GRB 070824

Combined SuperAGILE/IPN Localization of the long and bright GRB 070824


I. Donnarumma, E. Del Monte, F. Lazzarotto, M. Feroci, A. Giuliani,
M. Marisaldi, on behalf of the AGILE Team, 

B. Preger, on behalf of the AGILE Science Data Center, 

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

K. Hurley on behalf of the IPN Teams, 

J. Cummings, D. Palmer and H. Krimm, on behalf of the Swift/BAT team,

D. M. Smith, R. P. Lin, J. McTiernan, R. Schwartz, C. Wigger, W. Hajdas, 
and A. Zehnder, on behalf of the RHESSI GRB team, 

report: 

GRB 070824 triggered the SuperAGILE ground software trigger on 24
August 2007, at 20:50:09 UT. The GRB was seen as a 3 s long event,
preceded by a weaker 2 s precursor 5 s earlier. The SuperAGILE T90 for 
this event was 9.5 s.  The event was observed by SuperAGILE at 48.1 deg 
off-axis,  which is in a region of the instrument field of view with only 
one-dimensional imaging. In spite of this, the event was observed on all 
SuperAGILE detectors; due to its hardness,  it passed through the 
collimator shields. In addition, its arrival direction was partially 
obscured by the Earth's atmosphere for AGILE. For these reasons, the event 
was imaged by SuperAGILE with only 4.6 sigma significance. The 
SuperAGILE-only error box is thus a long, narrow strip, limited by the 
instrument field of view and Earth occultation in one direction and by the 
imaging uncertainty in the other. For the latter we assumed a 20 arcmin 
error radius, including statistics and systematics, due to the low 
statistics of the detection and the far off-axis position. The resulting 
error box is:		

RA	  Dec
===============
172.281 -27.240 
171.938 -26.648
147.942 -34.944 
148.056 -34.281

The SuperAGILE light curve may be found at the AGILE Science Data Center 
and AGILE-Team web pages.

This event was independently detected by other IPN experiments, namely 
Konus-Wind, Swift/BAT, RHESSI, AGILE/Minicalorimeter, and the
AGILE/Anticoincidence, as well as by Konus-A (Cosmos-2421). Their
detection combined to provide further constraints to the burst
localization, as shown in the figure at
www.ssl.berkeley.edu/ipn3/070824. 

The final localization of GRB 070824 is thus limited by the
corners of a triangular area:

   RA      Dec 	
==================
1: 172.055 -26.864
2: 171.188 -27.715
3: 172.281 -27.240

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

The bright long GRB 070824 localized by SuperAGILE/IPN
(Donnarumma et al., GCN 6767
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 ) triggered Konus-Wind
at T0=74999.585 s UT (20:49:59.585).

The Konus-Wind light curve shows two pulses
with a total duration of ~12 s.

As observed by Konus-Wind the burst
had a fluence of 2.84(-0.26, +0.09)x10^-5 erg/cm2,
and a 16-ms peak flux measured from T0+8.368 s
of 2.85(-0.38, +0.29)x10^-5 erg/cm2/s
(both in the 20 keV - 2 MeV energy range).

The time-integrated spectrum of the burst
(from T0 s to T0+16.986 s) is well fitted
(in the 20 keV - 2 MeV range)
by a power law with exponential cutoff model:
dN/dE ~ E^(-alpha)*exp(-E*(2-alpha)/Ep)
with alpha = 1.051 (-0.081, +0.078)
and Ep = 253 (-19, +22) keV (chi2 = 66.2/60 dof).
Fitting by a GRBM (Band) model yields:
the low-energy photon index alpha = -1.025 +/- 0.084,
the high energy photon index beta < -2.55,
the peak energy Ep = 243(-20, +24) keV (chi2 = 64.5/59 dof).

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

The Konus-Wind light curve of this GRB is available
at http://www.ioffe.rssi.ru/LEA/GRBs/GRB070824_T74999/