[vsnet-grb-info 7504] GRB 090323: Further TLS detections - a break?

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  9063
SUBJECT: GRB 090323: Further TLS detections - a break?
DATE:    09/04/01 02:22:46 GMT
FROM:    Alexander Kann at TLS Tautenburg  <kann at tls-tautenburg.de>

D. A. Kann, U. Laux, F. Ludwig and S. Stecklum (TLS Tautenburg) report:

We observed the afterglow (Updike et al., GCN 9026, Cenko et al. GCN 9027) 
of the intense Fermi GBM/LAT GRB 090323 (Ohno et al., GCN 9021) with the 
1.34m Schmidt telescope of the TLS Tautenburg observatory at several 
epochs.

At ~6 days after the GRB, conditions were bad (low transparency and 
passing clouds). We obtained 21 Rc frames of 600 seconds exposure time 
each. Only four of these are usable. The afterglow is faintly detected in 
the complete stack.

Using the same comparison star as Kann et al. (GCN 9033), we measure the 
following afterglow magnitude:

days after trigger	Exposure	Rc	dRc (statistical)

5.89127			4 x 600		23.13	0.50

At ~7 days after the GRB, conditions were very bad (almost complete 
overcast). We obtained 1 Rc frames of 600 seconds exposure time which had 
good quality. The afterglow is not detected.

days after trigger	Exposure	Rc

6.99881			1 x 600		> 22.50

This limit is not constraining.

At ~9 days after the GRB, conditions were very good (good transparency, 
good seeing, but influence of moonlight). We obtained 12 Rc frames of 600 
seconds exposure time each. The afterglow is faintly detected in the 
complete stack.

days after trigger	Exposure	Rc	dRc (statistical)

8.90324			12 x 600	23.64	0.36

Compared to the flattening of the decay noted by de Ugarte Postigo et al. 
(GCN 9051) and already hinted at in earlier TLS and Lick data (Kann et 
al., GCN 9041, Perley, GCN 9042), the decay has steepened again, an 
indication of a (jet?) break. On the other hand, the last TLS detection 
agrees well with the extrapolation of the earlier decay (Kann et al., GCN 
9041), indicating that the plateau may be a rebrightening/optical flare 
spanning a few days.

Due to increasing moonlight, no further TLS observations are planned.

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