TITLE: GCN CIRCULAR NUMBER: 7845 SUBJECT: GRB 080605: TLS RRM Analysis, Plateau/Rebrightening, Red OT DATE: 08/06/07 02:27:14 GMT FROM: Alexander Kann at TLS Tautenburg 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. This message may be cited.