GRB 081224

Colleen Wilson-Hodge (MSFC) and Valerie Connaughton (UAH) on behalf
of the Fermi-GBM team,
Francesco Longo (INFN Trieste) and Nicola Omodei (INFN Pisa) on behalf
of the Fermi LAT team.

"On 21:17:55.41 UT of Dec 24 2008 GBM detected a bright GRB.
Preliminary analysis of the real-time Fermi GBM  data from Trigger number
251846276/ GRB 081224887 / GRB 081224
shows that this is a FRED (Fast Rise Exponential Decay) lasting
approximately 50 seconds with a peak of FWHM 15 sec long.    It is
located to a position of RA=206.2, Dec= 73.3  with statistical
uncertainty 1 deg (and an estimated systematic uncertainty of 2-3 deg)
which places it at 16 deg to the LAT boresight.

The Large Area Telescope (LAT) also provided a tentative onboard
localization:
RA=213.367, DEC=74.233 (J2000)
corresponding to:
RA: +14h 13m 28s  (J2000),
DEC: +74d 13' 60" (J2000)
with on board estimated error of 36 arcmin (statistical only).
The current estimated onboard systematic error is 1 deg.

The estimated LAT position is  2.2 deg from the ground GBM location
and may be  considered more reliable. 
Accurate LAT localization and spectral analysis will
await the downlink of both LAT and GBM data which could take several
hours.

We further report that the Fermi Observatory executed a maneuver
following  this trigger and will track the burst location for the next
5 hours, subject  to Earth-angle constraints.

We encourage follow up observations.
The Fermi LAT point of contact for this burst is Francesco Longo
(francesco.longo@ts.infn.it).
The Fermi GBM point of contact is Valerie Connaughton
(valerie@nasa.gov)

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."

K.L. Page and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT 
team:

Swift performed a Target of Opportunity follow-up observation of the Fermi 
GBM and LAT burst GRB 081224 (Wilson-Hodge et al., GCN Circ. 8723
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 ), 
starting 18 ks after the trigger. 

There is a faint X-ray source (~6.7x10^-3 count s^-1) within the XRT field 
of view, detected in the 3.6 ks of data collected so far. Using 1193 s of 
XRT Photon Counting mode data and 1 UVOT image, we find an astrometrically 
corrected X-ray position (using the XRT-UVOT alignment and matching UVOT 
field sources to the USNO-B1 catalogue): RA, Dec = 213.1494, 74.1999, 
which is equivalent to:

RA (J2000):   14 12 35.85
Dec (J2000): +74 11 59.5

with an uncertainty of 3.4 arcsec (radius, 90% confidence).

However, we note that the uncertainty on the LAT position (36 arcmin) is 
larger than the XRT field of view, so Swift has not observed the complete 
error circle. We also cannot say whether the XRT source is fading at this 
time.

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

Julie McEnery (NASA/GSFC) reports on behalf of the LAT collaboration

On-ground analysis of GRB 081224 did not confirm the tentative LAT 
on-board detection reported in GCN 8723
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 . The apparent onboard excess was 
likely a statistical fluctuation.

H. Hayashi E. Sonoda, M. Yamauchi, H. Tanaka, R. Hara, 
N. Ohmori, K. Kono (Univ. of Miyazaki)
A. Endo, M. Tashiro, Y. Terada, Y. Urata, K. Onda,
N. Kodaka, K. Morigami, T. Sugasahara, W. Iwakiri (Saitama U.),
T. Enoto, K. Nakazawa, K. Makishima (Univ. of Tokyo), S. Sugita,
K. Yamaoka (Aoyama Gakuin U.), M. Ohno, M. Kokubun, M. Suzuki,
T. Takahashi (ISAS/JAXA), Y.E. Nakagawa, T. Tamagawa (RIKEN),
Y. Hanabata, T. Uehara, T. Takahashi, Y. Fukazawa, C. Kira 
(Hiroshima U.), S. Hong (Nihon U.), 
on behalf of the Suzaku WAM team, report:

The long, bright GRB 081224 (Fermi-GBM trigger #251846276; Colleen et
al., GCN 8725
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 ) triggered the Suzaku Wide-band All-sky Monitor (WAM)
 which covers an energy range of 50 keV - 5 MeV at 2008-12-24 21:17:55.214 UT (=T0).

The observed light curve shows a FRED-like peak structure
starting at T0-0.2s, ending at T0+19.8s with a duration (T90) of about 10.3 seconds.
The fluence in 100 - 1000 keV was 2.87(+0.09, -0.04) erg/cm^2.
The 1-s peak flux measured from T0+2s - T0+3s was 16.95(+0.91, -0.55) photons/cm^2/s in the same
 energy range.

Preliminary result shows that the time-averaged spectrum from
T0-0.2s to T0+19.8s is well fitted by a power-law with exponential cutoff model
  dN/dE ~  E^{-alpha} * exp(-(2-alpha)*E/Epeak) with
  alpha       0.83(+0.27, -0.28), and
  Epeak       447(+57, -40) keV (chi^2/d.o.f. = 19/22).

All the quoted errors are at statistical 90% confidence level,
in which the systematic uncertainties are not included.

The light curves for this burst are available at:
 http://www.astro.isas.jaxa.jp/suzaku/HXD-WAM/WAM-GRB/grb/trig/grb_table.html