Dr Lauren Rhodes

Abstract:

We present the discovery and extensive follow-up of a remarkable fast-evolving optical transient, AT 2022aedm, detected by the Asteroid Terrestrial impact Last Alert Survey (ATLAS). In the ATLAS o band, AT 2022aedm exhibited a rise time of 9 & PLUSMN; 1 days, reaching a luminous peak with M g & AP; -22 mag. It faded by 2 mag in the g band during the next 15 days. These timescales are consistent with other rapidly evolving transients, though the luminosity is extreme. Most surprisingly, the host galaxy is a massive elliptical with negligible current star formation. Radio and X-ray observations rule out a relativistic AT 2018cow-like explosion. A spectrum in the first few days after explosion showed short-lived He ii emission resembling young core-collapse supernovae, but obvious broad supernova features never developed; later spectra showed only a fast-cooling continuum and narrow, blueshifted absorption lines, possibly arising in a wind with v & AP; 2700 km s-1. We identify two further transients in the literature (Dougie in particular, as well as AT 2020bot) that share similarities in their luminosities, timescales, color evolution, and largely featureless spectra and propose that these may constitute a new class of transients: luminous fast coolers. All three events occurred in passive galaxies at offsets of & SIM;4-10 kpc from the nucleus, posing a challenge for progenitor models involving massive stars or black holes. The light curves and spectra appear to be consistent with shock breakout emission, though this mechanism is usually associated with core-collapse supernovae. The encounter of a star with a stellar-mass black hole may provide a promising alternative explanation

Abstract:

Gamma-Ray Bursts (GRBs) are brief flashes of gamma-rays that last from a fraction of second up to a few hundreds of seconds, during which a significant amount of isotropic equivalent energy is released (ranging from 10^48 to 10^54 erg). These explosive transients are associated with the catastrophic explosion of an isolated massive star (long-duration GRBs), or with the merger of compact objects (short-duration GRBs). Both scenarios lead to the formation of a highly magnetised neutron star or a spinning, stellar-mass black hole, which are thought to accrete material and launch two relativistic jets, causing the observed gamma-ray emission via magnetic processes or internal shocks. These jets interact with the surrounding material, producing the afterglow emission that extends from gamma-rays to radio waves. To understand GRB formation and evolution, a standard model involving an ultra-relativistic outflow is commonly employed. However, even sophisticated models face degeneracy in the multi-dimensional parameter space. To alleviate or possibly break the degeneracy, broad-band observations across the electromagnetic spectrum are crucial. In particular, the Very Long Baseline Interferometry technique (VLBI) has proven to be a unique asset, providing direct evidence of apparent superluminal expansion (for on-axis GRBs), centroid displacement of the outflow (for slightly off-axis GRBs) and the first confirmation that merger events can launch successful jets. In this Thesis, we employed radio and VLBI observations to characterise and constrain the outflow, the circum-burst medium, and the properties of the progenitors of GRBs. This included studies on individual events (GRB201015A and GRB221009A), which are important to test the predictions of current models, GRB host galaxies (GRB200716C), which are fundamental to constrain the nature of the progenitor through the characterisation of the surrounding environment, and the statistical properties of GRB afterglows, in order to verify the existence of potential GRB sub-populations

Abstract:

The newest generation of radio telescopes is able to survey large areas with high sensitivity and cadence, producing data volumes that require new methods to better understand the transient sky. Here, we describe the results from the first citizen science project dedicated to commensal radio transients, using data from the MeerKAT telescope with weekly cadence. Bursts from Space: MeerKAT was launched late in 2021 and received ∼89 000 classifications from over 1000 volunteers in 3 months. Our volunteers discovered 142 new variable sources which, along with the known transients in our fields, allowed us to estimate that at least 2.1 per cent of radio sources are varying at 1.28 GHz at the sampled cadence and sensitivity, in line with previous work. We provide the full catalogue of these sources, the largest of candidate radio variables to date. Transient sources found with archival counterparts include a pulsar (B1845-01) and an OH maser star (OH 30.1–0.7), in addition to the recovery of known stellar flares and X-ray binary jets in our observations. Data from the MeerLICHT optical telescope, along with estimates of long time-scale variability induced by scintillation, imply that the majority of the new variables are active galactic nuclei. This tells us that citizen scientists can discover phenomena varying on time-scales from weeks to several years. The success both in terms of volunteer engagement and scientific merit warrants the continued development of the project, while we use the classifications from volunteers to develop machine learning techniques for finding transients.