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GCN Circular 10612

Subject
GRB 100418A: Swift detection of a burst with a possible optical afterglow
Date
2010-04-18T21:27:58Z (14 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
F. E. Marshall (NASA/GSFC), A. P. Beardmore (U Leicester),
J. M. Gelbord (PSU), S. T. Holland (CRESST/USRA/GSFC),
E. A. Hoversten (PSU), J. A. Kennea (PSU),
O. M. Littlejohns (U Leicester), C. B. Markwardt (CRESST/GSFC/UMD),
P. T. O'Brien (U Leicester), C. Pagani (U Leicester),
K. L. Page (U Leicester), D. M. Palmer (LANL),
A. Rowlinson (U Leicester), M. C. Stroh (PSU),
T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU) report on behalf of the
Swift Team:

At 21:10:08 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 100418A (trigger=419797).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 256.376, +11.464 which is 
   RA(J2000) = 17h 05m 30s
   Dec(J2000) = +11d 27' 50"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 4 sec.  The peak count rate
was ~700 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 21:11:27.1 UT, 79.1 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
256.3635, 11.4619 which is equivalent to:
   RA(J2000)  = 17h 05m 27.24s
   Dec(J2000) = +11d 27' 42.7"
with an uncertainty of 3.1 arcseconds (radius, 90% containment). This
location is 44 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of
4.78e+20 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 1.27e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White   
filter  starting 87 seconds after the BAT trigger. A possible   
afterglow candidate has  been found in the initial data products at   
the location of the XRT afterglow.  The preliminary UVOT position is  

   
    RA(J2000.0) =  17:05:26.96  =  256.36233 (deg)   
   Dec(J2000.0) = +11:27:41.9   =  +11.46164 (deg)

with an  estimated uncertainty of 1.0 arcsec (radius, 90% confidence,  
statistical + systematic).  The estimated white magnitude is  20.7 +/-
0.3.  No correction has  been made for the expected extinction  
corresponding to E(B-V) of   0.07. 

Burst Advocate for this burst is F. E. Marshall (marshall AT milkyway.gsfc.nasa.gov). 
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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