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GCN Circular 8908

Subject
GRB 090217: Suzaku WAM observation of the prompt emission
Date
2009-02-19T04:55:52Z (15 years ago)
From
Yoshitaka Hanabata at Hiroshima U <hanabata@hep01.hepl.hiroshima-u.ac.jp>
Y. Hanabata, Y. Fukazawa, T. Takahashi, T. Uehara, C. Kira (HiroshimaU.),
Y. Terada, M. Tashiro, Y. Urata, A. Endo, K. Onda, N. Kodaka,
K. Morigami, T. Sugasahara, W. Iwakiri (Saitama U.), M. Ohno,
M. Kokubun, M. Suzuki, T. Takahashi (ISAS/JAXA), K. Yamaoka,
S. Sugita (Aoyama Gakuin U.), T. Enoto, K. Nakazawa, K. Makishima
(Univ. of Tokyo), Y.E. Nakagawa, T. Tamagawa (RIKEN),
E. Sonoda, M. Yamauchi, H. Tanaka, R. Hara, N. Ohmori, K. Kono,
H. Hayashi (Univ. of Miyazaki), S. Hong (Nihon U.),
N. Vasquez (Tokyo Tech), on behalf of the Suzaku WAM team, report:

The bright, long Fermi-LAT detected GRB 090217 (Fermi-GBM
trigger #256539404 ; von Kielin et al., GCN 8902;
Fermi-LAT detection ; Ohno et al., ; GCN 8903)
triggered the Suzaku Wide-band  All-sky Monitor (WAM) which covers an
energy range of 50 keV - 5 MeV at 04:56:42.98 UT (=T0).
The observed light curve shows a multi-peaked structure
starting at T0-3s, ending at T0+19s with a duration
(T90) of about 14 seconds. The fluence in 100 - 1000 keV
was 1.56(+0.05,-0.09)x10^-5 erg/cm^2.  The 1-s peak flux measured
from T0+7s was 3.0 (+0.2,-0.3) photons/cm^2/s in the same  energy range.
Preliminary result shows that the time-averaged spectrum from
T0-3s to T0+20s is well fitted by a power-law with exponential cutoff model:

 dN/dE ~  E^{-alpha} * exp(-(2-alpha)*E/Epeak) with
 alpha       0.81(+0.26,-0.31), and
 Epeak       780(+187,-126) keV (chi^2/d.o.f. = 41.0/24).

The GRB Band model also gives a good fit with only
an upper limit on the high-energy photon index as follows:

the low-energy photon index alpha: -0.75(+0.50,-0.28),
the high-energy photon index beta: < -2.33,
and the peak energy Epeak: 748(+195,-175) keV (chi^2/d.o.f = 40.4/23).

Please note that this GRB comes from the WAM 2 on-axis direction, which
has a large uncertaintiy only in absolute flux.
All the quoted errors are at statistical 90% confidence level,
in which the systematic uncertainties are not included.

The light curves for this burst will be available soon at:

http://www.astro.isas.jaxa.jp/suzaku/HXD-WAM/WAM-GRB/grb/trig/grb_table.html
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