[vsnet-grb-info 7971] GRB 090618: Swift detection of a bright burst with optical afterglow

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9512
SUBJECT: GRB 090618: Swift detection of a bright burst with optical afterglow
DATE:    09/06/18 08:48:39 GMT
FROM:    Jamie A. Kennea at PSU/Swift-XRT  <kennea at astro.psu.edu>

P. Schady (MSSL-UCL), W. H. Baumgartner (GSFC/UMBC),
A. P. Beardmore (U Leicester), S. Campana (INAF-OAB),
P.A. Curran (MSSL-UCL), C. Guidorzi (U Ferrara), J. A. Kennea (PSU),
J. Mao (INAF-OAB), R. Margutti (Univ Bicocca&OAB),
J. P. Osborne (U Leicester), K. L. Page (U Leicester),
P. Romano (INAF-IASFPA), M. H. Siegel (PSU), G. Stratta (ASDC) and
T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team:

At 08:28:29 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 090618 (trigger=355083).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 294.021, +78.353 which is 
   RA(J2000) = 19h 36m 05s
   Dec(J2000) = +78d 21' 13"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed multi peak
structure with a duration of about 130 sec.  The peak count rate
was ~40,000 counts/sec (15-350 keV), at 80 sec after the trigger. 

The XRT began observing the field at 08:30:30.8 UT, 120.9 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 293.9897, +78.3576 which is equivalent to:
   RA(J2000)  = 19h 35m 57.52s
   Dec(J2000) = +78d 21' 27.3"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 28 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

UVOT took a finding chart exposure of 150 seconds with the White   
filter  starting 128 seconds after the BAT trigger. There is a   
candidate afterglow in  the rapidly available 2.7'x2.7' sub-image at 

  RA(J2000)  =	19:35:58.69 = 293.99456    
  DEC(J2000) = +78:21:24.3  = 78.35676      with a 90%-confidence
error radius of about 0.74 arc sec. This position   is 4.6  arc sec. 
from the center of the XRT error circle. The estimated   magnitude is
 14.36 with  a 1-sigma error of about 0.01. No correction has   been
made for the expected extinction corresponding to E(B-V) of 0.09. 

Burst Advocate for this burst is P. Schady (ps AT mssl.ucl.ac.uk). 
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.)