Search and multi-frequency follow-up studies of radio transients: novel approaches and large campaigns

One of the most fascinating phenomena in modern radio astronomy is related to the Fast Radio Bursts (FRBs). FRBs are Jy-intense, ms-duration radio transients of extra-galactic origin, whose nature is not assessed yet. To date, FRBs have been observed only in the radio band. Despite their origin has not been disambiguated yet, observational facts point towards highly magnetised neutron stars, such as magnetars, as the putative sources behind at least some of them. In particular, on April 2020 the Galactic magnetar SGR J1935+2154 emitted a radio flash closely resembling the ones produced by the FRBs, with simultaneous detections in the X-rays. This result remarkably strengthened the FRB/magnetar link and strongly motivates panchromatic campaigns towards FRB sources in order to find, as in the case of SGR J1935+2154, their high-energy and also, possibly, optical/infrared counterparts. On one side, a multi-wavelength (MWL) detection of a FRB would lead to the confirmation of the aforementioned FRB/magnetar connection and/or to the evidence that other classes of astrophysical objects/events could be also responsible for a fraction of these radio transients. On the other hand, the observation of a MWL burst (outside the radio band) might discriminate between various proposed emission models, some of which predict MWL emission(s) simultaneous with the radio, while others prescribe that the MWL emission should occur well before or after the radio burst. However, any panchromatic detection of a FRB will mainly rely on the detection of a burst in radio. In order to detect as many radio bursts as possible, dedicated instruments and tailored search algorithms are a fundamental asset. MWL observations of FRBs are indeed the main driver of this PhD Thesis. On the other hand, MWL campaigns strongly depend on the capability to detect bursts in the radio band, which can happen with the deployment of proper instruments and with the use of dedicated search algorithms. In view of that, the PhD work presented in this Thesis, revolves around the following three main themes: (i) the analysis and possible improvement of the FRB detection algorithms; (ii) the development of radio facilities tailored for the FRB observations, and lastly (iii) a series of large MWL campaigns targeted to some of the FRB sources.