KilonovaRank: gravitational wave counterparts and exotic transients with next-generation surveys

Project Information

KRANK

Grant agreement ID: 948381

DOI

10.3030/948381

EC signature date 25 August 2020

Start date 1 September 2021

End date 30 April 2027

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 478 343,00

EU contribution

€ 1 478 343,00

Coordinated by

THE QUEEN'S UNIVERSITY OF BELFAST
United Kingdom

Periodic Reporting for period 3 - KRANK (KilonovaRank: gravitational wave counterparts and exotic transients with next-generation surveys)

Reporting period: 2024-09-01 to 2026-02-28

Summary of the context and overall objectives of the project

Time-domain astronomy is now entering its golden age. Modern sky surveys find thousands of transients—sources that suddenly appear then fade forever—every year. This is revolutionising our understanding of the dynamic Universe and stellar evolution, revealing unexpected and energetic channels by which stars can end their lives. Simultaneously, we have begun to probe the gravitational sky, thanks to the remarkable success of Advanced LIGO and Virgo. The most valuable transients are those which are poorly understood, since by definition they have
the most to teach us about physical processes in new regimes. Unsurprisingly, these tend to be the rarest events that have been difficult to study historically. For such objects, each additional event adds a new piece to the puzzle.

Three types of rare transient events in particular offer wide-ranging perspectives on how compact objects (black holes and neutron stars) influence stellar death, and on the physics of the compact objects themselves:

Kilonovae are the radioactive glows from new heavy elements created by the collision and merging of neutron stars in a close binary system. These also produce gravitational waves, and are so far the only sources in the Universe detected in both electromagnetic and gravitational emission. Understanding the physics of these explosions gives unique insight to the creation of about half of the periodic table, as well as the behaviour of sub-atomic particles above nuclear densities.

Superluminous supernovae are the brightest explosions in the Universe and point to missing physics in our understanding of stellar evolution. They may be powered by the rotation of fast-spinning, magnetic neutron stars formed during the collapse of their stellar cores, allowing us to understand compact object formation and the evolution of the most massive stars.

Tidal disruption events are flares from stars shredded by supermassive black holes. They can reveal how black holes grow in the centres of galaxies, and help to untangle the very complicated physics of astrophysical accretion disks. Each of these populations has only a few tens of known examples, and in the case of kilonovae only a handful.

The goal of this project is to capitalise on the unprecedented power of wide-field telescopes combined with Data Science and Machine Learning approaches, to discover, follow up and analyse large samples of these rare explosions.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The main objective to date has been to develop the search methods and machine learning code and start to compile our samples or rare transients. We have developed two codes: one for identifying slow transients like superluminous supernovae and tidal disruption events (NEEDLE) and one for identifying fast transients like kilonovae (FastFinder). These models are trained and have been deployed publicly via the UK Lasair alert broker. This is a website allowing anyone to interact with data from current transient surveys. Our codes publicly classify any promising events as likely kilonovae, superluminous supernovae of tidal disruption events, enabling rapid and detailed follow up observations. We have obtained significant telescope time to follow these events, both through large collaborations and from time awarded to our group via competitive proposals, including with space telescopes such as Hubble, James Webb and Chandra.

We have conducted significant work on analysing the most interesting transients discovered by this approach. Group members have led investigations into some of the most important kilonovae, supernovae, tidal disruption events and rare fast transients discovered to date. Much of this work is now already submitted or published in refereed journals. We have been compiling the largest ever samples of superluminous supernovae and tidal disruption events, and statistical analysis is under way. The biggest results so far are the discovery of the first kilonova associated with a long gamma-ray burst, and a proof of the link between tidal disruption events and another rare type of galaxy that emits regular bursts of X-rays.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Before this project, no transient search employed a machine learning algorithm that used both the time evolution of a source and the detailed images of the object and its host galaxy together. Our NEEDLE code is the first to do this and is successfully running in real time to identify likely rare and energetic transient events. This opens up many possibilities for follow up observations and new approaches in astronomical data science.

Only two well-studied kilonovae were known at the beginning of this project. We have doubled this sample in just a couple of years through detailed follow-up of gamma-ray bursts, and in particular long duration bursts that previously were thought to arise only from supernovae. This is despite the lack of neutron star merger detections so far from the gravitational wave detectors during the course of the project. These detectors are about to switch back on for another observing run, increasing the chances of more kilonova detections over the next year.

The next phase of the project has two main goals. We will complete detailed statistical analyses of the superluminous supernova and tidal disruption event populations gathered from all current and historical sky surveys. At the same time, the Vera Rubin Observatory will shortly begin its all-sky survey, and we will apply our methodology to the most powerful sky survey ever undertaken. Assuming that Rubin is operational by the end of the project period, we expect to rapidly increase the known samples of rare events with our machine learning framework, and hopefully find the first kilonovae detected in Rubin follow-up of gravitational wave events.