New insights from gravitational waves combined with electromagnetic light

Multimessenger astronomy - the use of light combined with non-photonic messengers such as gravitational waves or neutrinos - has come of age in the past year, thanks to the discovery of emission across the electromagnetic spectrum from a gravitational wave detected merging neutron star binary, and the identification of at least one source of very high energy neutrinos. Here we propose to exploit the unique UV to near-IR abilities of HST, in combination with a large ground-based campaign, to pair gravitational wave and EM information for a range of merger events with varying inclinations and mass ratios. We will map out the diversity of transient behaviour, addressing many central questions in contemporary astrophysics, including; i) what is the contribution of binary neutron stars to heavy element enrichment across the Universe? ii) what is the local Hubble constant measured from gravitational waves, and how can HST determined distances enhance its accuracy? iii) what are the properties of jets launched during the merger, do all mergers create short GRBs for some observers? iv) what are the environments of the mergers, and what does this tell us about the channels to create the binaries? v) how do EM properties depend on binary parameters, particularly if one component is a black hole? The multimessenger era offers the ability to answer these questions for the first time. The capabilites of Hubble to observe outside atmospheric windows, with the spatial resolution to resolve point soures from complex backgrounds, and to track them beyond the limits of ground-based instrumentation will enable us to make pivotal contributions to this new, emergent field.