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Transient Engine Driven Explosions

Periodic Reporting for period 4 - TEDE (Transient Engine Driven Explosions)

Reporting period: 2020-09-01 to 2022-03-31

Stars end their lives in many ways. Some fade away, others explode. More rarely they may collide with another star, or be torn apart by the strong gravity from a black hole. It is an irony of stellar evolution that many stars are actually at their brightest at the moment of their death, creating signatures which can be seen at great distances – much further than the star could be seen during its life. This means that stellar death provides a unique window both into the nature of the stars and their evolution, but also into the extreme physics which acts upon them at the moment of their demise. The extreme brightness of these stellar deaths also means they can be used as lighthouses to probe the distant Universe.

This ERC project is focussed on understanding the nature of stellar deaths which are powered by a compact object (usually a neutron star or a black hole) which resides at the centre of the explosion. The energy released from this central engine can transform the explosion, making it brighter than it may otherwise be or allowing other extreme physical processes, such as the synthesis of heavy elements, to occur. The aim of this project is to understand how these central engines impact explosions, from their observational appearance, to a test of the ubiquity of black holes in galaxies, to a route to testing the origin of the heaviest elements.

There were for key areas for work identified.

1) The origin of the longest duration gamma-ray bursts. How are they created? What is their relation to "normal" gamma-ray bursts?
2) The nature of tidal disruption events where stars are shredded by massive black holes. Measuring the diversity of their observational properties and the ubiquity of black holes in the nuclei of galaxies.
3) The nature of short-duration gamma-ray bursts. Are they caused by the merger of compact objects such as neutron stars and black holes?
4) The first optical counterparts of binary mergers seen with gravitational waves.
Over the first part of this grant, progress has been made in all of these areas. A particular highlight is the discovery of one source -- a pair of merging neutron stars – in both gravitational waves and light (a central goal of goals 3 & 4 above). My team was centrally involved in this work, both in the initial discovery of the source with light and with in-depth follow-up, leading observations from the Very Large Telescope and the Hubble Space Telescope. Team members presented these results at press conferences in Munich and Washington DC. Further work has demonstrated the link between these sources and short-duration gamma-ray bursts, linked them with the synthesis of very heavy elements and provided a new route to measuring the expansion rate of the Universe.

We have also made progress in understanding the nature of tidal disruption events which produce material moving close to the speed of light (item 1). We have published the first time-resolved polarisation of the jet associated with one of these events, as well as Hubble Space Telescope imaging of another example demonstrating it to arise from a merging galaxy, consistent with the idea that such mergers increase the rate of tidal disruptions. More recently we have also undertaken studies of very long-lived GRBs, and used these as a probe of extreme physics, with some very high energy photons being detected from ground-based telescopes minutes to hours after the event. We have also capitalised on recent theoretical work which suggests long-lived gamma-ray bursts may be responsible for some heavy element production and have launched a major campaign at a local long-duration GRB from August.
Going forward, this project will continue to search for and characterise the electromagnetic counterparts of gravitational wave mergers, in particular, those identified during the current observational run of the LIGO and VIRGO detectors. This will complete in the summer of 2020, and after that, the focus will switch to being on the analysis data taken during this period, and on other areas of the grant. We have an ongoing programme of observations of both an unusual GRB that may arise from a tidal disruption event (project 2) and a very long GRB from a stellar collapse which is a promising site for heavy element production. These observational campaigns run well into 2020 and will be published when they are complete.
GW170817 as seen by our Hubble Space Telescope observations (credit, Levan, Tanvir, NASA/ESA)