GRB 100504A

J. Mao (INAF-OAB), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
A. de Ugarte Postigo (INAF-OAB), J. M. Gelbord (PSU),
S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), F. E. Marshall (NASA/GSFC),
P. T. O'Brien (U Leicester), D. M. Palmer (LANL),
P. Romano (INAF-IASFPA), B. Sbarufatti (INAF-OAB/IASFPA),
M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC),
G. Tagliaferri (INAF-OAB) and E. Troja (NASA/GSFC/ORAU) report on
behalf of the Swift Team:

At 19:20:59 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 100504A (trigger=421124).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 255.572, -35.599 which is 
   RA(J2000) = 17h 02m 17s
   Dec(J2000) = -35d 35' 55"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a double-peaked
structure with a duration of about 30 sec.  The peak count rate
was ~2500 counts/sec (15-350 keV), at ~0 and ~10 sec after the trigger. 

The XRT began observing the field at 19:22:02.1 UT, 62.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright, fading,
uncatalogued X-ray source located at RA, Dec 255.57053, -35.58712 which
is equivalent to:
   RA(J2000)  = 17h 02m 16.93s
   Dec(J2000) = -35d 35' 13.6"
with an uncertainty of 4.1 arcseconds (radius, 90% containment). This
location is 42 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
3.61e+21 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 5.58e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 71 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. Because of the density of catalogued stars, further
analysis is required to report an upper limit for any afterglow in the
sub-image. The coverage of the XRT error circle by the 8'x8' region for the
list of sources generated on-board is uncertain because the large number of
sources filled the available telemetry. Because of the density of catalogued
stars, further analysis is required to report an upper limit for any afterglow
in the region. No correction has been made for the large, but uncertain
extinction expected. 

Burst Advocate for this burst is J. Mao (jirong.mao AT brera.inaf.it). 
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. Mao (INAF-OAB) reports on behalf of the Swift-XRT team:

The Swift-XRT observation of GRB 100504A started on 2010 May 4 at 
19:22:02.1 UT,
62.6 s after the BAT trigger. We detected a bright, single X-ray source. 
The UVOT-enhanced position is:

RA (J2000):  17h 02m 17.22s
Dec (J2000): -35d 35' 14.4''

with an uncertainty of 2.2 arcsec (radius, 90% confidence).

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

Adria C. Updike and Dieter H. Hartmann (Clemson University) report:

We observed the field of GRB 100504A (Mao et al., GCN 10712
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 ) using the
0.6m SARA-South telescope located at CTIO under good conditions and high
airmass beginning 6 hours and 42 min after the trigger.  We do not detect
a new source at the location of the UVOT-enhanced XRT position (Mao, GCN
10713
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 ) to a limiting magnitude of R = 20 mag (on comparison to the USNO
B1.0 catalog).  We note that the extinction at this galactic latitude is
significant; E(B-V) = 0.840 mag (Schlegel et al. 1998).

D. M. Palmer (LANL), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC),
J. R. Cummings (GSFC/UMBC), N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA),
J. Mao (INAF-OAB), C. B. Markwardt (GSFC/UMD), T. Sakamoto (GSFC/UMBC),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), T. N. Ukwatta (GWU)
(i.e. the Swift-BAT team):
 
Using the data set from T-239 to T+363 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 100504A (trigger #421124)
(Mao, et al., GCN Circ. 10712
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 ).  The BAT ground-calculated position is
RA, Dec = 255.575, -35.590 deg, which is 
   RA(J2000)  =  17h 02m 18.1s 
   Dec(J2000) = -35d 35' 23.0" 
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 92%.
 
The mask-weighted light curve shows two overlapping peaks followed by a
weak third peak, starting at ~T-10 sec, peaking at T+0, T+8, and ~T+90 sec,
and ending at ~T+100 sec.  T90 (15-350 keV) is 97.3 +- 7.3 sec
(estimated error including systematics).
 
The time-averaged spectrum from T-9.7 to T+102.2 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
1.76 +- 0.10.  The fluence in the 15-150 keV band is 2.4 +- 0.1 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+7.60 sec in the 15-150 keV band
is 1.7 +- 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/421124/BA/

P. Schady, F. Olivares, R. Filgas and J. Greiner (MPE Garching) report
on behalf of the GROND team:

We observed the field of GRB 100504A (Swift trigger 421124; Mao et
al., GCN #10712
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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 MPI/ESO telescope at La
Silla Observatory (Chile).

Observations started at 01:30 UT on May 5 (about 6.2 hours after the
GRB trigger). They were performed at a high airmass.

We do not detect a source within the Swift UVOT-enhanced XRT error
circle (Mao et al. GCN #10713
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 ) in any of the GROND filters. Coadded
images of 24 min of integration in g'r'i'z' and 20 min in JHK yield
the following 3-sigma upper limits:

g' > 23.2
r' > 22.2
i' > 21.6
z' > 21.2
J > 19.4
H > 19.1
K > 18.9

We note the presence of two uncatalogued sources 3" North-West of the
enhanced XRT position with magnitudes r' = 19.8 and J = 18.0 for the
more southern of the two, and r' = 20.2 and J = 18.6 for the second
source. Observations are continuing.

