TITLE:   GCN CIRCULAR
NUMBER:  3425
SUBJECT: XRF 050509c optical afterglow
DATE:    05/05/16 10:55:28 GMT
FROM:    Jens Hjorth at U.Copenhagen  <jens@astro.ku.dk>

J. Gorosabel (IAA-CSIC), B. L. Jensen (NBI), G. Galaz (Pont. Uni. de Chile),
R. Salinas (Uni. de Concepcion), J. Hjorth, J. P. U. Fynbo, H. Pedersen,
D. Watson, P. Jakobsson, J. M. Castro Cer�n (NBI), report:

"We have carried out R-band observations of XRF 050509c (HETE trigger #3751; 
GCN 3402) error box with the 1.54m Danish telescope (+DFOSC) on La Silla, 
as follows:

          ======================================
               Date UT     t-to   Texp  Seeing   
               May 2005    (hr)   (s)    (")     
          --------------------------------------
           10.1545-10.1703  5.1  2x600   1.2
           14.0490-14.0829 98.8  4x600   1.3
          ======================================

Both observations are centred in the SXC error box covering ~80% of it. 
Comparison of the two epoch images reveals, well centred in the SXC error 
box, a fading point-like object with coordinates:

     RA(J2000)   =  12:52:53.94 
     Dec(J2000)  = -44:50:04.1   (internal astrometric error of 1")

which we propose as the XRF 050509c optical afterglow.

Assuming R = 17.9 for the USNO star placed at (12:52:52.34, -44:49:23.1)
we estimate a decay from R~20.1 to R~24.0  between the two epochs. This
would imply a power-law decay index of -1.2 typical of GRB afterglows.

Finding charts can be found at:

http://www.dsri.dk/~jgu/grb050509c/FCs/grb050509c.1.54dk.epoch1.gif
http://www.dsri.dk/~jgu/grb050509c/FCs/grb050509c.1.54dk.epoch2.gif

Further observations are planned."