GRB 060813

A. Moretti (INAF-OAB), S. D. Barthelmy (GSFC),
J. R. Cummings (NASA/UMD), C. Guidorzi (Univ Bicocca&INAF-OAB),
C. B. Markwardt (GSFC/UMD), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL), P. Romano (INAF-OAB) and D. E. Vanden Berk (PSU)
report on behalf of the Swift Team:

At 22:50:22 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 060813 (trigger=224364).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA,Dec 111.868, -29.851 {07h 27m 28s, -29d 51' 01"} (J2000)
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single,
roughly square peak with a duration of about 10 sec.  The peak count rate
was ~10,000 counts/sec (15-350 keV), at ~7 sec after the trigger. 

The XRT began observing the field at 22:51:38 UT, 76 seconds after the
BAT trigger. XRT found a variable, uncatalogued X-ray source
located at RA(J2000) = 07h 27m 34.9s, Dec(J2000) = -29d 50' 54.6", with an
estimated uncertainty of 5.8 arcseconds (90% confidence radius). 
This location is 87 arcseconds from the BAT on-board position, within
the BAT error circle. The initial flux in the 2.5s image was
6.5e-10 erg/cm2/s (0.2-10 keV). 

UVOT took a finding chart exposure of 100 seconds with the White
(160-650 nm) filter starting 83 seconds after the BAT trigger. No
afterglow candidate has been found in the initial data products. The
2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical
3-sigma upper limit has been about 18.5 mag. 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. The
field is fairly crowded.  No correction has been made for the 
expected extinction corresponding to E(B-V) of 0.33.

J. Cummings (GSFC/UMBC), L. Barbier (GSFC), S. D. Barthelmy (GSFC),
E. Fenimore (LANL), N. Gehrels (GSFC), D. Hullinger (BYU-Idaho),
H. Krimm (GSFC/USRA), M. Koss (GSFC/UMD), C. Markwardt (GSFC/UMD),
A. Moretti (INAF-OAB), D. Palmer (LANL), A. Parsons (GSFC),
T. Sakamoto (GSFC/ORAU), G. Sato (GSFC/ISAS), M. Stamatikos (GSFC/ORAU),
J. Tueller (GSFC)
on behalf of the Swift-BAT team:

Using the data set from T-239 to T+543 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 060813 (trigger #224364)
(Moretti, et al., GCN Circ. 5442
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 ).  The BAT ground-calculated position is
RA,Dec = 111.890, -29.844 deg {7h 27m 33.6s, -29d 50' 40.2"} (J2000)
+- 0.6 arcmin, (radius, sys+stat, 90% containment).  The partial coding was 39%.
 
The mask-weighted lightcurve shows a roughly square-shaped peak starting
at T-1 sec and lasting until T+9 sec.  It is then followed a softer component,
roughly constant but weaker intensity, lasting out to ~T+30 sec.
T90 (15-350 keV) is 14.9 +- 0.5 sec (estimated error including systematics).
 
The time-averaged spectrum from T-0.5 to T+27.8 is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 1.00 +- 0.19, 
and a poorly-constrained Epeak of 176 +- 83 keV (chi squared 35.17 for 56 d.o.f.).
For this model the total fluence in the 15-150 keV band is
5.5 +- 0.1 x 10^-6 erg/cm2 and the 1-sec peak flux measured
from T+3.59 sec in the 15-150 keV band is 9.0 +- 0.4 ph/cm2/sec.
A fit to a simple power law gives a photon index of 1.35 +- 0.04
(chi squared 45.73 for 57 d.o.f.).  All the quoted errors are at the 90%
confidence level.

A. Moretti (INAF-OAB), C. Guidorzi (Univ. Bicocca & INAF-OAB),
P. Romano (INAF-OAB) report on behalf of the Swift-XRT team:

We have analysed the first 4 orbits of Swift-XRT data obtained for GRB
060813 (trigger number 224364). The data consist of 611 s in Windowed
Timing (WT) mode, starting 76 seconds after the BAT trigger and 7 ks
in Photon Counting (PC) mode.  Using PC data we obtain a refined
position of:

RA(J2000) =   07 27 35.36
Dec(J2000) = -29 50 54.2

with an estimated uncertainty radius of 3.5 arcsec (90% containment).
This is at 27" from the refined BAT position (Cummings et al., GCN
Circ. 5443
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 ).

The light-curve shows a shallow decay with alpha = 0.58+/- 0.05, in
the first part up to a break at time=861+/-180 s. After the break
data show a decline with alpha = 1.10+/- 0.05.

The spectrum formed from all the WT data before the light-curve break
can be modelled with a power-law of photon index Gamma = 1.98 +/-
0.08, with an absorbing column of NH = (4.4 +/- 0.4)e21 cm^-2
marginally consistent with the Galactic value of 3.1e21 cm^-2.
The spectrum formed from the PC data after the light-curve break is
maginally consistent with the pre-break spectrum: it can be modelled
with a power-law of photon index Gamma = 1.80 +/- 0.10, with an
absorbing column of NH = (3.2 +/- 0.5)e21 cm^-2.

