TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1369 SUBJECT: GRB020405: predicted break DATE: 02/04/13 16:03:45 GMT FROM: Daniele Malesani at Obs.Astro. di Brera D. Malesani, G. Ghisellini, S. Covino (Brera Observ., Milan) and D. Lazzati (IoA Cambridge, UK) report: According to the idea of Frail et al. (2001, ApJ, 562, L55) that the jet energies of GRBs are universal, an achromatic jet break in the afterglow lightcurve is predicted for all bursts at the time t_jet = 20.4 (1+z) / (E_gamma/1e52) days, where E_gamma is the observed (i.e. isotropic) energy of the prompt emission. This result is independent of the unknown efficiency and circumburst density. For GRB020405, with z=0.69 (GCN 1330 and GCN 1340) and E_gamma>4.3e52 erg (assuming H_0=65, Omega_m=0.3, Omega_Lambda=0.7 and taking the fluence to be larger than the reported value in the 25-100 keV band: 3e-5 erg/cm2, GCN 1325), we have t_jet<8 days. Hence the break should have occured before April 13. Our V-band data (GCN 1337, 1345) connect very well with a previous measurement by Palazzi et al. (GCN 1328), who use the same calibration stars. The decay slope is hence very well defined (1.52+-0.04), making easier the possible detection of the predicted break (see the lightcurve at: http://www.merate.mi.astro.it/~gabriele/020405.html ). Since both the detection or the non-detection of the break would be very important to constrain Frail et al. idea, we urge photometric observations of the afterglow of this burst. This message can be cited.