{
  "bibcode": "2016GCN.20346....1D",
  "body": "S. Dado, A. Dar and A. De Rujula report:\n\nHigh resolution VLA/VLBI follow-up observations of the radio\nafterglow of GRB161219B [1,2] may provide direct measurements\nof the hyperluminal motion and of the initial Lorentz factor of\nthe afterglow's source, relative to its parent supernova [3].\n\nThe small isotropic energy Eiso~1.6xE52 erg and the peak energy\nE'p~104 keV of the relatively nearby GRB161219B measured by Konus-\nWind [4] indicate, in the cannonball model of GRBs, that this is\nan ordinary GRB viewed far off-axis [5]. In this model ordinary\nGRBs viewed on or near the axis of their relativistic ejecta\nsatisfy E'p~150 (Eiso/E52)^{1/2} keV. The Konus-Wind measurements\nimply [5] a viewing angle @~3/Gamma and a linear polarization\nP = 2 @^2 Gamma^2/(1+@^4 Gamma^4)~20% where Gamma is the bulk-motion\nLorentz factor of the jet which produced GRB161219B in a broad-line\nIc supernova explosion akin to that of  SN1998bw [3].\n\nThe bright radio afterglow of the relatively nearby GRB161219B [1,2]\nat redshift z=0.1475 [6] provides an excellent opportunity to measure\nthe apparent superluminal speed V of its jet in the plane of the sky.\nAs long as @^2 x Gamma^2 >> 1, V=2c/(1+z)@ and a measurement of V yields\nGamma~3(1+z)V/2. If the jet decelerates in a constant density environment\nand reaches  @^2 x Gamma^2 << 1 before its superluminal speed could be \nmeasured, then its late-time apparent superluminal speed in the plane of \nthe sky, which is expected to decrease like t^{-1/2), if measured,\nmay be extrapolated to obtain an estimate of Gamma at early time [5].\n\n[1] K. D. Alexander, et al.  GCN 20313 \n[2] A. J. Nayana & and P. Chandra,  GCN 20344 \n[3] A. de Ugarte Postigo, et al. GCN 20342\n[4] D. Frederiks, T. Laskar, E. Berger, GCN 20323 \n[5] S. Dado, A. Dar, A. De Rujula, arXiv:1610.01985\n[6] N. R. Tanvir, et al. GCN 20321\n\n[GCN OPS NOTE(29dec16): The suffix \"B\" was added to the GRB name\nin the SUBJECT-line.]",
  "circularId": 20346,
  "createdOn": 1482997768000,
  "email": "arnon@physics.technion.ac.il",
  "subject": "GRB161219B - Anticipated hyperluminal evidence",
  "submitter": "Arnon Dar at Technion-Israel Inst. of Tech  <arnon@physics.technion.ac.il>",
  "eventId": "GRB 161219B"
}