GRB 091031

S. McBreen (UCD/MPE) and V. Chaplin (UAH)
report on behalf of the Fermi GBM Team:

"At 12:00:28.85 UT on 31 October 2009, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 091031 (trigger 278683230 / 091031500).

The on-ground calculated location, using the GBM trigger
data, is RA = 70.6, DEC = -59.1 (J2000 degrees,
equivalent to 04 h 42 m, -59 d 06 '), with an uncertainty
of 1 degrees (radius, 1-sigma containment,
statistical only; there is additionally a systematic
error which is currently estimated to be 2 to 3 degrees).
The angle from the Fermi LAT boresight is 22 degrees.

The GBM light curve consists of a number of pulses with
sub-structure and has a duration (T90) of about 35 s (8-1000 keV).
The time-averaged spectrum from T0-0.003 s to T0+36.609 s is
best fit by a Band function with Epeak = 503.10 (+47.30/-46.60) keV,
alpha = -0.91 +/- 0.04, and beta = -2.34 (+0.19/-0.31)
(CSTAT 674 for 602 d.o.f.).

The event fluence (8-1000 keV) in this time interval is
(2.05 +/- 0.04)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+6.9 s in the 8-1000 keV band
is 7.5 +/- 0.3 ph/s/cm^2.

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

[GCN OPS NOTE(01nov09): Per author's request, the missing minus sign
to the value for alpha was added.]

F. de Palma (University and INFN Bari), N. Omodei (INFN Pisa), Julie 
McEnery (NASA/GSFC) and
V. Vasileiou (NASA GSFC/UMBC) report on behalf of the Fermi LAT team:

At 12:00:28 (UT) on 31 October 2009 , the Fermi Large Area Telescope 
(LAT) detected gamma rays
from the long GRB 091031, which was triggered and located by the Fermi 
Gamma-ray Burst Monitor
(GBM) (trigger 278683230 / 091031500,GCN 10115). The angle of the GBM 
best position (RA, Dec=
70.58, -59.08) with respect to the LAT boresight was ~22 degrees at the 
time of the trigger,
which is well inside our field of view.

The Fermi LAT events after the GBM trigger are spatially and temporally 
correlated with the
emission measured by GBM. In the time interval T0-T0+100s using 
likelihood methods
on photons above 100 MeV this burst has a significance of more than 3 
sigma; using counting
methods the significance is well over 6 sigma in the full energy band 
(20 MeV - 300 GeV).
More than 30 photons above 100 Mev and 2 photons above 1 GeV were 
observed in the same
time interval in an energy dependent ROI centered on the GBM position.
The best LAT on-ground localization is found to be (RA, Dec = 71.7, 
-57.5) with a 90%
containment radius of 0.3 deg (statistical; 68% containment radius: 0.2 
deg, preliminary
systematic error is less than 0.1 deg) which is consistent with the GBM 
localization.

Further analysis is ongoing.

The point of contact for this burst is
Francesco de Palma : francesco.depalma@ba.infn.it

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.

This message can be cited.

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

The long GRB 091031 (Fermi-GBM trigger 278683230/091031500,
McBreen and Chaplin, 10115) localized by Fermi-LAT
(de Palma et al., GCN 10163
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 ) triggered
Konus-Wind at T0=43232.687s UT (12:00:32.687) and
Konus-RF onboard Coronas-Photon s/c at T0=43230.291s UT (12:00:30.291).

The burst light curve shows a complex structure with a total
duration of ~40 s. The Konus-Wind light curve of this GRB
is available at http://www.ioffe.ru/LEA/GRBs/GRB091031_T43232/

As observed by Konus-Wind the burst had a fluence
of (1.94 � 0.2)x10-5 erg/cm2,
and a 256-ms peak flux measured from T0 + 4.608s
of (2.74 � 0.35)x10-6 erg/cm2/s
(both in the 20 keV - 2 MeV energy range).

The time-integrated spectrum of the burst (from T0 to T0+41.216 s)
is well fitted in the 20 keV - 2 MeV range by a power law
with exponential cutoff model, for which:
alpha = 0.93 (-0.08, +0.09), and
Epeak = 477 (-91, +138)  keV, Chi2 = 31/58 dof.

Fitting by a GRB (Band) model yiels the same alpha and Epeak
values with high energy spectral index beta of -9.4 (<-2.4)

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