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Final Report Summary - FAST TRANSIENTS (A Panchromatic Approach to Unlocking the Transient Radio Universe)

A fundamental change in observational astrophysics has occurred over the last decade with the emergence of high-cadence, time-resolved capabilities in astronomical facilities. This important paradigm shift has been driven by the fact that our Universe is far from static on short time scales; some of the most energetic events in the Universe such as gamma-ray bursts (GRBs) and black hole mergers light up the sky regularly. Observatories at optical wavelengths and high energies have already been searching and monitoring transients for a number of years and the revolution is now spreading to include the radio regime, which is finally benefitting from technological advances suitable to probe large sky volumes at fast cadence.

This International Incoming Marie Curie Fellowship entitled “Fast Transients” aimed to develop the technology for real time searching and rapid response to fast transients for the new generation of radio telescopes. This incorporated a way of automating interactions with astronomical facilities operating over the whole range of the electromagnetic spectrum in order to quickly process their transient alerts and trigger radio follow ups, as well as the development of a commensal survey system for fast radio transients to equip a radio interferometer.

This project was motivated by the discovery, a few years ago, of fast radio bursts (FRBs), which are short duration events of extra-galactic origin. Despite the fact that the origin of FRBs is still unknown, their possible use as probes for cosmology and the extra-galactic medium are potentially game changing. Tapping on this new resource requires to detect more of them but also, crucially, to detect their location in the sky in order to determine their distance and identify possible counterparts in other regions of the electromagnetic spectrum. Separate but related are other known fast transients such GRBs as well as pulsars presenting transitional and intermittent behaviours, for which the immediate (prompt) radio signature is missed due to the long delay in setting up follow-ups.

Key to the success of multi-wavelength monitoring, especially when it comes to fast transients, is the rapidity at which follow-ups can be executed. In this respect, my project first focused on the Low Frequency Array (LOFAR), which is a revolutionary radio telescope array able to observe in multiple directions of the sky simultaneously and reconfigure its pointing via software within seconds. I have developed a system for the UK LOFAR international station which is able to connect to the live feed of observatories around the world to monitor for broadcast messages (called VOEvents) alerting the community about the discovery of transients. The goal was to trial this system by monitoring GRBs discovered by high-energy satellites and trigger automated follow-up with the UK LOFAR facility. In a trial run spreading over a few months, it was possible to achieve an automated response for 10 GRBs, the fastest response time corresponding to a 48 second interval from the event itself and a 17 second latency from the time the alert was broadcasted. This demonstrated the feasibility of performing fast responses to transients without the intervention of a human. The system was then extended to allow the triggering of additional international LOFAR stations such as the one in France. Processing of the recorded data proved to be more difficult than originally planned due to their volume. An on-site solution is now available and will avoid transferring the raw data across to an external facility. Data analysis will take place beyond the extent of the fellowship. Once the data reduction is automated, the plan is to start operating the monitoring and triggering system on a continuous basis.

The second major aim of the project has been to develop an instrument to enable the search for fast radio transients such as FRBs with the e-MERLIN array, a 7-element radio interferometer spread across the United Kingdom. This work included the design and feasibility study of the new instrument, and its initial implementation and commissioning in order to collect data from the array. The goal was to produce a technology which can operate commensally to the existing science operation in order to maximise the potential survey uptime, given that the expected transients do not present a preferential direction. Additionally, this will also allow for a new high-time resolution recording mode. By the end of the fellowship, the new instrument has been put in place and successfully recorded data during commissioning observations. The system in place has also been adapted in order to allow for commensal recording. Some further work beyond the fellowship will be needed in order to finish implementing the automated pipeline that will search the data stream for transients in real time. Science operations are expected to start in the coming year.

In parallel to the two highly demanding technical projects described above, the fellowship also led to significant scientific contributions in the field of time-domain astrophysics. More specifically, it involved the study of various binary systems that show regular time variability (eclipses, orbital flux and colour modulations) as well as unexpected events (state transitions, outbursts, intermittency). Most notable is the first direct dynamical evidence of a sub-stellar donor star harboured in a binary system called cataclysmic variable. In these systems a low-mass star transfers mass to a compact object known as white dwarf. It had long been suspected that the depletion in mass might bring the donor star below the limit to sustain nuclear reactions in its core, but a direct dynamical measurement of the donor mass to prove it had never been obtained until then. Another project led to the detection of a transient radio source in LOFAR imaging data which turned out to be an eclipsing binary millisecond. This work enabled us to show that the data are consistent with the pulsar disappearing completely during the eclipses, thus putting constraints on the possible mechanisms causing them. Additionally, it shows the potential of continuum imaging transient surveys to detecting binary millisecond pulsars which display radio eclipses.

Beyond the academic impact of the technical and science work achieved here, this fellowship also made an important impact to the general public. The work about the cataclysmic variable system presented earlier was published in Nature and was publicised in the media via a press release. Additionally, a citizen science project called Pulsar Hunters has been co-developed by Dr. Breton. Aimed at allowing the general public to help astronomers discover new pulsars, Pulsar Hunters was featured as the science theme of the 2016 BBC 2 Stargazing Live TV show. Several tens of thousands of people contributed to examining over 60,000 pulsar candidates, which resulted in 2.6 millions classifications events recorded in just over 48 hours. Finally, Dr. Breton co-organised a major city-wide outreach campaign called #astrotram as part of the Manchester European City of Science activities, which aimed at engaging the users of the city’s public transit with astronomy. The campaign included posters, decorated trams, science busking and an augmented-reality smart phone app. #astrotram featured for about three weeks, reaching over 1.8 million commute journeys, whilst the online Twitter presence recorded 2.5 million impressions.

For further information:
Website of Dr. Breton: www.renebreton.org
Paper on the cataclysmic variable system: https://doi.org/10.1038/nature17952
Paper on the eclipsing millisecond pulsar: https://doi.org/10.1093/mnras/stw794
Papers published in during the period of the fellowship: https://ui.adsabs.harvard.edu/#search/q=%20author%3A%22Breton%2C%20R.%20P.%22%20year%3A2015-2016&sort=date%20desc%2C%20bibcode%20desc
Link to Pulsar Hunters: www.pulsarhunters.org
Link to #astrotram: http://www.manchester.ac.uk/discover/news/tram-passengers-transported-to-another-world-for-esof/

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