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 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. This message may be cited.