Description:

Speaker: Dr. Kelly Gourdji, Swinburne University

Title: Gravitational wave merger events on milliarcsecond scales with VLBI

Abstract: Short gamma-ray bursts (SGRBs) offer an opportunity to study extreme examples of transient jets, providing insight into the jet physics and the merger progenitors. While SGRBs have traditionally been seen only on-axis and at typically high redshift, the advent of gravitational-wave merger (GWM) detections means that much closer, off-axis events can also be identified and studied. The prototypical example of this new category was the landmark detection of the multiwavelength counterpart to binary neutron star merger GW170817, which provided an unprecedented view of an SGRB. Its relative proximity enabled very long baseline interferometric (VLBI) observations that identified superluminal proper motion of the merger’s relativistic outflow. This provided largely orthogonal information to the multiwavelength light curve that enabled degeneracies between models to be broken and hence much tighter constraints on key jet parameters. However, such VLBI data will often be of low signal-to-noise and includes systematic sources of uncertainty that are difficult to quantify. This complicates the estimation of jet parameters and especially the associated uncertainty. In this talk I will describe these challenges and present a novel Bayesian VLBI model fitting framework that we have developed to address them. I will present a major reanalysis of GW170817 VLBI data using this framework and the resulting improved constraints on the key jet parameters and, by extension, H0.