GRB 130504C

P. D'Avanzo (INAF-OAB), A.Y. Lien (NASA-GSFC), P.A. Evans (U. Leicester)
report on behalf of the Swift-XRT team:

We have analysed 12.3 ks of XRT data for the Fermi-LAT-detected burst GRB
130504C (Kocevski et al. GCn Circ 14574
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 ),  from 65.6 ks to 278.7 ks after
the  Fermi-LAT trigger. The data are entirely in Photon Counting (PC)
mode.  The X-ray source reported by D'Avanzo et al. (GCN Circ. 14584
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 ,
14588
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 ) displays a fading behaviour.

The overall light curve can be modelled with a power-law decay with a
decay index of alpha=1.39 (+0.32, -0.25).

We confirm it as the X-ray afterglow of GRB 130504C.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00020267.

T. Yasuda, M. Tashiro, Y. Terada, Y. Ishida, H. Ueno, S. Sugimoto
(Saitama U.), M. Ohno, K. Takaki, T. Kawano, R. Nakamura, S. Furui,
Y. Fukazawa (Hiroshima U.), M. Yamauchi, N. Ohmori, M. Akiyama
(Univ. of Miyazaki), K. Yamaoka (Nagoya U.),  S. Sugita (Ehime U.),
Y. E. Nakagawa, M. Kokubun, T. Takahashi (ISAS/JAXA), W. Iwakiri
(RIKEN), Y. Hanabata (ICRR), Y. Urata (NCU), K. Nakazawa,
K. Makishima (Univ. of Tokyo) on behalf of the Suzaku WAM team, report:


The bright, long GRB 130504C (GCN 14574
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 ; Fermi-LAT detection : Kocevski
et al.) triggered the Suzaku Wide-band  All-sky Monitor (WAM) which
covers an energy range of 50 keV - 5 MeV at 23:29:00.540 UT (=T0).

The observed light curve shows a multi-peaked structure starting at T0-9
s, ending at T0+110 s with a duration (T90) of about 67 seconds.
The fluence in 100 - 1000 keV was 9.11 (+0.14/-0.51) x10^-5 erg/cm^2.
The 1-s peak flux measured from T0+ 27 s was
15.9 (+0.3/-4.0) photons/cm^2/s in the same energy range.

Preliminary result shows that the time-averaged spectrum from
T0-9 s to T0+110 s is well fitted by a GRB Band model as follows.
the low-energy photon index alpha:  -1.59 (+0.17/-0.12),
the high-energy photon index beta: -2.43 (+0.17/-0.33),
and the peak energy Epeak: 845 (+227/-159) keV (chi^2/d.o.f = 77.0/50).

Due to the brightness of this burst, a 1% systematic error was added for
low energy channels.
All the quoted errors are at statistical 90% confidence level.

The light curves for this burst are available at:
http://www.astro.isas.jaxa.jp/suzaku/HXD-WAM/WAM-GRB/grb/trig/grb_table.html

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

We have analysed 7.7 ks of XRT data for the Fermi-LAT-detected burst: GRB
130504C,  from 65.6 ks to 93.3 ks after the  Fermi-LAT trigger. The data
are entirely in Photon Counting (PC) mode. The  X-ray source reported by
D'Avanzo et al. (GCN Circ. 14584
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 ) displays behaviour consistent with
fading. We propose it as the X-ray afterglow of GRB 130504C.

The refined XRT position is RA, Dec = 91.63038, +3.8339 which is
equivalent to:

RA (J2000): 06 06 31.29
Dec(J2000): +03 50 02.0

with an uncertainty of 3.6 arcsec (radius, 90% confidence).

The light curve can be modelled with  a power-law decay with a decay index
of alpha=1.3 (+/-1.0).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.8 (+/-0.3). The best-fitting
absorption column is  5.1 (+2.0, -1.6) x 10^21 cm^-2, in excess of the
Galactic value of 2.5 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 5.0 x 10^-11 (7.9 x 10^-11) erg cm^-2 count^-1.

A summary of the PC-mode spectrum is thus:
Total column:             5.1 (+2.0, -1.6) x 10^21 cm^-2
Galactic foreground: 2.5 x 10^21 cm^-2
Excess significance: 2.7 sigma
Photon index:             1.8 (+/-0.3)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00020267.

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

[GCN OPS NOTE(06may13): Per author's request, the extra "further" was removed
from the ubject-line.]

S. Golenetskii, R.Aptekar, D. Frederiks, E. Mazets, V. Pal'shin,
P. Oleynik, M. Ulanov, D. Svinkin, and T. Cline on behalf
of the Konus-Wind team, report:

The long-duration intense GRB 130504C
(Fermi-LAT detection: Kocevski et al., GCN 14574
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 ;
Fermi-GBM detection: Burgess et al., GCN 14583
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 )
triggered Konus-Wind at T0=84544.491s UT (23:29:04.491)

The light curve shows multiple partly overlapped peaks
from ~T0-15 s to ~T0+105s.
The emission is seen up to 12 MeV.

