GRB 050713

A. Falcone (PSU), D. Palmer (LANL), A. Blustin (MSSL), S. Barthelmy
(GSFC), J. Cummings (GSFC/NRC), H. Krimm (GSFC/USRA), D. Burrows, D.
Morris, C. Gronwall (PSU), C. Markwardt (GSFC/UMD), K. Page
(Leicester), N. Gehrels (GSFC)

At 04:29:02.39 UT, the Swift Burst Alert Telescope (BAT) triggered and
located on-board GRB050713 (trigger=145675).  The spacecraft slewed
immediately and was on target at approximately 70 seconds. The
flight-determined location is RA,Dec 320.536,+77.072 {+21h 22m 09s, +77d
04' 20"} (J2000) with an uncertainty of 3 arcmin (radius, stat+sys).
This is a bright burst with a peak count rate of 6000 cts/sec in the
15-350 keV band.  The brightest part of the burst duration is ~20
seconds, followed by smaller peaks at T+50, T+65 and T+105 seconds.

The spacecraft slewed immediately and the XRT began observing the burst
at 04:30:14.9 UT (72.6 s after the BAT trigger).  XRT found a very
bright, uncataloged, fading X-ray source at:

RA:   +21h 22m 09.6s (J2000),
DEC: +77d 04' 30.3" (J2000).

This position is 11 arcseconds from the BAT position.  The estimated
uncertainty is 6 arcseconds radius (90% containment).

The Swift Ultra Violet/Optical Telescope (UVOT) observations began at
04:30:17.7 UT, 75 seconds after the BAT trigger.  The first data taken
after the spacecraft settled was a 100 sec exposure using the V filter
with the midpoint of the observation at 125 sec after the BAT trigger.
Based on comparisons to the DSS and USNO, we detect no new source at the
XRT position. The 3-sigma upper limit in the V-filter is approximately
17.81 mag.

D. Malesani (SISSA), P. D'Avanzo (INAF-OAB), E. Palazzi (INAF/IASF, Bo), 
G.L. Israel (INAF, OARm), G. Chincarini (Univ. Milano-Bicocca), L. 
Stella (INAF, OARm), M. Pedani (INAF, TNG) report on behalf on a larger 
collaboration:

We observed the field of GRB 050713 (Falcone et al., GCN 3581
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 ) with the 
Italian TNG telescope, located at the Canary Islands.

The object was observed after twilight, with a seeing of ~1.4", starting 
at 5.274 UT (0.8 h after the GRB). A 2-minutes exposure was acquired in 
the I filter.

Inside the XRT error circle, we detect one source at the coordinates 
(J2000):

  alpha = 21:22:09.6,  delta = +77:04:29
   
with an uncertainty of ~1".

We note two further objects lying just outside the nominal XRT error circle:

  1: alpha = 21:22:11.8,  delta = +77:04:27
  2: alpha = 21:22:08.4,  delta = +77:04:39

All these object are fainter than the DSS limit, so we cannot confirm if 
any of them is related to the GRB. Further analysis is in progress.

A finding chart can be found here:
http://www.sissa.it/~malesani/GRB/050713/GRB050713_finder.jpg

This message can be cited.

F. Hearty (Colorado), G. Stringfellow (Colorado), D. Q. Lamb (Chicago), D.
G. York (Chicago), G. Wallerstein (Washington), V. Woolf (Washington), S.
Anderson (Washington), J. Dembicky (APO), J. Barentine (APO), R. McMillan
(APO), B. Ketzeback (APO), and D. Reichart (North Carolina) report on
behalf of the ARC GRB team of the FUN GRB collaboration:

We began observations of the localization of GRB 050713 (Falcone et al.,
GCN 3525
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 ) with NIC-FPS on the 3.5m ARC at APO beginning 53 min after the
burst.  We detect all three of the candidates identified by Malesani et al.
(GCN 3582
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 ) in J,H,Ks in 80-sec integrations, and identify their first
candidate as fading.

NIC-FPS is currently in its commissioning phase.

This message may be cited.

S. Guziy, A. J. Castro-Tirado, A. de Ugarte Postigo,
J. Gorosabel (IAA-CSIC Granada), J. de León Cruz
(IAC Tenerife),  O. Bogdanov (Nikolaev State Univ.)
and M. Jelínek (IAA-CSIC),

report:

"Following the detection of GRB 050713 by Swift
(Falcone et al. GCNC 3581), we obtained R-band
images at the 2.5m Nordic Optical Telescope
(+ ALFOSC) starting on June 13.295 UT (about
47 min after the burst onset). We confirm the
presence of an optical source fainter than the DSS-2
limiting magnitude within the reported Swift/XRT
error box, at RA(2000) = 21 22 09.53,  Dec(2000) =
+77 04 29.5 (+/- 0.4"), consistent with the position
reported by Malesani et al. (GCNC 3582). Further
observations are needed in order to confirm if this
is the optical afterglow to GRB 050713. "

This message can be cited.

