THE DIVERSITY OF MASSIVE STELLAR DEATH AND RELATIVISTIC JETS: UNRAVELING A COSMIC TANDEM

Abstract

Stellar engine-driven transients are spectacular cosmic explosions that occur when a massive star undergoes a core-collapse supernova (SN) and collapses into a black hole or neutron star (compact object), launching relativistic jets. These jets are beams of plasma that are the fastest moving objects in the Universe, moving at 99.999% of the speed of light. Studies of stellar engine-driven transients probe extreme physics, allowing tests of gravity, dense matter, accretion, and particle acceleration in environments impossible to recreate on Earth. This dissertation delves into different observational studies on various classes of stellar engine-driven transients, aiming to shed light on a fundamental question in astrophysics: how do some compact objects generate the fastest and most powerful outflows in the Universe?

This dissertation begins with a brief overview of the landscape of stellar engine-driven transients, and is broken into four parts, corresponding to four distinct classes of stellar engine-driven phenomena. Part 1 features two chapters on single-object studies studying SNe accompanying long gamma-ray bursts (LGRBs). Historically, the most studied class of stellar engine-driven transients is LGRBs, which are discovered via their high-energy gamma-ray prompt emission. They are followed by afterglows that light up the entire EM spectrum for months. LGRBs are usually found at distances too far to detect their associated SNe – only ∼ 50 LGRBs have robust SN associations (LGRB-SNe) compared to thousands of LGRB discoveries, making every new LGRB-SN discovery significant. Chapter 2 focuses on GRB 221009A, the brightest and most energetic GRB ever detected. Chapter 3 focuses on GRB 230812B, another nearby and highly energetic LGRB. In addition to characterizing their associated SNe, I performed a statistical analysis of the most updated sample of LGRB–SNe ever presented in the literature, revealing no correlation between LGRB energetics and their associated SN brightness. This demonstrated that the physics powering stellar collapse is decoupled from the mechanisms that launch relativisticjets in LGRB-SNe.

Nearly all LGRB-SNe are broad-lined Type Ic SNe (SNe Ic-BL). These SNe originate from massive stars stripped of their outer hydrogen and helium envelopes and have high ejecta velocities. The local LGRB rate is 1e−3 – 1e−6 times the local SNe Ic-BL rate, and even after accounting for viewing angle effects, it is clear most SNe Ic-BL do not produce relativistic out-flows. To probe why, Part 2 and Chapter 4 of this dissertation feature the largest systematic study of SNe Ic-BL to date, analyzing 36 events detected by the Zwicky Transient Facility (ZTF). A subset of these events was observed at radio wavelengths. Radio observations probe non-thermal emission associated with the fastest moving ejecta, which could be due to relativistic jets in SNe Ic-BL. Through a joint optical–radio analysis of the subsample, I surprisingly found no correlations between optical and radio properties.

Wide-field optical surveys such as ZTF are discovering afterglows without corresponding gamma-ray triggers. Some of these events have no associated gamma-ray counterparts, and are thus known as orphan afterglows. It is not well understood why – viewing angle effects, complex jet structures, and baryon-loaded GRBs or “dirty fireballs” have all been proposed scenarios. Part 3 and Chapter 5 of this dissertation features a multi-wavelength study of AT 2023sva, the sixth orphan afterglow with a redshift measurement. Through detailed light-curve modeling, I showed its missing gamma-ray emission is best explained by observing a structured jet slightly off-axis, with its energy concentrated in a narrow core, dropping off at wider angles. This placed AT2023sva among a small set of engine-driven events with direct evidence for angular jet structure.

X-ray flashes (XRFs) are engine-driven explosions with unknown progenitors whose spectra peak at lower energies than those of LGRBs (hundreds of keV), but higher energies than those of SNe (tens of eV). Their past discoveries were limited by the lack of sensitive all-sky soft X-ray instruments. However, the recently launched Einstein Probe (EP) is well-equipped to discover XRFs, with over 100 times the field of view and vastly greater sensitivity than previous missions. Part 4 of this dissertation features two chapters presenting multi-wavelength studies on two SNe accompanying XRFs (XRF-SNe). Chapter 6 features EP250108a/SN 2025kg, one of the earliest EP XRF–SN discoveries. I found the progenitor system was likely a black hole formed after stellar collapse that launched a relativistic jet, which interacted strongly with an extended circumstellar medium (CSM). Chapter 7 features EP250827b/SN 2025wkm. I determined the event showed direct evidence for prolonged central engine activity and was likely powered by a magnetar remnant and CSM interaction.