[AAVSO-HEN] GRB 090227: Fermi GBM detection

[grb at aavso.org]

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TITLE:   GCN CIRCULAR
NUMBER:  8917
SUBJECT: GRB 090227: Fermi GBM detection
DATE:    09/02/27 19:20:13 GMT
FROM:    Elisabetta Bissaldi at MPE  <ebs at mpe.mpg.de>

E. Bissaldi (MPE) reports on behalf of the Fermi GBM Team: 


"At 07:25:57.00 UT on 27 February 2009, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 090227 (trigger 257412359 / 090227310).
The on-ground calculated location, using the GBM trigger
data, is RA = 3.2, DEC = -43.1 (J2000 degrees,
equivalent to 00h 13m, -43d 06'), with an uncertainty
of 1.2 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 21 degrees.

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

The GBM light curve shows one main FRED-like pulse with substructure.
Emission continues to at least 80 seconds.
T90 (8-1000 keV) is about 50 s and T50 (8-1000 keV) is about 20 s.
The time-averaged spectrum from T0 to T0+16.3 s is
adequately fit by a power law function with an exponential
high energy cutoff.  The power law index is -0.91 +/- 0.06 and
the cutoff energy, parameterized as Epeak, is 1355 +/- 259 keV
(chi squared 748 for 729 d.o.f.).

The event fluence (8-1000 keV) in this time interval is
(9.0 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+1.9 s in the 8-1000 keV band
is 4.57 +/- 0.13 ph/s/cm^2.

A Band function fits the spectrum equally well (chi squared 
748 for 728 d.o.f.) with Epeak = 1357 +/- 284 keV, 
alpha = -0.92 +/- 0.06 and beta = -3.6 +/- 2.9.


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