[vsnet-grb-info 506] GRB 021004: VRI observations at TNG: possible break

[GCN Circulars <gcncirc@lheawww.gsfc.nasa.gov>]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  1645
SUBJECT: GRB 021004: VRI observations at TNG: possible break  
DATE:    02/10/19 10:14:00 GMT
FROM:    Stefano Covino at Brera Astronomical Observatory  <covino@merate.mi.astro.it>

Malesani D., Stefanon M., Covino S., Ghisellini G. (INAF-OAB), Lazzati D., 
Rossi E. (IOA, Cambridge, UK), Fugazza D., Guerra J.C., Pedani M. (INAF-TNG), 
Masetti N. (IASF-CNR), Pian E. (INAF-OATs), on behalf of a larger Italian 
collaboration, report:


On Oct ~16.9 UT, we observed the optical counterpart of GRB 021004 (Shirasaki 
et al., GCN 1565; Fox, GCN 1564) using the 3.6m TNG telescope in the Canary 
Islands; observations were performed with V (20 min), R (9 min) and I (15 
min) filters under mediocre conditions (seeing 1.5-1.8 arcsec, thin clouds). 
With respect to the comparison star used by Fox and calibrated by Henden (GCN 
1583), the source has the following magnitudes:


UT      Band  Mag    Error
--------------------------
16.979  I     21.65  0.12
16.942  R     22.38  0.20
16.962  V     22.49  0.07


The object is quite faint in our images. Actually, it lies about ~0.7 mag 
below the powerlaw extrapolation of the previously published data (R band), 
if we include the three early-time data points of Fox (t < 20 min). If 
instead we let the powerlaw fit to begin ~2 hours after the trigger, there is 
no disagreement. A light curve with the relevant fits is reported at 
http://www.merate.mi.astro.it/~malesani/GRB/021004 .


If indeed a break has occured between Oct 10 and Oct 17 (6 to 13 days after 
GRB), this is in agreement with the prediction of GCN 1607.


Further observation are encouraged to assess the nature of this break.


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