TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  2788
SUBJECT: GRB041006: break in the light curve
DATE:    04/10/08 18:34:22 GMT
FROM:    Daniele Malesani at SISSA-ISAS,Trieste,Italy  <malesani@sissa.it>

P. D'Avanzo, S. Covino, L.A. Antonelli, D. Fugazza, D. Malesani, F. 
Fiore, F. Cocchia, N. Masetti, E. Pian, L. Stella (on behalf of the 
Italian CIBO collaboration); V. Lorenzi and R. Barrena (INAF-TNG), report:

We observed again the optical afterglow (Da Costa, Noel & Price, GCN 
2765; Maeno et al. GCN 2772) of GRB 041006 (Galassi et al., GCN 2770) 
with TNG. Photometry was performed in the UBVRI bands under good 
observing conditions (seeing 1.2").

We measure the following magnitude for the afterglow on Oct 8.10 UT 
(1.59 days after the GRB):

R = 22.12 +/- 0.08

A comparison with our previous measurement (Fugazza et al., GCN 2782) 
implies a dimming of 1.36 +- 0.09 mag. The corresponding decay index is 
alpha = 1.20 +- 0.08 (F = K t^-alpha), significantly steeper than the 
early-time value (alpha ~ 0.7: Fox, GCN 2768; Price, Da Costa & Noel, 
GCN 2771; Yost et al., GCN 2776). A break in the light curve must 
therefore be present, consistent with the suggestion of Kahharov et al. 
(GCN 2775). The fading is confirmed in the other bands and is achromatic 
within the errors.

We also observed again Landolt standard stars, finding the same 
zeropoint than in our previous observation (Fugazza et al., GCN 2782) to 
within 0.03 mag. We conclude that our calibration is correct and both 
nights were photometric.

The light curve of this event, including all the published photometry, 
can be found at the following URL:

http://www.sissa.it/~malesani/GRB/041006

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