Radio sky maps of the GRB 170817A afterglow from simulations

TL;DR

This paper uses hydrodynamic simulations to generate synthetic radio images of the GRB 170817A afterglow, exploring how radio centroid motions can distinguish between successful and choked jets over time.

Contribution

It demonstrates that centroid motion alone cannot determine jet success, but combining multiple observations can differentiate models, with specific timing signatures indicating jet success or choking.

Findings

Centroid exhibits superluminal motion for at least 300 days.

Reversal of centroid trajectory before 600 days indicates a successful jet.

Successful jets have smaller angular extent (~4.5 mas) compared to choked jets (~7 mas).

Abstract

We present synthetic radio images of the GRB170817A afterglow, computed from moving-mesh hydrodynamic simulations of binary neutron star merger outflows. Having expanded for nearly a year, the merger remnant is expected to subtend roughly 5 milli-arcseconds (mas) on the sky, potentially resolvable by very long baseline radio imaging techniques. Any observations revealing the radio centroid to be offset from the line-of-site to the merger would be the smoking gun of a jetted outflow. However, our results indicate that a measurement of the centroid position alone cannot independently determine whether that jet escaped successfully from the merger debris cloud, or was "choked," yielding a quasi-spherical explosion. We find that in both scenarios, the centroid exhibits superluminal proper motion away from the merger site at roughly 4 - 10 mas per day for at least the first 300 days. We argue that a successful strategy for differentiating among the explosion models will need to include multiple observations over the coming months - years. In particular, we find the time at which the centroid attains its maximum offset, and begins heading back toward the merger site, is considerably later if the jet was choked. Detecting a reversal of the centroid trajectory earlier than 600 days would uniquely identify a successful jet. Our results indicate the source might be resolved using VLBI radio observing techniques with roughly 1 mas resolution starting at roughly 400 days post-merger, and that the the angular extent of a successful jet is significantly smaller than that of a choked jet (4.5 versus 7 mas respectively).