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GCN Circular 9955

Subject
GRB 090927, Swift-BAT refined analysis of the possibly short burst
Date
2009-09-27T19:36:02Z (15 years ago)
From
Jay R. Cummings at NASA/GSFC/Swift <james.r.cummings@nasa.gov>
M. Stamatikos (OSU), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (GSFC/UMBC),
E. E. Fenimore (LANL), N. Gehrels (GSFC), D. Grupe (PSU),
H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC/UMD),
D. M. Palmer (LANL), A. M. Parsons (GSFC), T. Sakamoto (GSFC/UMBC),
G. Sato (ISAS), J. Tueller (GSFC), T. N. Ukwatta (GWU)
(i.e. the Swift-BAT team):

Using the data set from T-239 to T+963 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 090927 (trigger #370846)
(Grupe, et al., GCN Circ. 9945).  The BAT ground-calculated position is
RA, Dec = 343.929, -70.973 deg which is
    RA(J2000)  =  22h 55m 42.9s
    Dec(J2000) = -70d 58' 24.2"
with an uncertainty of 1.6 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 55%.

The mask-weighted light curve shows a single irregular peak with an
overall roughly FRED shape.  T90 (15-350 keV) is 2.2 +- 0.4 sec 
(estimated error including systematics).

The time-averaged spectrum from T+0.0 to T+2.4 sec is best fit by a
simple power-law model.  The power law index of the time-averaged
spectrum is 1.80 +- 0.20.  The fluence in the 15-150 keV band is
2.0 +- 0.3 x 10^-07 erg/cm2.  The 1-sec peak photon flux measured from
T-0.01 sec in the 15-150 keV band is 2.0 +- 0.2 ph/cm2/sec.  All the
quoted errors are at the 90% confidence level.

The classification of this burst as short or long is not clear.  The
burst appears to have significant lag between energy bands, and
moreover is not particularly hard.  We expect to perform a more
quantitative lag analysis and report the results in an additional
circular.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/370846/BA/
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