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GCN Circular 14392

Subject
GRB 130418A, Swift-BAT refined analysis
Date
2013-04-19T13:49:19Z (11 years ago)
From
Hans Krimm at NASA-GSFC <hans.krimm@nasa.gov>
J. R. Cummings (GSFC/UMBC), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), M. De Pasquale (UCL-MSSL), N. Gehrels (GSFC),
H. A. Krimm (GSFC/USRA), A. Y. Lien (NASA/GSFC/ORAU),C. B. Markwardt (GSFC),
D. M. Palmer (LANL), T. Sakamoto (AGU), M. Stamatikos (OSU),
T. N. Ukwatta (MSU) (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 130418A (trigger #553847)
(De Pasquale, et al., GCN Circ. 14377). The BAT ground-calculated position is
RA, Dec = 149.045, 13.674 deg which is
    RA(J2000)  =  09h 56m 10.8s
    Dec(J2000) = +13d 40' 28.2"
with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 89%.

It appears that BAT triggered on the decay phase of a burst already in progress
when the source position came into the BAT field of view during a pre-planned 
slew. Therefore we do not know the true time of the start of the burst, but it 
was at least 50 seconds before T0, here defined as the trigger time. From T-50 
sec, when the burst became visible, there is a steady decline punctuated by two 
peaks at approximately T-20 and T+50, the first being harder than the second. 
The decay continued to roughly T+300 sec.  The burst location was no longer 
visible after T+450, when another slew took it out of the BAT field. T90 cannot
be determined since BAT did not see the start of the burst, but T90 is at least
300 seconds.

The time-averaged spectrum from T-40.16 to T+285.59 sec is best fit by a simple
power-law model.  The power law index of the time-averaged spectrum is
2.07 +- 0.17.  The fluence in the 15-150 keV band is 1.8 +- 0.2 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T-37.68 sec in the 15-150 keV band
is 0.6 +- 0.2 ph/cm2/sec.  All the quoted errors are at the 90% confidence
level.  Note that these values are calculated only for the part of the burst
detected by the BAT, so the fluence should be considered a lower limit.

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/553847/BA/
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