A perspective on the high-energy transient sky: from gamma-ray bursts to the search for fast radio burst counterparts

My work of thesis focuses on two protagonists of the transient sky, gamma-ray bursts (GRBs) and fast radio bursts (FRBs), tackling both the open issues behind their emission and the technological challenges connected with their observation. In the context of an in-depth understanding of GRBs I studied the popular ”Ep,i–Eiso” correlation. It is fundamental to shed light on the peculiar behaviour of a few events, that appear to be important outliers of the Ep,i–Eiso correlation. In the first part of thesis, I investigate if the locations in the Ep,i/Eiso plane of these bursts may be due to the fact that a substantial fraction of their energy was released in the soft X-ray band, and consequently missed by the instruments that detected. My result suggests that if the two bursts would have been observed by Swift and by eXTP, they may have matched the Ep,i–Eiso relation. This provides strong support to the idea that instrumental biases can make some events in the lower-left corner of the Ep,i–Eiso plane appearing as outliers. GRB prompt emission mechanism is still an open issue, despite the tremendous progress made in the last decades in the comprehension of the GRB phenomenon. It has been realised that only a very broadband analysis (from soft X–rays to MeV) provides a stringent test to the proposed models in the literature. I tackled the problem performing a systematic, broad-band, temporally-resolved spectral analysis of a number of GRBs to test three out of the most popular models: the empirical Band function, and more physically grounded models like the synchrotron (in the form of a double broken power–law), and the Comptonisation model “grbcomp”. I successfully modelled about 3/4 of the entire sample even if, interestingly, roughly 20% spectra are truly problematic for any model. Adding X–ray data turnedout essential (i) to assess the frequent presence of extra components, like in the case of the Band function; (ii) to validate physical models like synchrotron and grbcomp, at the same time, emphasising their limits. In the era of multi-messenger astronomy, the exploration of the early emission from transients is a key task for understanding the encoded physics, while current generation networks of fully-robotic telescopes provide new opportunities in terms of fast followup and sky cover-age. In this context, I designed a robotic pipeline for robotic optical followup of gamma-ray bursts with the Las Cumbr ́es Observatory network aimed at automatically submitting a requestfor observations within 3 minutes from the discovery alert. Via Telegram the pipeline keeps theusers informed, allowing them to take control upon request. The last part of my thesis focuses on FRBs, millisecond-long bursts uniquely detected atradio frequencies, with the only possible exception of FRB 131104, for which aγ–ray transient positionally and temporally consistent was claimed. The aim of my work was testing the systematic presence of an associated transient high-energy counterpart throughout a sample of the FRB population. My result excludes about 94% of Fermi/GBM detected long gamma–ray bursts and about 96% of Fermi/GBM detected short gamma–ray bursts. My result excludes a γ–ray counterpartas fluent as the one possibly associated with FRB 131104 to be a common feature of FRBs.