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GCN Circular 6415

GRB 070518: Swift detection of a burst with optical afterglow
2007-05-18T14:50:19Z (17 years ago)
David Palmer at LANL <>
C. Guidorzi (Univ Bicocca&INAF-OAB), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), D. N. Burrows (PSU),
S. Campana (INAF-OAB), M. M. Chester (PSU),
G. Cusumano (INAF-IASFPA), P. A. Evans (U Leicester),
N. Gehrels (NASA/GSFC), C. Gronwall (PSU),
S. T. Holland (CRESST/GSFC/USRA), J. A. Kennea (PSU),
H. A. Krimm (GSFC/USRA), F. E. Marshall (NASA/GSFC),
A. Moretti (INAF-OAB), P. T. O'Brien (U Leicester),
D. M. Palmer (LANL), J. L. Racusin (PSU),
P. Romano (Univ. Bicocca & INAF-OAB), T. Sakamoto (NASA/ORAU),
G. Sato (GSFC/ISAS), B. Sbarufatti (INAF-IASFPA),
M. Stamatikos (NASA/ORAU), R. L. C. Starling (U Leicester),
E. Troja (INAF-IASFPA), D. E. Vanden Berk (PSU) and
S. D. Vergani (DIAS-DCU) report on behalf of the Swift Team:

At 14:26:21 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 070518 (trigger=279592). Swift slewed immediately to the
burst. The BAT on-board calculated location is  RA, Dec 254.223, +55.297 which 
   RA(J2000) = 16h 56m 54s   
   Dec(J2000) = +55d 17'47"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve shows a single
(FRED-like) peak with a duration of about 7 sec. The peak count rate
was ~1500 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 14:27:31 UT, 70 seconds after the
BAT trigger. XRT found a variable, uncatalogued X-ray source
located at RA, Dec 254.1979, +55.2974 which is
   RA(J2000)  =  16h 56m 47.5s
   Dec(J2000) =  55d 17' 42.5"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). 
This location is 52 arcseconds from the BAT on-board position,
within the BAT error circle. The initial flux in the 2.5s image
was 5.8e-10 erg/cm2/s (0.2-10 keV). 

UVOT took a finding chart exposure of 100 seconds with the White
(160-650 nm) filter starting 82 seconds after the BAT trigger.  We
see a fading afterglow located at RA, Dec 254.1986, 55.2951 which is   
  RA(J2000) = 16h 56m 47.7s   
  Dec(J2000) = 55d 17' 42.3" 
with an uncertainty of 1 arcsecond (radius, 90% containment)
with an estimated magnitude of 18. No correction
has been made for the expected extinction corresponding to E(B-V) of
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