Presentation #115.24 in the session Multi-Messenger Astrophysics.
Observations of the gamma-ray burst (GRB) afterglows of gravitational wave (GW) detected binary neutron star (BNS) mergers are likely to be valuable and, in the immediate future, rare. Valuable, in part, because the afterglow encodes information about the inclination between the jet and Earth, which can aid GW cosmology by breaking the distance-inclination degeneracy. Rare because isotropy implies these BNS mergers will predominantly occur at large inclination: Earth will generally lie far off the jet axis and the afterglow emission will be delayed and dim. The observable afterglows will be prime targets for intensive and long-term follow-up. We present an updated theoretical modelling framework designed to make the best use of both photometric and astrometric observations of GW-GRB afterglows, particularly at late times. As an application we perform a comprehensive analysis of the GW170817 afterglow, directly including high precision VLBI astrometry, faint source statistics, and modelling of the trans-relativistic evolution of the electron population. This analysis produces a measurement of the GW170817 viewing angle accurate to ~10% and allows a robust assessment of whether a new X-ray emitting source is contributing to the late-time observations. Finally, we explore what we may be able to learn from observations in LIGO O4 and beyond.