[vsnet-grb-info 1850] GRB050724: VLT observations of the variable source

[GCN Circulars]

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  <davanzo at merate.mi.astro.it>

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.

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