TITLE:   GCN CIRCULAR
NUMBER:  22196
SUBJECT: GRB171205A hyperluminal radio afterglow?
DATE:    17/12/07 11:28:34 GMT
FROM:    Arnon Dar at Technion-Israel Inst. of Tech  <arnon@physics.technion.ac.il>

Dado, Dar and De Rujula report:

The very bright radio afterglow^1 of the far off-axis (low luminosity)
GRB171205A at the outskirts of its host galaxy^2 at redshift z=0.0368
(agular distance Da~ 150 Mpc), provides another rare opportunity, like
GRBs^3 980425 and 030329, to measure the apparent hyperluminal separation
of the jet-produced afterglow from the anticipated supernova, which
launched it^3. The CB model relation Ep \propto Eiso^{1/3} for far 
off-axis GRBs^4, and its estimated isotropic energy release in the 
observed 15-150 keV band^5, Eiso~ 5.7E49 erg, yield, in the CB model^3, 
Ep~100 keV, viewing angle of @~(2/Ep(eV))^1/2~4.5 mrad, and a hyperluminal
velocity^3 V~ 2c/@ ~450c.

With such a hyperluminal velocity, the radio afterglow separation from the
SN, which launched it, will reach ~ 200 mas! after a year, easily resolved 
in VLBI and VLA radio follow-up observations.

If the afterglow of GRB171205A was a SN-less^6 GRB, produced by a highly
relativistic jet launched in a phase transition of a neutron star
(to quark star ?), the above estimate will not be valid for the separation
of the radio afterglow from the GRB position, if its late-time afterglow
was produced by a highly magnetized millisecond pulsar^7 (MSP).

We strongly urge VLA and VLBI follow up observations of the radio 
afterglow of GRB171205A.

1. A. de Ugarte Postigo, et al. GCN 22187
2. L. Izzo, et al. GCN 22180
3. S. Dado, A. Dar and A. De Rujula, arXiv:1610.01985
4. S. Dado and A. Dar, arXiv:1708.04603
5. S. D. Barthelmy, et al. GCN 22184
6. D. A. Perley and K. Taggart GCN 22194
7. S. Dado, A. Dar  arXiv:1710.02456