TITLE: GCN CIRCULAR NUMBER: 20346 SUBJECT: GRB161219B - Anticipated hyperluminal evidence DATE: 16/12/29 07:49:28 GMT FROM: Arnon Dar at Technion-Israel Inst. of Tech S. Dado, A. Dar and A. De Rujula report: High resolution VLA/VLBI follow-up observations of the radio afterglow of GRB161219B [1,2] may provide direct measurements of the hyperluminal motion and of the initial Lorentz factor of the afterglow's source, relative to its parent supernova [3]. The small isotropic energy Eiso~1.6xE52 erg and the peak energy E'p~104 keV of the relatively nearby GRB161219B measured by Konus- Wind [4] indicate, in the cannonball model of GRBs, that this is an ordinary GRB viewed far off-axis [5]. In this model ordinary GRBs viewed on or near the axis of their relativistic ejecta satisfy E'p~150 (Eiso/E52)^{1/2} keV. The Konus-Wind measurements imply [5] a viewing angle @~3/Gamma and a linear polarization P = 2 @^2 Gamma^2/(1+@^4 Gamma^4)~20% where Gamma is the bulk-motion Lorentz factor of the jet which produced GRB161219B in a broad-line Ic supernova explosion akin to that of SN1998bw [3]. The bright radio afterglow of the relatively nearby GRB161219B [1,2] at redshift z=0.1475 [6] provides an excellent opportunity to measure the apparent superluminal speed V of its jet in the plane of the sky. As long as @^2 x Gamma^2 >> 1, V=2c/(1+z)@ and a measurement of V yields Gamma~3(1+z)V/2. If the jet decelerates in a constant density environment and reaches @^2 x Gamma^2 << 1 before its superluminal speed could be measured, then its late-time apparent superluminal speed in the plane of the sky, which is expected to decrease like t^{-1/2), if measured, may be extrapolated to obtain an estimate of Gamma at early time [5]. [1] K. D. Alexander, et al. GCN 20313 [2] A. J. Nayana & and P. Chandra, GCN 20344 [3] A. de Ugarte Postigo, et al. GCN 20342 [4] D. Frederiks, T. Laskar, E. Berger, GCN 20323 [5] S. Dado, A. Dar, A. De Rujula, arXiv:1610.01985 [6] N. R. Tanvir, et al. GCN 20321 [GCN OPS NOTE(29dec16): The suffix "B" was added to the GRB name in the SUBJECT-line.]