[vsnet-grb-info 18895] GRB161219 - Anticipated hyperluminal evidence

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  20346
SUBJECT: GRB161219 - Anticipated hyperluminal evidence
DATE:    16/12/29 07:49:28 GMT
FROM:    Arnon Dar at Technion-Israel Inst. of Tech  <arnon at physics.technion.ac.il>

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