[vsnet-grb-info 6236] GRB 080603B - TLS 2nd Epoch: Another Break

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  7865
SUBJECT: GRB 080603B - TLS 2nd Epoch: Another Break
DATE:    08/06/09 02:33:11 GMT
FROM:    Alexander Kann at TLS Tautenburg  <kann at tls-tautenburg.de>

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

We observed again the field of the Swift GRB 080603B (Mangano et al., GCN 
7794) with the TLS 1.34m Schmidt telescope. The observing conditions 
changed rapidly, from passing clouds and moonlight to excellent 
conditions. We obtained a total of 11 x 600 sec images in the R band. The 
first five images were lost due to high background as well as passing 
clouds. We stacked the final six images, which have a mid-time of June 
8.98085. The afterglow (Rujopakarn, Guver, & Smith, GCN 7792) is not 
detected anymore in the stack. Using the same comparison star as Kann et 
al. (GCN 7823), we derive a 2 sigma upper limit of R > 23.7 at 5.132644 
days after the GRB.

Comparing this value with the one determined at 1.16 days after the GRB by 
Kann et al. (GCN 7823) (R = 21.32), we find a decay slope alpha > 1.47. 
While this does not allow any assertion on the reality of the steep decay 
slope (alpha ~ 2.5) reported by Kann et al. (GCN 7823), it is steeper than 
the slope reported (alpha_R=-1.14+/-0.09) by Zhuchkov et al. (GCN 7803) 
and confirmed by Xin et al. (GCN 7814), indicating that another break in 
the light curve must have occurred, independent of calibration issues. As 
the slope reported by Zhuchkov et al. (GCN 7803) is much too shallow to be 
a post-jet break slope, this indicates that the second break is very 
probably a jet break. The slope reported by Kann et al. (GCN 7823) is in 
accordance with a post-jet break slope. This then leads to the interesting 
question of the origin of the first break Zhuchkov et al. detected, as it 
is much to strong to be a cooling break (Delta alpha_R = 0.55 versus the 
theoretical value of Delta alpha = 0.25 for a cooling break).

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