GCN Circular 3699

K. Wiersema (U. of Amsterdam), E. Rol (U. of Leicester),
R. Starling (U. of Amsterdam), N. Tanvir (U. of Hertfordshire),
D. S. Bloomfield, H. Thompson (Queen's University Belfast)

report:

We have observed the position of the short burst
GRB 050724 (Covino et al. GCN 3665
Not found
 
 
) with the William Herschel
Telescope at the Observatorio del Roque de los Muchachos on
La Palma, using the Aux port Imager.

We observed two epochs, with midpoints 0.424 days and 3.405
after burst. The epoch 1 observations consisted of
4 x 15 + 5 minutes exposure time in R band. Epoch 2
consisted of 4 x 15 minutes in R band.
At the first epoch the weather conditions were good with an
average seeing of 0.8 arcsec. During the second epoch the seeing
was considerably worse, with an average seeing ~1.3 arcsec.
PSF-matched image subtraction of the two epochs using the
ISIS code (Alard et al. 1999) reveals a clearly fading source.
This behaviour is confirmed in a subtraction of the first and
last image from the first epoch. The position of the fading
source matches that described in e.g. GCNs 3690
Not found
 
 
 and 3694
Not found
 
 
,
as well as the radio (GCN 3684
Not found
 
 
) and X-ray afterglow position
(GCN 3683
Not found
 
 
, 3697
Not found
 
 
).

We performed aperture photometry of the host galaxy plus the OT
with respect to two unsaturated USNO stars: 0624-0502999 and
0624-0503050, whose values we take from the USNO-B catalogue.
We find a magnitude difference of approximately 0.2 magnitudes
between the two epochs.
Assuming the contribution of the afterglow to the total flux
at the second epoch to be negligible, we estimate the afterglow to
be approximately R ~22.1 at our first epoch.

A jpg image showing the position of the afterglow and the host
can be found on:
http://remote.science.uva.nl/~kwrsema/grb050724/

We thank the staff of the WHT for outstanding support for these
observations.