Assuming the light-curve continues to decay with alpha ~ 1.10, the
count rate at 24 hours is predicted to be 0.023 count s^-1. This
corresponds to an observed (unabsorbed) flux of 1.4e-12 (1.9e-12)
erg cm^-2 s^-1.

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

A. Cucchiara, P. Brown, A. Moretti report on behalf of the
Swift/UVOT team:

The Swift UVOT began observing GRB 060813 (trigger #224364,
A. Moretti et al., GCN Circ. 5442
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 ) 83 seconds after the BAT
trigger. No optical afterglow is detected in the XRT refined error
circle (A. Moretti et al., GCN Circ. 5444
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 ).

The 3-sigma limiting magnitudes for the finding chart and the
coadded images for the UVOT filters are listed below:

Finding chart:

Filter        T_range (s)    Exposure (s)    3sigma UL
White         83-181         98              19.46

Coadded images:

Filter          T_range (s)      Exposure (s)   3sigma UL
WHITE           83-12239         1394           20.89
V               183-29590        1235           19.66
B               391-18023        1822           21.03
U               366-23807        2203           20.76
W1              343-23609        2200           20.82
M2              300-22702        1613           20.86
W2              419-28770        1279           20.93

T_range is calculated from the time of the burst. No correction has
been made for the Galactic reddening of E(B-V) = 0.33 mag
(Schlegel et al. 1998) along the line of sight to the burst.

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

The long GRB 060813 (Swift-BAT trigger #224364;
Moretti et al., GCN 5442
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 ; Cummings et al., GCN 5443
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 )
triggered Konus-Wind at T0=82221.576 s UT (22:50:21.576).

As observed by Konus-Wind the burst shows
a single pulse with a duration of ~6 s,
followed by a weak soft tail seen up to T-T0 ~13 s.
The burst fluence is (1.18 +/- 0.07)x10^-5 erg/cm2,
and the 64-ms peak flux measured from T0+5.136 s
(2.96 +/- 0.58)x10^-6  erg/cm2/s
(both in the 20 keV - 1 MeV energy range).

The time-integrated spectrum of the burst
(from T0 to T0+6.656 s; in the 20 keV - 1 MeV range)
is well fitted (in the 20 keV - 1 MeV range)
by GRBM (Band) model for which:
the low-energy photon index is alpha = -0.53 (-0.14,+0.16),
the high energy photon index beta = -2.65(-0.49, +0.26),
the break energy E0 = 130(-23, +27) keV (chi2 = 58/47 dof).
The peak energy Ep = 192(-18, +20) keV.

All the quoted errors are at the 90% confidence level.

A. Pelangeon & J-L. Atteia (LATT-OMP) report:

We have used the spectral parameters of GRB 060813
provided by Golenetskii et al. (GCNC 5446) to
compute the spectral pseudo-redshift** of this burst
detected by SWIFT-BAT (Moretti et al., GCNC 5442).

We find a pseudo-redshift pz= 2.38 +/- 0.40

This is a typical value for a GRB.


** cf. http://www.ast.obs-mip.fr/grb/pz


[GCN OPS NOTE(15aug06): This Circular was delayed by ~2 hrs due to problems
at the GCN end.]

K.Yamaoka, S.Sugita (Aoyama Gakuin U.), M.Ohno, T.Takahashi, 
T.Asano, T.Uehara, Y.Fukazawa (Hiroshima U.), Y.Terada, T.Tamagawa, 
M.Suzuki (RIKEN), K.Abe, K.Onda, M.Suzuki, M.Tashiro (Saitama U.), 
S.Hong (Nihon U.) K.Nakazawa, T.Takahashi (ISAS/JAXA), T.Enoto, 
R.Miyawaki, K.Kokubun (Univ. of Tokyo) 
on behalf of the Suzaku HXD-II team report:

The bright, long burst, GRB 060813 (Swift-BAT trigger #224364; 
Morettim et al., GCN5442
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 ; Cummings et al. GCN 5443
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 ), triggered 
the Suzaku Wideband All-sky Monitor (WAM) which covers an energy 
range of 50 keV - 5 MeV at 22:50:24.295 (UT). 
The observed prompt emission exhibits a single-peaked lightcurve
with a duration (T90) of 6.1 seconds. 
The fluence in 100 - 1000 keV was (1.0 +/- 0.1)X10^-6 erg/cm2. 
The 1-s peak flux was 5.6 +/- 0.2 photons/cm2/s 
in the same energy range.

Preliminary result shows that the time-averaged spectrum 
is well-fitted by a power law with an exponential 
cutoff having the following parameters:
alpha = 0.93 +- 0.31,
E0 = 229 (+82, -53) keV, and 
Epeak = 248 +- 20 keV.
All the quoted errors are at statistical 90% confidence level, 
 not included in the systematic errors.

Further detailed analyses are in progress.

