Real-time analysis of gamma-ray transients: developing software and statistical methods for the CTAO

The Cherenkov Telescope Array Observatory (CTAO) is going to be the leading observatory for very-high-energy astrophysics in the next decades. It is going to be a powerful instrument for the exploration of the violent, variable Universe thanks to its unprecedented sensitivity to short timescale phenomena. The CTAO will be equipped with a real-time-analysis pipeline, the Science-Alert-Generation (SAG) pipeline, which will perform scientific monitoring of the observed field of view and generate candidate Science Alerts when detecting transient phenomena. We developed the scientific pipeline of SAG, SAG-SCI, that performs high-level analysis of gamma-ray data using traditional analysis methods. We developed SAG-SCI respecting CTAO software quality requirements and scientific strategies to optimise the analyses on different types of targets and generate candidate Science Alerts. We used SAG-SCI to analyse CTAO simulated data under several observational scenarios, to validate the software algorithms, investigate systematic uncertainties and present an overview of the most relevant applications of SAG-SCI. When the analyses must be performed in a low-photon-statistics regime, the traditional methods are not valid, thus we developed a Burst Analysis statistically valid in this regime. We applied the Burst Analysis to observations of SGR 1935+2154, a Galactic magnetar, performed with the CTAO Large-Sized Telescope Prototype, LST-1. We searched for possible TeV counterpart of its X-ray persistent and burst emission. While neither persistent nor transient emission was detected in our search, we could provide, for the first time, TeV upper limits to the emission of a short magnetar burst simultaneous to its soft gamma-ray signal. Our development of SAG-SCI will allow the CTAO to become a fundamental facility for time-domain astrophysics, and enable multi-wavelength and multi-messenger approaches leading to a deeper understanding of the Universe. Our development of the Burst Analysis will allow CTAO to analyse several interesting classes of fast transients in the low-photon-statistics regime.