TITLE:   GCN CIRCULAR
NUMBER:  12046
SUBJECT: GRB 110530A: Swift detection of a burst with an optical afterglow
DATE:    11/05/30 15:49:56 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

P. D'Avanzo (INAF-OAB), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), S. Campana (INAF-OAB),
V. D'Elia (ASDC), J. M. Gelbord (PSU), B. Gendre (ASDC),
C. Gronwall (PSU), S. T. Holland (CRESST/USRA/GSFC),
J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL),
V. Mangano (INAF-IASFPA), F. E. Marshall (NASA/GSFC),
A. Melandri (INAF-OAB), P. T. O\'Brien (U Leicester),
K. L. Page (U Leicester), D. M. Palmer (LANL),
P. Romano (INAF-IASFPA), B. Sbarufatti (INAF-OAB/IASFPA),
M. H. Siegel (PSU), C. A. Swenson (PSU) and T. N. Ukwatta (MSU) report
on behalf of the Swift Team:

At 15:31:02 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 110530A (trigger=454473).  Swift slewed after 11 minutes
because of an observing constraint. 
The BAT on-board calculated location is 
RA, Dec 282.045, +61.932, which is 
   RA(J2000)  =  18h 48m 11s
   Dec(J2000) = +61d 55' 54"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single peak
with a duration of about 20 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 15:38:16.7 UT, 434.0 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 282.06718, 61.92861 which is equivalent
to:
   RA(J2000)  = 18h 48m 16.12s
   Dec(J2000) = +61d 55' 43.0"
with an uncertainty of 3.9 arcseconds (radius, 90% containment). This
location is 39 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 5.65
x 10^20 cm^-2 (Kalberla et al. 2005). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 437 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	18:48:16.35 = 282.06814
  DEC(J2000) = +61:55:45.1  =  61.92920
with a 90%-confidence error radius of about 0.71 arc sec. This position is 2.8
arc sec. from the center of the XRT error circle. The estimated magnitude is
20.31 with a 1-sigma error of about  0.18. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.06. 

Burst Advocate for this burst is P. D'Avanzo (paolo.davanzo AT brera.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.)