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Amsterdam-ASTRON Radio Transient Facility And Analysis Centre: Probing the Extremes of Astrophysics

Final Report Summary - AARTFAAC (Amsterdam-ASTRON Radio Transient Facility And Analysis Centre: Probing the Extremes of Astrophysics)

The AARTFAAC project’s overall goal was to study extreme objects in the universe that make their presence known via unpredictable, highly energetic outbursts of radiation. These outbursts manifest themselves through a variety of radiation types, and can thus in principle be studied with many different telescopes. In this project we focused our observational efforts in low-frequency radio waves, because this is relatively unexplored terrain, and because very wide-field observations have become possible there, which allow us to search for very rare events. We also studied theoretically the formation of explosions that are powered by black holes and neutron stars, because in the neighbourhood of such objects one naturally finds the most energy, and thus the strongest explosions with the most extreme physics.

In terms of radio observations, there are a few major achievements to report for the project. First, we played a major role in the commissioning and initial use of the LOFAR telescope’s capability in finding transients. This led to the creation of a well-documented and published transient detection pipeline (named ‘TraP’), which is now used also by other major radio telescopes in the world such as MeerKAT, ASKAP, and MWA. Using this, we discovered a number of previously known types of radio transient, such as an eclipsing millisecond pulsar and a blazar. Furthermore, we found a new, strange type of radio transient that lasts only minutes and is very bright. Its nature is a complete mystery thus far, and we are preparing longer and deeper surveys in order to find more of them and try to characterize them. The other major observational effort was the construction of the radio equivalent of a fish-eye lens, to monitor the full sky for very rare, bright transients. This telescope, also called AARTFAAC, was harder to construct and calibrate than we thought, but in the end we did manage to make it work. However, commissioning and scientific exploration with this instrument only began in the final months of the ERC project, and we do not yet have any firm results. Fortunately, the project has been promising enough and has sufficient technical implications for future large radio telescopes (such as the SKA) that we were able find funding to pursue further science and development of the AARTFAAC telescope concept.

On theory side, we contributed significantly to the development of modelling of gamma-ray bursts as explosions where matter is ejected with nearly the speed of light, and then collides with ambient material to cause the strong emission of all wavelengths of light. We also explored the emission of matter and light from ultra-magnetised neutron stars, called magnetars. These sources could be the origin of bright flashes of radio waves, but this has not yet been observed. In order to probe this possibility, we explored what can be learned about the theory of magnetar outbursts from the well-observed phenomena in X and gamma rays. We conclude that during major outbursts, there is likely strong ejection of relativistic material from the magnetic poles, as this would explain the strong directionality of the high-energy emission from magnetar outbursts.

Both the observational and theoretical lines of work started in this project continue to be pursued at the Anton Pannekoek Institute and ASTRON, so the legacy of the AARTFAAC project will not only be the results obtained during the project, but also the research lines in radio telescope observations and techniques, and in high-energy theory that were founded or supported by AARTFAAC.