GCN Circular 7705

D. A. Kann, C. Hoegner, R. Filgas and S. Klose (TLS Tautenburg) report:

We observed again the location of the candidate afterglow (Kann et al., 
GCN 7701
Not found
 
 
) of the AGILE GRB 080507 (Lapshov et al., GCN 7697
Not found
 
 
), which was 
also detected as a bright X-ray source (Mangano et al., GCN 7699
Not found
 
 
). We 
obtained 20 x 180 sec images in the R band under excellent atmospheric 
conditions but with elevated sky background due to twilight and the waxing 
moon. The resulting stacked image is roughly half a magnitude deeper than 
the image from the first epoch (23.5 vs. 23, respectively).

We find that the candidate afterglow has significantly faded over the 
course of a day. Using the same comparison star as Kann et al. (GCN 7701
Not found
 
 
), 
we find:

time (days)	Rc	dRc

2.5578		22.69	0.15

We also use SExtractor (Bertin & Arnouts 1996) to derive magnitudes, and 
find Rc = 21.95 +/- 0.14 and Rc = 22.65 +/- 0.20 in the first and second 
epoch, respectively, in accordance with our other results.

This fading behaviour is in contrast to the plateau phase or even rising 
behaviour reported from UVOT White images (Breeveld & Baumgartner, GCN 
7702
Not found
 
 
), which may imply a non-monotonic light curve evolution.

With the clear detection of fading, we consider the identification of this 
source as the optical afterglow of GRB 080507 as secure.

Finally, we note that GRB 080507 was also detected by INTEGRAL SPI-ACS, 
showing a flat-topped structure, with possible multiple peaks, of 40 
seconds duration, in full agreement with the AGILE report (Lapshov et al., 
GCN 7697
Not found
 
 
). The public light curve can be found here:

http://isdc.unige.ch/cgi-bin/cgiwrap/~beck/ibas/spiacs/ibas_acs_web.cgi/
?trigger=2008-05-07T07-44-58.8545-18523-00007-0

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