TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3699 SUBJECT: GRB 050724: WHT optical observations DATE: 05/07/29 17:58:10 GMT FROM: Klaas Wiersema at GRACE/U of Amsterdam 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) 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 and 3694, as well as the radio (GCN 3684) and X-ray afterglow position (GCN 3683, 3697). 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.