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GCN Circular 7845

Subject
GRB 080605: TLS RRM Analysis, Plateau/Rebrightening, Red OT
Date
2008-06-07T02:27:14Z (16 years ago)
From
Alexander Kann at TLS Tautenburg <kann@tls-tautenburg.de>
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, Kann et al., GCN 7829, and 
Gomboc et al., GCN 7831). 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).

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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