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GCN Circular 9912

Subject
GRB 090915 Swift-BAT detection of a burst in ground analysis
Date
2009-09-16T03:11:44Z (14 years ago)
From
Jay R. Cummings at NASA/GSFC/Swift <james.r.cummings@nasa.gov>
J. R. Cummings (NASA/GSFC/CRESST) and
C. B. Markwardt (NASA/GSFC/CRESST)
on behalf of the Swift-BAT team

At 15:35:36 Swift-BAT triggered on GRB 090915. This was
probably the same event as Fermi-GBM trigger 274721737. No
source was found onboard. A weak but significant source was
found in ground analysis, 15 degrees from the GBM ground-
calculated (2nd version) position as reported at
http://gcn.gsfc.nasa.gov/fermi_grbs.html. The GBM position
was also in the BAT FOV.

The BAT ground-calculated position is
RA, Dec = 237.990, +15.480 deg which is
    RA(J2000)  =  15h 51m 57.6s
    Dec(J2000) = +15d 28' 48"
with an uncertainty of 3 arcmin, (radius, sys+stat, 90%
containment). The partial coding was 12%.

The mask-weighted light curve shows a single weak peak.
T90 (15-350 keV) is 8 +- 2 sec
(estimated error including systematics).

The time-averaged spectrum from T-3 to T+5 sec is best fit
by a power law. This fit gives a photon index 1.0 +- 0.2.
For this model the total fluence in the 15-150 keV band
is 5 +- 2 x 10-7 erg/cm2 and the 1-sec peak flux measured
from T+2 sec in the 15-150 keV band is 0.8 +- 0.3 ph/cm2/sec.
All the quoted errors are at the 90% confidence level.

A Swift TOO has been requested and approved. The narrow-
field instruments will observe the location approximately
13 hours after the burst.
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