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GCN Circular 14377

Subject
GRB 130418A: Swift detection of a burst with an optical counterpart
Date
2013-04-18T19:16:51Z (11 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
M. De Pasquale (UCL-MSSL), W. H. Baumgartner (GSFC/UMBC),
A. P. Beardmore (U Leicester), M. M. Chester (PSU),
S. T. Holland (STScI), J. A. Kennea (PSU),
H. A. Krimm (CRESST/GSFC/USRA), N. P. M. Kuin (UCL-MSSL),
C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC),
A. Maselli (INAF-IASFPA), P. T. O'Brien (U Leicester),
K. L. Page (U Leicester), D. M. Palmer (LANL),
B. Sbarufatti (INAF-OAB/PSU), E. Sonbas (NASA/GSFC/Adiyaman Univ.),
M. C. Stroh (PSU), C. A. Swenson (PSU) and E. Troja (NASA/GSFC/UMCP)
report on behalf of the Swift Team:

At 19:00:53 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 130418A (trigger=553847).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 149.043, +13.649 which is 
   RA(J2000) = 09h 56m 10s
   Dec(J2000) = +13d 38' 58"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is usual with image triggers, the BAT light 
curve shows no significant features. 

The XRT began observing the field at 19:03:03.2 UT, 129.7 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 149.0377, 13.6654 which is equivalent to:
   RA(J2000)  = +09h 56m 9.05s
   Dec(J2000) = +13d 39' 55.4"
with an uncertainty of 5.3 arcseconds (radius, 90% containment). This
location is 61 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 138 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	09:56:08.92 = 149.03718
  DEC(J2000) = +13:40:02.8  =  13.66744
with a 90%-confidence error radius of about 0.74 arc sec. This position is 7.6
arc sec. from the center of the XRT error circle. The estimated magnitude is
14.96 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.03. 

This source was detected on the first image trigger after it came
within the BAT's field of view, so its start time may be significantly
before the nominal trigger time.  There is no significant fading
in the XRT over the ~300 second XRT observation time.  Thus
it is likely that this is a very long duration event.  The position
of the source is 47 degrees from the Galactic plane, but that
doesn't preclude a nearby source.  Further determination of the
characteristics of the source will await further data analysis. 

Burst Advocate for this burst is M. De Pasquale (m.depasquale AT ucl.ac.uk). 
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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