TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3690 SUBJECT: GRB050724: VLT observations of the variable source DATE: 05/07/27 09:01:59 GMT FROM: Paolo D'Avanzo at INAF-OAB P. D'Avanzo, S. Covino (INAF/OABr), L.A. Antonelli, A. Melandri (INAF/OAR), D. Malesani (SISSA), A. Moretti, P. Romano, G. Tagliaferri (INAF/OAB), S. Piranomonte (INAF/OAR), G. Chincarini (Univ. Milano- Bicocca), and L. Stella (INAF/OAR) report on behalf of the MISTICI collaboration We observed the afterglow of GRB050724 (Covino et al., GCN 3665) with the ESO-VLT equipped with the FORS1 camera, starting on 2005 Jul 25.01 and Jul 25.98 (12 and 35 hours after the burst respectively). The seeing was 1.0" and 0.8" in the two epochs respectively. Observations were carried out in the R and I filters. We clearly detect all the sources listed by Bloom et al. (GCN 3672) and Castro-Tirado et al. (GCN 3673). PSF-matched image subtraction was performed with the ISIS package (Alard & Lupton, 1998). A highly significant variable source is detected superimposed to the galaxy named as source "D" in GCN 3672 (for a figure see http://www.merate.mi.astro.it/~covino/GRB050724). The source is slightly offset with respect to the galaxy center. We give below the 0.2" uncertainty position of the galaxy and the OT: Galaxy centroid: 16:24:44.405 -27:32:26.95 OT centroid: 16:24:44.400 -27:32:27.90 Assuming the object has completely disappeared in our second epoch, the magnitude of the variable source was I = 20.85 +/- 0.05 at our first epoch. This value was derived by adding artificial stars of known brightness to the reference frame, calibrated with Landolt standard stars. This source faded between the two epochs in both the R and I bands. Therefore, we confirm the variability reported by Gal-Yam et al. (GCN 3681). However, our source appears (in both the R and I bands) 1" South with respect to the core of the host galaxy. This is at variance with the picture posted by Gal-Yam et al. (GCN 3681), where the variable object appears North of the host galaxy centroid. After the rebrightening episode reported by them, the source has therefore clearly faded. This message may be cited.