Unveiling the Local Population of Compact Binary Mergers

In 2017, for the first and so far only time, a collision between two neutron stars was detected simultaneously in gravitational waves (ripples in space-time) and electromagnetic light. From this one event, we learned that merging neutron stars launch powerful jets at nearly the speed of light, and forge some of the heaviest elements in the periodic table.

The newly-formed heavy elements are highly unstable, and glow with radioactive energy as they decay to stable configurations. This glow can be detected by optical and infra-red telescopes as a 'kilonova' - the observational signature of new heavy elements being created. Neutron star mergers are now thought to be the Universe's primary factories of culturally significant elements like gold, and are the only source of rare-Earth and trans-Uranium elements that we know of.

Unfortunately, kilonovae are faint and short-lived, making them hard to find. Fewer than 10 candidate events are known, and only 3 have good observational coverage. With such small numbers, we are currently unable to assess exactly how big a contribution neutron star mergers make to the Universal heavy element census.

We will solve this problem by developing new ways to discover kilonovae in the local Universe, then model them to learn about their nucleosynthesis yields.

The first pathway is through a deeper understanding of the jets launched by neutron star mergers. If pointed towards Earth, we detect these as short gamma-ray bursts. However, we can still detect a short gamma-ray burst when Earth is only grazed by the jet; the 2017 event was viewed up to 30 degrees away from the powerful core.

This is only possible when the merger is nearby, but we expect hundreds of sufficiently nearby mergers every year. We therefore likely already detect some without realising. We will learn to tell them apart from regular 'direct hit' short gamma-ray bursts by studying archival events and developing theoretical expectations for their appearance, then testing these expectations by looking for tell-tale signs of late-rising radio, optical and X-ray signals following new short gamma-ray bursts.

Learning to recognise the gamma-ray signatures of very nearby mergers in real time will enable comprehensive studies of their kilonovae.

The second pathway is by sifting through the data streams of two next-generation wide-field telescopes: the Vera Rubin Observatory's Legacy Survey of Space and Time (LSST), and the Gravitational-wave Optical Transient Observer (GOTO). They present two different challenges: LSST will be very sensitive, and is expected to identify lots of nearby kilonovae.

However, they will be buried in millions of unrelated transients like supernovae or stellar flares. We will use models we have developed to simulate key observable characteristics of kilonovae, and apply them as filters to the LSST data stream to filter out contaminants and identify kilonovae for further study. GOTO will be less sensitive, but will specialise in searching for targets whose position on the sky is not precisely known.

We will use this unique skillset to search for optical light from short gamma-ray bursts discovered by the Fermi satellite, which finds them at a high rate but with poor localisation precision. Once found, we will search for and study the associated kilonovae.

Our goal is to understand the contribution mergers make to heavy element production, but we will also learn about the structure of the jets they launch, and get tighter constraints on how often mergers occur in our Universe. We also aim to discover the first light ever detected from a neutron star merging with a black hole, where jets and kilonovae can be produced if the neutron star is ripped apart before plunging over the event horizon.

We will investigate this possibility to gain insights on how easy to shred neutron stars are (the so-called 'equation-of-state'), and whether neutron star - black hole mergers contribute to heavy element production.