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GCN Circular 7972

Subject
GRB 080710: TLS observations, steepening afterglow decay
Date
2008-07-11T14:14:53Z (16 years ago)
From
Alexander Kann at TLS Tautenburg <kann@tls-tautenburg.de>
S. Schulze, D. A. Kann, A. Rossi, E. Gonsalves, C. Hoegner and B. Stecklum 
(TLS Tautenburg) report:

We observed the optical afterglow location (Li et al., GCN 7959) of Swift 
GRB 080710 (Sbarufatti et al., GCN 7957) with the TLS 1.34m Schmidt 
telescope under inclement but improving conditions. 600 second B and V 
observations were affected by passing clouds and yielded no detections and 
shallow upper limits only. We detect the afterglow clearly in a single Rc 
image as well as in four Ic images (600 seconds each) before dawn shut us 
down. We measure afterglow magnitudes against eight USNOB1.0 stars in each 
case:

time (days)	filter	magnitude	exposure

0.7275		Rc	20.05 +/- 0.12	600

0.7468		Ic	19.75 +/- 0.13	4 x 600

In comparison with the magnitude as well as the slope reported from the 
Faulkes Telescope North (Bersier et al., GCN 7963) (R = 17.8 at 4.4h, 
alpha = 0.82), our measurement implies a significant steepening of the 
decay, we find alpha = 1.5 between 4.4 and 17.5 hours after the GRB. This 
implies that a break must have occurred inbetween, and possibly the slope 
during the time of our observations is already > 2 and the break is a jet 
break.

Using the redshift of 0.845 (Perley et al., GCN 7962) as well as the 
values derived in the BAT refined analysis (Tueller et al., GCN 7969) and 
estimating the peak energy following equation 3 of Liang et al. (2007, 
ApJ, 670, 565), we derive a bolometric isotropic energy release of 6.8 
+2.0 -1.9 x 10^51 erg. If there is a jet break before 17 hours, this would 
imply a low collimation-corrected energy release.

No further observations from TLS are possible due to an instrument change.

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