GCN Circular 7845

D. A. Kann, U. Laux and S. Ertel (TLS Tautenburg) report:

We continue observations of the afterglow of GRB 080605 (independently 
discovered by Sbarufatti et al., GCN 7828
Not found
 
 
, Kann et al., GCN 7829
Not found
 
 
, and 
Gomboc et al., GCN 7831
Not found
 
 
). We acquired one 300 sec image each in V, R, and 
I around 0.95 days after the GRB, and further imaging 0.1 days later until 
dawn. Astonishingly, the afterglow is still clearly visible in R and I 
bands.

As mentioned in the above-cited GCNs, the afterglow is close to a star and 
affected by its PSF in images with low-quality seeing or large pixel 
scale, such as the TLS camera. To obtain a qualitative description of the 
afterglow evolution, we perform aperature photometry in a 3 pixel 
aperature, and derive the aperature correction for each image by comparing 
the counts for a reference star in the 3 pixel aperature with that in a 7 
pixel (which is typically seeing-matched) aperature. As a reference star, 
we use the star to the south of the afterglow, USNOB1.0 ID 0940-0289655 at 
(catalog position): RA = 17:28:29.920, Dec. = +04:00:36.51, which has R2 = 
15.90 mag, I = 14.54 mag. We derive the following magnitudes for selected 
images in the I band (the error is estimated to be 0.1 magnitudes in all 
cases):

dt		I

0.00478791	16.74
0.00674681	17.18
0.00869923	17.36
0.01064872	17.61
0.01849396	18.19
0.02433042	18.19
0.05922051	18.48
0.96109933	18.51
1.05275325	18.71

The early afterglow decay (0.00479 to 0.01849 days) is well-described by a 
power-law with decay slope alpha=0.97. After this, the afterglow decay 
seems to flatten. Between 0.06 days and 0.95 days, the afterglow magnitude 
remains unchanged, indicating a plateau phase. As the afterglow has 
resumed its decay 0.1 days later, this probably implies that the 
afterglow underwent a strong rebrightening which we caught after the 
peak. This is reminiscent of GRB 071003, which lies at a very similar 
redshift (Perley et al. 2008, ApJ, submitted, arXiv:0805.2394) of ~ 1.6 
(Jakobsson et al., GCN 7832
Not found
 
 
).

Furthermore, we determined the R band magnitude at two epochs:

dt		R

0.01655957	19.87
0.95457023	20.75

Corrected for the moderate Galactic foreground extinction of E(B-V) = 
0.136, we find R - I ~ 2 mag. This is a very red color for an afterglow, 
indicating strong internal reddening due to dust. We caution that the 
USNOB1.0 comparison star may have incorrect photometry, on the other hand, 
a simple image comparison shows that the R band magnitude is much fainter 
than the I band magnitude.

The strong rebrightening combined with the large correction for dust 
extinction that is needed imply that this is an intrinsically extremely 
luminous afterglow. Further follow-up, especially in the NIR bands, is 
highly encouraged.

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