All magnitudes given above are in the AB system and are based on the
GROND zeropoints and 2MASS field stars. No correction has been applied
for the large, uncertain Galactic foreground extinction corresponding
to a reddening of E_(B-V)~ 0.8 in the direction of the burst (Schlegel
et al. 1998).

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

Using 1124 s of XRT Photon Counting mode data and 3 UVOT
images for GRB 100504A, 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 = 255.57137, -35.58742 which is equivalent
to:

RA (J2000): 17h 02m 17.13s
Dec (J2000): -35d 35' 14.7"

with an uncertainty of 2.4 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.

J. Mao (INAF-OAB) reports on behalf of the Swift-XRT team:

We have analysed 8.7 ks of Swift XRT data for GRB 100504A (Mao et al. 
GCN Circ. 10712
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 ). The data comprise 120 s in Windowed Timing (WT) mode 
with the remainder in Photon Counting (PC) mode.

The light curve shows evidence of flaring in WT mode, peaking at about 
83 s, after that, the light curve can be modelled with a broken 
power-law in PC mode, with an initial decay index of 
alpha1=3.04(+/-0.57) and breaking to a decay of index 
alpha2=0.57(+/-0.06), the break time is about 450 s.

A spectrum formed from the WT mode data can be fitted with an absorbed 
power-law with a photon spectral index of 1.95 (+/-0.09). The 
best-fitting absorption column is 9.07 (+/-0.71) x 10^21 cm^-2, in 
excess of the Galactic value of 3.77 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 5.96 x 10^-11 (9.31 x 10^-11) erg 
cm^-2 count^-1.

The spectrum formed from the PC mode data can be fitted with an absorbed 
power-law with a photon spectral index of 2.40 (+/-0.37) and the 
absorption column density is consistent with the Galactic one in the GRB 
direction. The counts to observed (unabsorbed) 0.3-10 keV flux 
conversion factor deduced from this spectrum is 5.56 x 10^-11 (6.72 x 
10^-11) erg cm^-2 count^-1.

If the light curve continues to decay with a power-law decay index of 
0.57, the count rate at T+24 hours will be 6.9 x 10^-3 count s^-1, 
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.8 x 
10^-13 (4.6 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/00421124.

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

M. H. Siegel (PSU) and J. Mao (INAF-OAB) report on behalf
of the Swift/UVOT team.

The Swift/UVOT began settled observations of the field of
GRB 100504A 72s after the BAT trigger (Mao et al.,
GCN Circ. 10712
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 ).  Data summed from the first and second orbits
do not reveal a source at the enhanced position of the X-ray
afterglow (Mao et al., GCN Circ. 10713
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 ).

The 3-sigma upper limits for the finding chart (fc) and
summed exposures are reported below:

FILTER     T_start(s)  T_stop    Exposure     Mag/3UL
========================================================
white fc      72        222        147        >20.39
white         73       1705        391        >20.91
v            614       1755        136        >18.54
b            713       1680         97        >19.35
u           1463       1816         45        >18.43
uvw1         836       1804         97        >18.92
uvw2         762       1037         38        >18.43
=======================================================

The above magnitudes are not corrected for the Galactic extinction
corresponding to a reddening of E_{B-V} = 0.87 (Schlegel et al.,
1998, ApJS, 500, 525). The photometry is on the UVOT photometric
system described in Poole et al. (2008, MNRAS, 383, 627).

V.Chaplin (UAH) reports on behalf of the Fermi GBM Team:

"At 19:20:55.53 UT on 04 May 2010, the Fermi Gamma-ray Burst Monitor  
(GBM)
triggered on GRB 100504A (trigger 294693657 / 100504.806), which
was also seen by Swift,BAT (J.Mao et al., GCN 10712
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 ).  The calculated
GBM ground position is consistent with the Swift,BAT position.
The angle from the Fermi LAT boresight is 104 degrees.

The GBM light curve shows a single pulse with a duration (T90)
of about 16 s (50-300 keV). The time-averaged spectrum from T0-6.1 to  
T0+23.6 s
is best fit by a power-law with an exponential high-energy cutoff.
The power-law index is -0.82 (+0.17/-0.16), and the cutoff energy,
parameterized as Epeak, is 85.7 (8.0/-6.6) keV (C-stat 960.7 for 481  
DOF).
The fluence (10-1000 keV) in this time interval is (2.7 +/- 0.12)E-06  
erg/cm^2.

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

D. Kuroda (OAO, NAOJ),  H. Hanayama, T. Miyaji, J. Watanabe(IAO, NAOJ),
K. Yanagisawa (OAO, 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 100504A (Mao et al. GCNC 10712)
with the optical three color (g', Rc and Ic) CCD camera attached
to the Murikabushi 1m telescope of Ishigakijima Astronomical
Observatory.

The observation started on 2010-05-04 19:24:11 UT (~3.2 min after the
burst). We did not find any new point source within the XRT error
circle (Mao GCNC10713; Osborne et al. GCNC 10718) in all the three bands.

Three sigma upper limits of the OT are listed below. We used
GSC2.3 catalog for flux calibration.

T0+[day]   MID-UT   T-EXP[sec]    g'     Rc     Ic
------------------------------------------------------
0.00625    19:28:41   480.0      >20.1  >20.1  >18.3
------------------------------------------------------
T0+ : Elapsed time after the burst [day]
T-EXP: Total Exposure time [sec]