Petrecca, Vincenzo (2024) Timing and Demographic properties of AGN and SNe in preparation for the Legacy Survey of Space and Time (LSST). [Tesi di dottorato]

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Abstract

Time-domain astronomy is living a golden era with the advent of next-generation facilities designed to survey the changing universe across multiple wavelengths with unprecedented depth, sky coverage, and cadence. Among these, the Legacy Survey of Space and Time (LSST) at the Vera C. Rubin Observatory, expected to start in the second half of 2025, will totally revolutionize the field by observing the entire southern sky every few nights for 10 years with the biggest digital camera ever built. To get ready for the operations, the worldwide Rubin Community organised into eleven LSST Science Collaborations, working on both simulations and archival data to develop analysis tools and refine the final observing strategy. During my PhD, I contributed to this general effort by focusing on the study of Active Galactic Nuclei (AGN) and Supernovae (SNe), two completely different types of sources sharing both detection techniques and the important role in the general picture of galaxy evolution. The term AGN identifies a wide class of peculiar galaxies characterized by a central supermassive black hole with an accretion disc of infalling matter. Variability is one of their striking features, with timescales ranging from minutes to decades and a completely stochastic and aperiodic behaviour. It has proven to be a powerful selection tool, and an effective instrument to constrain the AGN inner structure. However, the details about the origin of variability and the exact interplay between different AGN emitting regions are still unknown. SNe, instead, are violent stellar explosions at the end of their life cycles. Type Ia SNe, among the other types, have a peak luminosity correlated with the duration of the event, which makes them standardizable candles and fundamental cosmological probes. Nonetheless, albeit there is general consensus that they result from a thermonuclear explosion of a carbon-oxygen white dwarf (WD) in a binary system, the details about the other companion are not yet clear. There are two main progenitor channels: a WD accreting matter from a non-degenerate star up to exceeding a critical mass limit, or two WDs spiralling together and eventually merging. For both sources, the LSST will dramatically increase both the size of the sample and the temporal coverage. However, to maximise the scientific outcome from the survey requires making predictions on the expected results and developing the proper analysis tools. This thesis describes part of the work done to study timing and demographic properties of AGN and SNe in preparation for the LSST. The first part is dedicated to AGN, with a modeling of UV/optical variability via ensemble power spectral density, analysis of correlations between variability and physical properties (e.g. accretion rate, black hole mass, rest-frame wavelength), and AGN selection through variability. The second part, instead, is dedicated to the measure of the SN Ia rate as a method to constraint the progenitors, with the measurement of the SN Ia rate on simulated LSST images to assess the impact of uncertainties due to photometric redshift, host-galaxy association and classification of light curves. All the tools developed for the analysis ready to used with the upcoming LSST data.

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