, -0.4) x 10^22 cm^-2
Galactic foreground:      8.1 x 10^21 cm^-2
Excess significance:      2.8 sigma
Photon index:		  1.7 (+/-0.4)

If the light curve continues to decay with a power-law decay index of
0.79, the count rate at T+24 hours will be 0.021 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.5 x
10^-12 (2.8 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/00555096.

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

D. M. Palmer (LANL), S. D. Barthelmy (GSFC),  W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), E. E. Fenimore (LANL), N. Gehrels (GSFC),
H. A. Krimm (GSFC/USRA), A. Y. Lien (NASA/GSFC/ORAU), C. B. Markwardt (GSFC),
T. Sakamoto (AGU), G. Sato (ISAS), M. Stamatikos (OSU), J. Tueller (GSFC),
T. N. Ukwatta (MSU)
  
Using the data set from T-61 to T+242 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 130504A (trigger #555096)
(Cummings, et al., GCN Circ. 14551
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 ).  The BAT ground-calculated position is
RA, Dec = 272.459, -16.320 deg which is
    RA(J2000)  =  18h 09m 50.1s
    Dec(J2000) = -16d 19' 13.1"
with an uncertainty of 1.7 arcmin, (radius, sys+stat, 90% containment).  The
partial coding was 100%.

This position is 5.3 arcmin away from the automated flight position reported
in GCN circular# 14551.  The imaging interval selected onboard was not optimum.
The refined error circle includes the position of the XRT afterglow candidate
reported by D'Avanzo et al. in GCN circular# 14553, hence we believe that
fading x-ray source to be the correctly identified afterglow of GRB 130504A.

The mask-weighted light curve shows multiple weak peaks.  T90 (15-350 keV) is
50 +- 10 sec (estimated error including systematics).
  
The time-averaged spectrum from T-31.07 to T+36.84 sec is best fit by a power
law with an exponential cutoff.  This fit gives a photon index 0.52 +- 0.68,
and Epeak of 84.5 +- 47.6 keV (chi squared 82.16 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 1.0 +- 0.1 x 10^-06 erg/cm2
and the 1-sec peak flux measured from T-0.06 sec in the 15-150 keV band is
0.9 +- 0.2 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.41 +- 0.15 (chi squared 88.43 for 57 d.o.f.).  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/555096/BA/

A. A. Breeveld (MSSL-UCL) and J. R. Cummings (GSFC/UMBC) report on 
behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 130504A 
89s after the BAT trigger (Cummings et al., GCN Circ. 14551
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 ), but the 
GRB position is outside the field of view for several exposures. No 
optical afterglow consistent with the XRT position (D'Avanzo et al. GCN 
Circ. 14553
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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 u filter 
finding chart (FC) exposure and subsequent exposures are:

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

u_FC               301          551          246         >19.8
white              858         1008          147         >20.5
w1                 680         6058          333         >20.2
m2                 655         7093          356         >20.4
w2                 606         6878          549         >20.8

The magnitudes in the table are not corrected for the Galactic 
extinction due to the reddening of E(B-V) = 3.22 in the direction of the 
burst (Schlegel et al. 1998).

P. D'Avanzo (INAF-OAB), P.A. Evans and M.R. Goad (U. Leicester) report on
behalf of the Swift-XRT team:

Using 3463 s of XRT Photon Counting mode data and 5 UVOT images, 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 =
272.45604, -16.31339 which is equivalent to:

RA (J2000):  18 09 49.45
Dec (J2000): -16 18 48.2

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 is an official product of the Swift-XRT team.

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

We imaged the field of GRB 130504A detected 
by SWIFT(trigger 555096) with the robotic 
telescopes of (IAU station 565) Bassano Bresciano 
Observatory, Italy.

The observations started automatically 2 min 42 sec after the 
GRB trigger, with our schmidt telescope D=400 mm F/D=3 
and our Newton telescope D=250mm F/D=5.5.

Weather conditions were good.

We co-added 2 series of 10 unfiltered CCD exposures of 120s each.

We did not found any optical counterpart in the error box of 
the XRTcandidate (Cummings et al. GCN 14551
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 ) and in error box of
X-ray afterglow candidate (P. D'Avanzo (INAF-OAB), A. Melandri (INAF-OAB)
et al. GCN 14553
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 ) at our following limiting magnitude:
.

Start      End       Vlim
2.7min     52min     18.0


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

The message may be cited.

D. Kuroda, K. Yanagisawa, Y. Shimizu, H. Toda (OAO, NAOJ),
S. Nagayama (NAOJ), M. Yoshida (Hiroshima), K. Ohta (Kyoto)
and N. Kawai(Tokyo Tech)
report on behalf of the MITSuME collaboration:

We observed the field of GRB 130504A (Cummings et al., GCNC 14551)
with the optical three color (g', Rc and Ic) CCD camera attached
to the MITSuME 50cm telescope of Okayama Astrophysical Observatory.

The observation started on 2013-05-04 15:09:24 UT (~13.1 h after the burst).
We did not find any new point source within the enhanced XRT circle
(D'Avanzo et al., GCNC 14558) in all the three bands.

Photometric results of the OT are listed below.
We used GSC2.3 catalog for flux calibration.

#T0+[day]  MID-UT    T-EXP[sec]   g'     Rc     Ic
-----------------------------------------------------
0.58630    16:09:50    6360.0   >19.9  >19.7  >18.6
-----------------------------------------------------
T0+ : Elapsed time after the burst [day]
T-EXP: Total Exposure time [sec]

J. Gorosabel (IAA-CSIC/UPV-EHU), A. de Ugarte Postigo (IAA-CSIC/DARK-NBI),
J.C. Tello (IAA-CSIC),  A.J. Castro-Tirado (IAA-CSIC), J. Cepa (IAC), D.
Jimenez-Mejias (IAC),  J.L. Doreste Caballero (HGT), D. Hernandez Ojados
(SECAT), R. Sanchez-Ramirez (IAA-CSIC),  report on behalf of a larger
collaboration:

We observed the GRB 130504A field (BAT trigger 555096; Cummings et al. GCN
14551
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 ) with the 0.82m IAC80 telescope. The data were acquired in the
I-band. The observations started at 02:22:30.6 UT (~17 min post burst) and
lasted ~1.5 hours.

An object, coincident with the enhanced XRT position (D'Avanzo et al.
14558), shows a clear fading in our images. The optical candidate is
placed at RA(J2000)=18:09:49.44, DEC(J2000)=-16:18:48.0 with a Vega
magnitude of I=18.1 (against USNO B1.0) in our first images.