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GCN Circular 14448

Subject
GRB 130427A: Swift detection of a very bright burst with a likely bright optical counterpart
Date
2013-04-27T08:24:13Z (11 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
A. Maselli (INAF-IASFPA), A. P. Beardmore (U Leicester),
A. Y. Lien (NASA/GSFC/ORAU), V. Mangano (INAF-IASFPA),
C. J. Mountford (U Leicester), K. L. Page (U Leicester),
D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the
Swift Team:

At 07:47:57 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 130427A (trigger=554620).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 173.139, +27.692 which is 
   RA(J2000) = 11h 32m 33s
   Dec(J2000) = +27d 41' 29"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows an extremely bright
complex peak about 20 seconds long starting at T-50, while Swift was 
slewing from the previous pre-planned target, followed by a smaller 
peak during the slew to the burst with emission through at least T+200. 
The peak count rate was ~ 100,000 counts/sec (15-350 keV), 
at T-40 sec, before the trigger. 

The XRT began observing the field at 07:50:17.7 UT, 140.2 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 173.1362, 27.7129 which is equivalent to:
   RA(J2000)  = +11h 32m 32.69s
   Dec(J2000) = +27d 42' 46.4"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 75 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 147 seconds after the BAT trigger.  A blurred bright source
appears to be located near the XRT position in the initial 2.7'x2.7'
sub-image.  However, the lack of reference stars and lack of star tracker
lock prevents a definitive identification and the measurement of a position
or magnitude. 

This is an extremely bright burst in all three instruments,
and it was also seen by Fermi/GBM. 

After the slew to the burst, the star trackers had trouble
locking on to give a spacecraft attitude, so the XRT position
may be inaccurate at the arcminute level. 

The apparent optical counterpart appears extremely blurred, 
possibly due to the lack of star tracker lock, but more likely to be 
due to instrumental effects on a very bright optical counterpart. 


Further follow-up is warranted for all ground-based observatories. 

Burst Advocate for this burst is A. Maselli (maselli AT ifc.inaf.it). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)
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