XMM-Newton will observe GRB 050713 at location 
(RA=21h 22m 09.6s, DEC=+77d 04' 30.3", J2000),
starting at 09:41:00 UT, on July 13, 2005,
for an exposure of 33000 seconds.

A. Monfardini, A. Gomboc, C. Guidorzi, C.G. Mundell, R.J. Smith,
I. A. Steele, C.J. Mottram, D. Carter, M.F. Bode (Liverpool JMU) report:

"The 2-m Liverpool Telescope followed up robotically the GRB050713
detected by SWIFT (GCN 3581
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 ). Three prompt images (2-4 minutes) have 
been acquired. No later time observations have been acquired.
We clearly detect the first object suggested by Malesani (GCN 3582
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 ) at:

RA = 21:22:09.6,  DEC = +77:04:29 (1 arcsec error)

At a mean epoch of 3 minutes after the reported GRB time we preliminarly
estimate a magnitude of r'=19.2 with large errors due to comparison
with USNOB1.0 and magnitude conversion. Further analysis is ongoing. 
The afterglow is detected on individual images.

This message can be cited."

Andrea De Luca (IASF Mi) on behalf of a larger collaboraton 
report:

We have analyzed the data from the XMM-Newton observation 
of GRB050713A, discovered by Swift on 2005, July 13 at 
04:29:02.39 UT (Falcone et al., GCN3581
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 ).

The XMM-Newton observation started on 2005, July 13 at 
10:18 UT and lasted for 30.7 ks. We report here on the 
analysis of data collected with the EPIC/pn detector, 
which started observing the field at 10:54 UT (~6h 20min 
after the GRB).

As reported by Loiseau et al. (GCN3594
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 ), the afterglow of 
GRB050713A is clearly detected in the pn image, at a 
position fully consistent with the refined Swift/XRT one 
(Morris et al., GCN3606
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 ). 
Extracting source events from a circle of 25 arcsec radius 
(containing ~80% of the total counts), the time-averaged, 
background-subtracted count rate in the 0.2-8 keV range 
is 0.547+/-0.005 cts/s.

The afterglow is clearly seen to fade along the XMM-Newton 
observation, spanning the time range 23.5-51.5 ks after 
the GRB. The background-subtracted light curve (0.5-5 keV) 
is well fitted (reduced chi2=0.9, 26 d.o.f.) by a power law 
decay with index delta=1.45+/-0.07 (90% c.l.). 
The afterglow decay has significantly steepened with 
respect to the epoch of the earlier Swift observation: 
Morris et al. (GCN 3606
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 ) observed an index delta=0.82+/-0.11
in the time range 5-10 ks after the burst using Swift/XRT data. 
This implies the presence of a break in the afterglow X-ray
light curve between 10 ks and 23.5 ks from the GRB.

We extracted the time-averaged spectrum and generated ad-hoc 
response and effective area files. We quote here errors at 
90% level for a single interesting parameter, unless otherwise 
specified.

A fit in the energy range 0.2-8 keV with an absorbed power 
law model yields a reduced chi2 of 1.25 for 172 d.o.f. 
The resulting NH=(3.25+/-0.15)x10^21 cm^-2 is higher than 
the expected Galactic value in the burst direction (NH=1.1x10^21 
cm^-2, Dickey & Lockman, 1990); the best fitting power law photon 
index is Gamma=2.16+/-0.05.
Such results are consistent with the XRT ones (Morris et al., 
GCN 3606
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 ), which implies no significant spectral evolution with
respect to the earlier phase of the afterglow.

A better fit to the pn spectrum (reduced chi2=0.97, 171 dof) 
may be obtained fixing the NH to the expected Galactic value 
(NH=1.1x10^21 cm^-2) and adding a neutral, redshifted absorber 
component to the spectral model. With a simple F-test we evaluate 
the chance occurrence probability of the improvement to be 
of 5x10^-11. The best fit value for the intrinsic NH is 4.0x10^21 
cm^-2, while the best fit value for the redshift z is 0.55.
At 90% c.l. for 2 parameters, we obtain the following ranges:
intrinsic NH=(0.4-3.2)x10^22 cm^-2; redshift z=(0.4-2.6).
Using such model, the resulting power law photon index is 
Gamma=2.04+/-0.05.
The observed flux is of 2.2x10^-12 erg cm^-2 s^-1 in 0.2-10 keV;
the corresponding unabsorbed flux is of 3.8x10^-12 erg cm^-2 s^-1.

As a last step, we divided the pn dataset into two time intervals 
of ~9500 s and ~14600 s (each containing about half of the counts 
from the afterglow) and we repeated the spectral analysis. 
We found no significant spectral changes in the two considered 
intervals.