GRB 170817A / GW 170817: constraining the relativistic outflow structure and the compact remnant
This talk will focus on what can be learned from this event about the properties
of the outflow that powered the prompt gamma-ray emission and the afterglow emission,
and briefly outline the constraints on the type of compact remnant (black hole or massive
neutron star) that was left in its aftermath. Both its prompt gamma-ray emission properties
(highly under-luminous but with a more typical peak energy) and the long lasting
(more than 130 days) rise of its (X-ray to radio) afterglow flux strongly challenge the
simple “top-hat” jet model — a uniform jet with sharp edges. These observations, and in
particular the afterglow, may instead be explained by either of two competing effects: (i)
angular energy distribution: an off-axis structured jet, and (ii) radial velocity
distribution (in a quasi-spherical outflow). Both types of models can reproduce the observed
afterglow flux up to the peak at around 150 days, but differ in their post-peak predictions.
Also, three additional and potentially powerful diagnostics will be discussed: the afterglow
polarization, flux centroid proper motion, and image size and shape. These can further help to
break the degeneracy between different models. Finally, this talk will discuss the recent VLBI
observations that have strongly constrained the structure of the outflow that powered GRB 170817A.