A. Moretti (INAF-OAB), P. Romano (INAF-OAB), C. Guidorzi
(Univ. Bicocca & INAF-OAB), D.N. Burrows (PSU), N. Gehrels (GSFC)
report on behalf of the Swift-XRT team:

Based on the most recent downlinked data on GRB060813 (trigger number
224364) we find clear evidence of a break in the light curve at
48(+/-10) ks from the burst trigger (see the light curve at
http://www.brera.inaf.it/~moretti/grbplot/xrt060813.gif )
which could be a jet break.
The light curve slope steepens from 1.20+/-0.10 to 2.50+/-0.30 .
(errors are quoted at 90% confidence level).
In order to convert the XRT count rate to absorbed (unabsorbed)
flux the conversion factor is 6.1E-11 (8.1E-11).

Therefore we encourage optical data collection on this GRB, when
possible.

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

T. Sakamoto (GSFC/ORAU), G. Sato (GSFC/ISAS), S. D. Barthelmy (GSFC), 
J. Cummings (GSFC/ORAU), N. Gehrels (GSFC), H. Krimm (GSFC/USRA), 
A. Parsons (GSFC), J. Tueller (GSFC), L. Barbier (GSFC), 
E. Fenimore (LANL), D. Hullinger (BYU-Idaho), M. Koss (GSFC/UMD), 
C. Markwardt (GSFC/UMD), D. Palmer (LANL), M. Stamatikos (GSFC/ORAU), 
on behalf of the Swift-BAT team, 

S. Golenetskii, R.Aptekar, E. Mazets, V. Pal'shin, D. Frederiks, 
and T. Cline on behalf of the Konus-Wind team, 

M. Ohno, T. Takahashi, T. Asano, T. Uehara, Y. Fukazawa (Hiroshima U.),
K. Yamaoka, S. Sugita (Aoyama Gakuin U.), 
Y. Terada, T. Tamagawa, M. Suzuki (RIKEN), 
K. Abe, K. Onda, Y. Urata, M. Suzuki, M. Tashiro (Saitama U.),
S. Hong (Nihon U.), K. Nakazawa, T. Takahashi (ISAS/JAXA), 
T. Enoto, R. Miyawaki, K. Kokubun, K. Makishima (Univ. of Tokyo)
on behalf of the Suzaku-WAM team, 

report:

We performed the Swift/BAT, Konus-Wind, and Suzaku/WAM joint fit spectral 
analysis of GRB 060813 (Swift-BAT trigger #224364; Moretti, et al., 
GCN Circ. 5442
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 ).  The time interval of the spectral data for each instrument 
is chosen as the same interval as the Konus-Wind time-averaged spectrum 
reported by Golenetskii et al. (GCN Circ. 5446
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 ).  The energy ranges which we 
used in the joint spectral analysis are 14-150 keV, 20-1000 keV, 
and 100-1000 keV, for Swift/BAT, Konus-Wind and Suzaku/WAM respectively.  
The spectral data of three instruments are fitted with the spectral model 
multiplied by the constant factor to take into account the systematic 
uncertainties in the response matrices of each instrument.  

The spectrum is well fitted with a power-law time exponential cutoff model 
(dN/dE ~ E^{alpha} * exp(-(2+alpha)*E/Epeak)).  The constant factors of each 
instrument agree within 10%.  No systematic residual from the best fit model 
is seen in the spectral data of each instrument.  The best fit spectral 
parameters are: alpha = -0.83 +- 0.05 and Epeak = 240 (-10/+11) keV 
(chi2/dof = 135/125).  Although there is no significant improvement in the 
fit, the best fit parameters in the Band function are: the low-energy 
photon index alpha = -0.79 +- 0.06, the high energy photon index 
beta = -2.71 (-0.70/+0.24) and Epeak = 230 (-14/+15) keV 
(chi2/dof = 133/124).  The energy fluence in the 15-1000 keV band calculated 
by the Band function is 1.22 +- 0.05 x 10^-5 erg/cm2 (in case the constant 
factor of the Konus-Wind is fixed to 1).  The spectral paramters in a 
power-law time exponential cutoff model for Swift/BAT and Suzaku/WAM were 
reported by Cummings et al. (GCN Circ. 5443
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 ) and by Yamaoka, et al. 
(GCN Circ. 5462
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 ).  The Konus-Wind best fit spectral parameters for a 
power-law time exponential cutoff model are:  alpha = -0.65 (-0.12, +0.11), 
Epeak = 214 (-15, +17) keV (chi2/dof = 81/59).  Our joint fit spectral 
parameters in a power-law time exponential cutoff model are consistent 
within 90% confidence level with the spectral parameters derived by individual 
instruments.  However, we do see a systematic difference in the spectral 
parameters in the Band function fit especially in the low energy photon 
index alpha.  We are still working on the cross-calibration among three 
instruments.  

All the quoted errors are at the 90% confidence level. 

[GCN OPS NOTE(25aug06): Per author's request, the last two words were added
to the Subject line.]