GRB 120911B

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 hard GRB 120911B
(IPN detection and localization: Hurley at al., GCN 13755
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 ;
Fermi-LAT detection: Racusin et. al., GCN 13756
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 ;
Fermi-GBM observation: Gruber et. al., GCN 13757
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 )
triggered Konus-Wind at T0=23172.213s UT (06:26:12.213)

The light curve shows a bright hard pulse with a duration
of ~30 s followed by a tail of weaker and softer extended
emission during the next ~150 s; additionally,
a soft weak precursor is observed around T0-55 s.
The emission in the main bursting episode is seen up to 9 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB120911_T23172/

As observed by Konus-Wind, the burst
had a fluence of 3.0(-0.1,+0.1)x10-4 erg/cm2,
and a 64-ms peak flux, measured from T0+15.232 s,
of 3.8(-0.4,+0.4)x10-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-integrated spectrum of the main emission episode
(measured from T0 to T0+34.048 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) function with the following model parameters:
the low-energy photon index alpha = -0.86 (-0.03, +0.03),
the high energy photon index beta = -2.6 (-0.2, +0.2),
the peak energy Ep = 950(-50, +50) keV,
chi2 = 78.4/82 dof.

The spectrum at the maximum count rate
(measured from T0+14.080 to T0+15.616 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) function with the following model parameters:
the low-energy photon index alpha = -0.63 (-0.08, +0.09),
the high energy photon index beta = -2.9 (-1.6, +0.3),
the peak energy Ep = 1000(-120, +130) keV,
chi2 = 81.1/82 dof.


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

David Gruber (MPE), W. Paciesas (USRA), V. P�lassa (UAH)
and V. Chaplin (UAH) report on behalf of the Fermi GBM Team:

"We report on the ground analysis of the IPN detected GRB 120911B (GCN 13755
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in the GBM data.

The GBM light curve consists of a precursor starting at ~ 06:25:14 UT (T0) 
which is followed by a bright and hard main emission ~ 50 s later, lasting
for another ~ 40 s.
The duration (T90) is of about 69 s (50-300 keV).
The time-averaged spectrum from T0 to T0+132 s is best fit by a Band function
with Epeak = 1200 +/- 55 keV, alpha = -1.01 +/- 0.01,
and beta = -2.72 +/- 0.14.

The event fluence (10-1000 keV) in this time interval is
(1.973 +/- 0.026)E-04 erg/cm^2. The 4-sec peak photon flux measured
starting from T0+70 s in the 10-1000 keV band
is 33.04 +/- 0.7 ph/s/cm^2.

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

J. L. Racusin (NASA/GSFC), G. Vianello (SLAC), D. Kocevski (NASA/GSFC), N. Omodei (Stanford), and M. Ohno (ISAS/JAXA), report on behalf of the Fermi-LAT team:

We report on ground analysis of the IPN detected GRB 120911B (GCN 13755
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 ) at 06:26:15 UT. The center of the IPN error box was at the edge of the LAT field of view, 67 degrees from the LAT boresight at the trigger time. The best LAT localized position is RA/Dec = 172.03, -37.51 (J2000), with a error radius of 0.3 deg (68% containment), using LAT source class events from 0-40 s after the trigger. This is within the IPN error box, 0.2 deg from the center.

The burst left the LAT field of view ~100 seconds after the trigger and returned ~4100 s later.

The LAT data show a significant increase in counts at energies >100 MeV, starting several seconds after the trigger, with a single peaked structure. The non-standard LAT Low Energy (LLE) event class shows two-peaks with a tail lasting to approximately T0+60 s.

A spectral fit of the LAT data in the 0-40 s window yields a photon index of -2.5 +/- 0.2 (68% containment, statistical only) and a flux of (2.0 +/- 0.4) x 10^-7 ph/s-cm^2 in the 100 MeV - 10 GeV energy range.

Fermi-GBM did not trigger on this GRB due to high geo-magnetic latitude, but it was clearly detectable in ground analysis. The GBM team will describe the observations in a subsequent circular.

The Fermi LAT point of contact for this burst is Masanori Ohno (ohno@hep01.hepl.hiroshima-u.ac.jp).

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.

K. Hurley on behalf of the Mars Odyssey and MESSENGER GRB teams,

I. G. Mitrofanov, D. Golovin, M. L. Litvak, and A. B. Sanin,
on behalf of the HEND-Odyssey GRB team,

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

W. Boynton, C. Fellows, K. Harshman, H. Enos, and R. Starr, on
behalf of the GRS-Odyssey GRB team,

G. Di Cocco, F. Fuschino, M. Galli, C. Labanti, and M. Marisaldi,
on behalf of the AGILE MCAL team, and

A. von Kienlin, X. Zhang, A. Rau, V. Savchenko, E. Bozzo, and C.
Ferrigno, on behalf of the INTEGRAL SPI-ACS GRB team, report:

The long-duration, intense GRB 120911B has been observed by INTEGRAL 
(SPI-ACS), AGILE (MCAL), Konus-Wind, and Mars Odyssey (HEND), so far, at 
about 23175 s UT (06:26:15).

We have triangulated it to a preliminary, 3 sigma error box whose
coordinates are:
  ---------------------------------------------
   RA(2000), deg                 Dec(2000), deg
  ---------------------------------------------
  Center:
   172.146 (11h 28m 35s) -37.696 (-37d 41' 46")
  Corners:
   172.616 (11h 30m 28s) -38.705 (-38d 42' 20")
   172.297 (11h 29m 11s) -38.543 (-38d 32' 34")
   171.709 (11h 26m 50s) -36.675 (-36d 40' 31")
   172.013 (11h 28m 03s) -36.848 (-36d 50' 53")
  ---------------------------------------------
The error box area is 0.380 sq. deg, and its maximum
dimension is 2.15 deg.

This box can be improved.

The time history and spectrum will be given in a forthcoming GCN Circular.