Past exam of the astrophysics course of the University of Oxford / 2018 / 3 / a

GW170817 supplied the first direct gravitational wave observation of a binary neutron star inspiral. The chirping waveform tests compact-binary general relativity, constrains masses and tidal effects, and links a class of known compact binaries to observable sources of gravitational waves. The electromagnetic counterpart identifies the host and distance, enabling complementary source and cosmological inference. This is distinct from the earlier binary-black-hole detections; it established a system involving matter that could also be observed electromagnetically.^[ref]

The short gamma-ray burst GRB 170817A followed the merger signal by about $1.7\mathrm{s}$, providing direct evidence that at least some short gamma-ray bursts arise from neutron star mergers. The association connects their high-energy emission and relativistic outflows to compact-binary coalescence. It does not establish that every short burst has this progenitor, nor that the observed gamma-ray luminosity is independent of viewing angle.^[ref]

The evolving optical and infrared kilonova was consistent with radioactive heating of neutron-rich merger ejecta. Rapid neutron capture, the r-process, can synthesize nuclei heavier than iron; decay of the newly formed unstable nuclei powers the transient. Its redward evolution and large inferred opacity supported production of heavy-element-rich ejecta. Thus mergers are a demonstrated important site for heavy-element production, while the fractions of individual elements and their total cosmic contribution remain model-dependent. The transient was not a direct identification of every very heavy element.^[ref]