UC Berkeley

Astronomical transients are sourced by some of the most cataclysmic and energetic events in the universe, such as the collapse of a massive star into a black hole. Understanding the mechanisms and energetics of these transients requires coverage across a broad range of timescales and wavelengths. High-angular-resolution cosmic microwave background (CMB) observatories are uniquely placed to provide such coverage at millimeter wavelengths where no other dedicated large sky area survey instruments exist. Many classes of galactic and extragalactic transients are known or predicted to have interesting time-variable behavior in these wavebands, including but not limited to gamma-ray bursts (GRBs), active galactic nuclei (AGN), and nearby flaring stars.

In this thesis, I describe two untargeted searches for millimeter-wave transients with the third generation SPT-3G camera on the South Pole Telescope. The first of these searches uncovered 15 stellar flares and 2 bright AGN flares, and established the existence of nearby stellar flares as the dominant population of millimeter-wave transients at 1--10 day timescales. The second search established the sensitivity limits of SPT-3G as a transient finding instrument, enabled by a detailed understanding of daily noise variations in SPT-3G data, and by the development of an accurate transient detection simulation suite. With these novel techniques we discover an additional 111 stellar flares, and use the absence of short-timescale extragalactic transients in SPT-3G data to place constraints on bright and brief reverse shock emission from simulated GRB populations. Together these results represent some of the first untriggered transient sources ever detected at millimeter wavelengths, and demonstrate the potential of CMB surveys to discover new transient sources and to place powerful observational constraints on extragalactic population models.