TITLE:   GCN CIRCULAR
NUMBER:  14460
SUBJECT: GRB 130427B: Swift detection of a burst with optical counterpart
DATE:    13/04/27 13:31:58 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

A. Maselli (INAF-IASFPA), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU),
N. Gehrels (NASA/GSFC), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL),
A. Y. Lien (NASA/GSFC/ORAU), V. Mangano (INAF-IASFPA),
C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC),
C. J. Mountford (U Leicester), D. M. Palmer (LANL) and
M. H. Siegel (PSU) report on behalf of the Swift Team:

At 13:20:41 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 130427B (trigger=554635).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 314.891, -22.530 which is 
   RA(J2000)  =  20h 59m 34s
   Dec(J2000) = -22d 31' 49"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows 3 peaks
with a total duration of about 40 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 13:21:59.1 UT, 77.4 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 314.89766, -22.54548 which
is equivalent to:
   RA(J2000)  = 20h 59m 35.44s
   Dec(J2000) = -22d 32' 43.7"
with an uncertainty of 4.4 arcseconds (radius, 90% containment). This
location is 59 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.  We
cannot determine whether the source is fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 4.41
x 10^20 cm^-2 (Kalberla et al. 2005). 

The initial flux in the 2.5 s image was 1.15e-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. There is a candidate afterglow in
the list of sources generated on-board at
  RA(J2000)  =	20:59:35.64 = 314.89848
  DEC(J2000) = -22:32:46.5  = -22.54626
with a 90%-confidence error radius of about 1.10 arc sec. This position is 7.6
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.19. No correction has been made for the expected extinction corresponding to
E(B-V) of 0.07. 

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.)