A possible hard precursor is seen in the 360-1400 keV
light curve at ~T0-50s, but its attribution to GRB 130504C
is yet unclear.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB130504_T84544/

As observed by Konus-Wind, the burst
had a fluence of (2.0 � 0.1)x10-4 erg/cm2,
and a 64-ms peak flux, measured from T0+21.440s,
of (2.6 � 0.2)x10-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum (measured from T0 to T0+105.216 s)
is best fit in the 20 keV - 15 MeV range
by the GRB (Band) function with the following model parameters:
the low-energy photon index alpha = -1.32 � 0.04,
the high energy photon index beta = -2.15 � 0.1,
the peak energy Ep = 452 � 49 keV,
chi2 = 110/97 dof.

The spectrum at the maximum count rate
(measured from T0+21.248 to T0+23.296 s)
is best fit in the 20 keV - 15 MeV range
by the GRB (Band) function with the following model parameters:
the low-energy photon index alpha = -0.74 � 0.12,
the high energy photon index beta = -1.93 � 0.06,
the peak energy Ep = 251 � 38 keV,
chi2 = 61/66 dof.

All the quoted results are preliminary.

P. D'Avanzo, A. Melandri (INAF-OAB) report on behalf of  the Swift team:

We have analysed 1.3 ks of Swift-XRT data for the Fermi 130504C (Kocevski
GCN Circ. 14574
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 ; Burgess et al. GCN Circ. 14583
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 ), from 65.6 ks to 70.1 ks
after the LAT trigger. The data are all in Photon Counting (PC) mode.

We detect an uncatalogued X-ray source inside the LAT error circle
(Kocevski GCN Circ. 14574
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 ), at the following position: RA, Dec=91.63047,
3.83388 which is equivalent to:

RA (J2000): 06 06 31.3
Dec (J2000): +03 50 01.96

with an uncertainty of 4.8 arcsec (radius, 90% confidence).
The count rate at this position is (4.4 +/- 0.7) x 10^-2 cts/s. At the
present stage, it is not possible to determine whether the source is
fading.

J. Michael Burgess (UAH), Valerie Connaughton (UAH) and Shaolin Xiong (UAH)
report on behalf of the Fermi GBM Team:

"At 23:28:57.518 UT on 04 May 2013, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 130504C (trigger 389402940 / 130504978).

High peak flux from the GRB caused GBM to issue a repoint request
that reoriented the satellite to place the GRB near the LAT boresight
for 2.5 hours, subject to Earth limb contraints.

The on-ground calculated location, using the GBM trigger
data, is RA = 90.71, DEC = 4.45 (J2000 degrees,
equivalent to 06 h 02 m, 4 d 27 '), with an uncertainty
of 1.0 degrees (radius, 1-sigma containment,
statistical only; there is additionally a systematic
error which is currently estimated to be 2 to 3 degrees).
This location is consistent with the LAT location.
The angle from the Fermi LAT boresight is 47 degrees.

This burst was also independently detected by INTEGRAL SPI-ACS.

The GBM light curve consists of about 5 peaks associated with the GRB and 1 peak 
associated with a solar flare about 100 seconds prior to T0.
The duration (T90) of the GRB is about 74 s (50-300 keV).
The time-averaged spectrum from T0-2.0 s to T0+120.0 s is
adequately fit by a Band function with 
Epeak = 637 +/- 34 keV, alpha = -1.23 +/- 0.01, 
and beta = -2.28 +/- 0.08

The event fluence (10-1000 keV) in this time interval is
(1.34 +/- 0.01)E-04 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+30.8 s in the 10-1000 keV band
is 43 +/- 0.5 ph/s/cm^2.


The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."

D. Kocevski (NASA/GSFC), M. Ohno (Hiroshima U.), J. Racusin (NASA/GSFC), G. Vianello (Stanford), V. Vasileiou (LUPM), and E. Troja (CRESST) report on behalf of the Fermi-LAT team:

At 23:30:15 UT on 04 May 2013, Fermi LAT detected high energy emission from GRB 130504C, which was also detected by Fermi-GBM (trigger 389402940 / 130504979).  The GBM detection triggered an autonomous repoint of the spacecraft.

The best LAT on-ground location is found to be RA=91.715, DEC=3.846 (J2000) with an error radius of 0.155 deg (68% containment, statistical error only). 

The burst was about 40 deg from the LAT boresight at the time of the trigger and the spacecraft slew brought the source within the LAT field of view for the next 2200 seconds. The data from the Fermi LAT show long lasting emission with >70 photons above 100 MeV observed out to 1000s seconds with a TS of >70.  Multi-peaked emission lasting roughly 40 seconds can be seen using the non-standard LAT Low Energy (LLE) with a significance of ~26 sigma. The highest energy LAT photon has an energy of ~5 GeV arriving 251 seconds after the trigger.

A Swift TOO request has been submitted.

A GBM circular is forthcoming.

The Fermi LAT point of contact for this burst is Daniel Kocevski (daniel.kocevski@nasa.gov).

The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.





Daniel Kocevski
NASA Goddard Space Flight Center
www.kocevski.com
510.316.3208