[vsnet-grb-info 6071] GRB 080507: E Pur Si Muove!

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  7705
SUBJECT: GRB 080507: E Pur Si Muove!
DATE:    08/05/09 23:48:40 GMT
FROM:    Alexander Kann at TLS Tautenburg  <kann at tls-tautenburg.de>

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) of the AGILE GRB 080507 (Lapshov et al., GCN 7697), which was 
also detected as a bright X-ray source (Mangano et al., GCN 7699). 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), 
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), 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). 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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