[vsnet-grb-info 7895] GRB 090530: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9438
SUBJECT: GRB 090530: Swift detection of a burst with an optical counterpart
DATE:    09/05/30 03:30:01 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at lheamail.gsfc.nasa.gov>

J. K. Cannizzo (NASA/UMBC), S. D. Barthelmy (GSFC),
W. H. Baumgartner (GSFC/UMBC), J. R. Cummings (NASA/UMBC),
C. Gronwall (PSU), S. T. Holland (CRESST/USRA/GSFC),
V. Mangano (INAF-IASFPA), C. B. Markwardt (CRESST/GSFC/UMD),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL),
T. Sakamoto (NASA/UMBC), B. Sbarufatti (INAF-IASFPA),
M. H. Siegel (PSU), M. Stamatikos (NASA/ORAU) and
T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team:

At 03:18:18 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090530 (trigger=353567).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 179.395, +26.584 which is 
   RA(J2000) = 11h 57m 35s
   Dec(J2000) = +26d 35' 01"
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 10 sec.  The peak count rate
was ~5000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 03:19:33.1 UT, 74.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 179.41927,
26.59282 which is equivalent to:
   RA(J2000)  = 11h 57m 40.62s
   Dec(J2000) = +26d 35' 34.2"
with an uncertainty of 3.9 arcseconds (radius, 90% containment). This
location is 84 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 in excess of the Galactic value (1.78e+20
cm^-2, Kalberla et al. 2005), with an excess column of 2.1
(+2.01/-1.71) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 6.42e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 83 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	11:57:40.50 = 179.41876
  DEC(J2000) = +26:35:38.6  = 26.59405
with a 90%-confidence error radius of about 0.75 arc sec. This position is 4.7
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.39 with a 1-sigma error of about 0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.02. 

Burst Advocate for this burst is J. K. Cannizzo (cannizzo 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.)