[vsnet-grb-info 7604] GRB 090418B: Detection of a bright burst from Swift-BAT slew data

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9159
SUBJECT: GRB 090418B: Detection of a bright burst from Swift-BAT slew data
DATE:    09/04/18 22:19:33 GMT
FROM:    Antonio Copete at Harvard U  <acopete at head.cfa.harvard.edu>

A. Copete, J. Grindlay (Harvard)
S. Barthelmy, C. Markwardt, N. Gehrels (NASA/GSFC)

We report the detection of a bright GRB by the BAT Slew Survey (BATSS)  
in the slew that started at 08:59:02 and lasted 151 sec. The ground- 
calculated position is RA, Dec = 225.910, +17.224 deg, which is
RA (J2000) = 15h 03m 38s
Dec(J2000) = +17d 13' 26"
with an uncertainty of 1.91 arcmin (90% confidence, including  
systematics). The detection was triggered by simultaneous independent  
detections of 86.0 sigma and 41.8 sigma from imaging in the 15-50 keV  
and 50-150 keV energy bands, respectively. The burst mask-tagged  
lightcurve lightcurve in the 15-150 keV band shows a strong and broad  
multi-peaked emission structure starting at T+25sec as the burst  
enters the BAT field-of-view, lasting 48 sec, and peaking 6 times at T 
+29, T+35, T+38, T+47, T+58 and T+62sec. This is followed by a weaker  
broad peak starting at T+87sec, lasting 28sec and peaking at T+100sec.  
T90 = 65.0sec and T50 = 17.0sec.

The time-averaged spectrum over the 120 sec exposure time of the  
source is best fit by a power law with an exponential cutoff, with  
photon index 1.42 and high-energy cut at 319 keV. The fluence in the  
15-150 keV band is 1.6 x 10^-5 erg/cm^2, and the peak 1-sec flux is  
6.5 x 10^-7 erg/cm^2/sec at T+38sec. A follow-up Swift ToO observation  
for this burst has been scheduled.