GRB 090831

Arne Rau (MPE) reports on behalf of the Fermi GBM Team:

"At 07:36:36.58 UT on 31 August 2009, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 090831 (trigger 273396998 / 090831317).

The on-ground calculated location, using the GBM trigger
data, is RA = 145.1, DEC = 51.0 (J2000 degrees,
equivalent to 09 h 40m m, +51 d 00 '), with an uncertainty
of 1.9 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 107 degrees.

This burst was also independently detected by INTEGRAL SPI-ACS.

The GBM light curve consists of two structured main pulses
with a duration (T90) of about 69.1 s (8-1000 keV).
The time-averaged spectrum from T+0.0 s to T0+47.7 s is
best fit by a power law function with an exponential
high energy cutoff.  The power law index is -1.57 +/- 0.03 and
the cutoff energy, parameterized as Epeak, is 399.6 +/- 86.2 keV
(chi squared 443 for 415 d.o.f.).

The event fluence (8-1000 keV) in this time interval is
(1.66 +/- 0.05)E-05 erg/cm^2. The 1.024-sec peak photon flux measured
starting from T0+8.784 s in the 8-1000 keV band is 9.4 +/- 0.4 ph/s/cm^2.

A Band function fits the spectrum equally well (chi squared 441 for 414 
d.o.f.)
with Epeak= 243.8 +/- 85.3 keV, alpha = -1.52 +/- 0.06 and beta = -1.96 
+/- 0.14.

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

M. Matsuoka (JAXA), S. Miyoshi, M. Nakajima, H. Negoro, R. Ishiwata (Nihon
Univ.), K. Kawasaki, S. Ueno, H. Tomida, M. Suzuki, M. Ishikawa (JAXA),
T. Mihara, M. Kohama, Y.E. Nakagawa, M. Sugizaki, T. Yamamoto (RIKEN), 
N. Kawai, M. Morii, K. Sugimori (Tokyo Tech), A. Yoshida, K. Yamaoka, 
S. Nakahira (Aoyama G. Univ.), H. Tsunemi, M. Kimura (Osaka Univ.), 
Y. Ueda, N. Isobe, S. Eguchi and K. Hiroi (Kyoto Univ.) report on behalf of
the MAXI team:

The Gas Slit Camera (GSC) of MAXI, the all-sky X-ray monitor on the ISS,
detected a bright X-ray source at the position consistent with GRB 090831
(Rau et al, GCN 9850
Loading...
 
 ) in the energy range of 2-30 keV.  
The nominal location of the source as measured by GSC is 

(R.A., Dec.) = (145.4 deg, +51.4 deg) = (09:41:36, +51:24:00) (J2000)

with an estimated systematic uncertainty of about 1 degree.

The source was detected for 60 seconds during the transit of GSC over 
the source starting approximately at 07:36:35 UT on 31 August 2009, 
very close to the Fermi GBM trigger time.
The X-ray flux at the peak of the transit was about 3 Crab.
There are significant time structure within the transit light curve, 
which would be simply triangular for a steady source.

The all-sky image and the transit light curve is shown at the MAXI web site
http://maxi.riken.jp/ in the "News" section.

MAXI is currently in the commissioning phase that runs for three months.
The systematic uncertainty in localization will be significantly improved
with the progress of the alignment calibration.