DOI

https://doi.org/10.48717/7gn0-7y30

Project

RP030

Title

e-EVN: EG102B, EG096E, RP030A, EL058E

Abstract

Last Dec 5 2017 one of the most nearby (D=150 Mpc), radio bright GRBs happened, GRB 171205A. This is a rare instance that brings us a unique opportunity to directly test, constrain, and deepen our understanding of the GRB phenomenon. VLA observations indicate that GRB 171205A is one of the brightest GRBs ever in the radio. We therefore request out-of-session, ToO EVN observations towards GRB 171205A, aimed at determining its angular size by means of deep K- and C-band observations. Those observations will directly inform us on the apparent expansion speed of the GRB, <beta_perpendicular> , thus directly constraining the opening angle of the GRB jet, <theta>, its intrinsic expansion speed, <beta>, and, very important for constraining GRB models, its Lorentz factor, <Gamma>. Intriguingly, if the Cannonball model applies to GRB 171205, the jet launched during its burst is expected to move hyperluminally, with an apparent expansion speed of ~450 c (!!) which implies an angular expansion of ~0.5 mas/day. Such a motion will be easily detected in a matter of a few days, or weeks at most, and therefore our requested observations will unambiguously test the hyperluminal motion predicted by the Cannonball model.