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Identifying the sites of r-process nucleosynthesis, a primary mechanism of heavy element production, is a key goal of astrophysics. The discovery of the brightest gamma-ray burst (GRB) to date, GRB 221009A, presented an opportunity to spectroscopically test the idea that r-process elements are produced following the collapse of rapidly rotating massive stars. Here we present James Webb Space Telescope observations of GRB 221009A obtained +168 and +170 rest-frame days after the gamma-ray trigger, and demonstrate that they are well described by a SN 1998bw-like supernova (SN) and power-law afterglow, with no evidence for a component from r-process emission. The SN, with a nickel mass of approximately 0.09 M ⊙, is only slightly fainter than the brightness of SN 1998bw at this phase, which indicates that the SN is not an unusual GRB-SN. This demonstrates that the GRB and SN mechanisms are decoupled and that highly energetic GRBs are not likely to produce significant quantities of r-process material, which leaves open the question of whether explosions of massive stars are key sources of r-process elements. Moreover, the host galaxy of GRB 221009A has a very low metallicity of approximately 0.12 Z ⊙ and strong H2 emission at the explosion site, which is consistent with recent star formation, hinting that environmental factors are responsible for its extreme energetics.

Solar flares are often accompanied by filament/prominence eruptions (~10⁴ K and ~10¹⁰⁻¹¹ cm⁻³), sometimes leading to coronal mass ejections that directly affect the Earth’s environment. ‘Superflares’ are found on some active solar-type (G-type main-sequence) stars, but the filament eruption–coronal mass ejection association has not been established. Here we show that our optical spectroscopic observation of the young solar-type star EK Draconis reveals evidence for a stellar filament eruption associated with a superflare. This superflare emitted a radiated energy of 2.0 × 10³³ erg, and a blueshifted hydrogen absorption component with a high velocity of −510 km s⁻¹ was observed shortly afterwards. The temporal changes in the spectra strongly resemble those of solar filament eruptions. Comparing this eruption with solar filament eruptions in terms of the length scale and velocity strongly suggests that a stellar coronal mass ejection occurred. The erupted filament mass of 1.1 × 10¹⁸ g is ten times larger than those of the largest solar coronal mass ejections. The massive filament eruption and an associated coronal mass ejection provide the opportunity to evaluate how they affect the environment of young exoplanets/the young Earth6 and stellar mass/angular momentum evolution.

We describe the science case, design and expected performances of the X/Gamma-ray Imaging Spectrometer (XGIS), a GRB and transients monitor developed and studied for the THESEUS mission project, capable of covering an exceptionally wide energy band (2 keV – 10 MeV), with imaging capabilities and location accuracy <15 arcmin up to 150 keV over a Field of View of 2sr, a few hundreds eV energy resolution in the X-ray band (<30 keV) and few micro seconds time resolution over the whole energy band. Thanks to a design based on a modular approach, the XGIS can be easily re-scaled and adapted for fitting the available resources and specific scientific objectives of future high-energy astrophysics missions, and especially those aimed at fully exploiting GRBs and high-energy transients for multi-messenger astrophysics and fundamental physics. © 2022 SPIE. All rights reserved.

The dawning of low--frequency gravitational wave (GW) astronomy via pulsar timing arrays and space--based GW interferometry will provide new opportunities for the study of the supermassive black hole (SMBH) binaries which form as galaxies merge through cosmic time. The onset of observational GW studies has coincided with the expansion of wide--field optical time--domain surveys such as the Zwicky Transient Facility (ZTF), which provide a complementary way to detect and analyze SMBHs when they accrete gas and emit at optical wavelengths. In this thesis I describe how high cadence surveys like ZTF can be used to discover rare populations of massive black holes which inform our understanding of early massive black hole seeding channels and their subsequent growth through mergers to produce the SMBH populations we see today. In the first part of this thesis I present a search for variable active galactic nuclei (AGN) which are spatially offset from their host galaxies using time-resolved imaging data from ZTF and deeper, higher resolution imaging data from the Legacy Surveys. I present a population of 52 variable AGN in merging galaxies in addition to 9 candidates for gravitational wave recoil of remnant SMBHs which may be used to constrain SMBH binary merger rates and spin alignment efficiencies. I also examine the dramatic rebrightening of a previous recoiling SMBH candidate SDSS1133, and conclude from spectroscopic follow--up that it is more likely an outbursting luminous blue variable star. In the second part of the thesis, I present a population of 190 low--mass AGN in dwarf galaxies discovered by their optical or mid--infrared variability in deep ZTF difference imaging and forward--modeled photometry of {\it WISE} image stacks. These intermediate mass black hole (IMBH) candidates can be used to constrain the low--mass end of the $M_{BH}-\sigma_*$ relation and dwarf galaxy occupation fractions in order to better understand the origins of the first massive black hol

We describe the science case, design and expected performances of the X/Gamma-ray Imaging Spectrometer (XGIS), a GRB and transients monitor developed and studied for the THESEUS mission project, capable of covering an exceptionally wide energy band (2 keV – 10 MeV), with imaging capabilities and location accuracy <15 arcmin up to 150 keV over a Field of View of 2sr, a few hundreds eV energy resolution in the X-ray band (<30 keV) and few micro seconds time resolution over the whole energy band. Thanks to a design based on a modular approach, the XGIS can be easily re-scaled and adapted for fitting the available resources and specific scientific objectives of future high-energy astrophysics missions, and especially those aimed at fully exploiting GRBs and high-energy transients for multi-messenger astrophysics and fundamental physics.

Gamma-ray bursts (GRBs) are brief flashes of γ-rays and are considered to be the most energetic explosive phenomena in the Universe1. The emission from GRBs comprises a short (typically tens of seconds) and bright prompt emission, followed by a much longer afterglow phase. During the afterglow phase, the shocked outflow—produced by the interaction between the ejected matter and the circumburst medium—slows down, and a gradual decrease in brightness is observed2. GRBs typically emit most of their energy via γ-rays with energies in the kiloelectronvolt-to-megaelectronvolt range, but a few photons with energies of tens of gigaelectronvolts have been detected by space-based instruments3. However, the origins of such high-energy (above one gigaelectronvolt) photons and the presence of very-high-energy (more than 100 gigaelectronvolts) emission have remained elusive4. Here we report observations of very-high-energy emission in the bright GRB 180720B deep in the GRB afterglow—ten hours after the end of the prompt emission phase, when the X-ray flux had already decayed by four orders of magnitude. Two possible explanations exist for the observed radiation: inverse Compton emission and synchrotron emission of ultrarelativistic electrons. Our observations show that the energy fluxes in the X-ray and γ-ray range and their photon indices remain comparable to each other throughout the afterglow. This discovery places distinct constraints on the GRB environment for both emission mechanisms, with the inverse Compton explanation alleviating the particle energy requirements for the emission observed at late times. The late timing of this detection has consequences for the future observations of GRBs at the highest energies

e-ASTROGAM is a concept for a breakthrough observatory space mission carrying a γ-ray telescope dedicated to the study of the non-thermal Universe in the photon energy range from 0.15 MeV to 3 GeV. The lower energy limit can be pushed down to energies as low as 30 keV for gamma-ray burst detection with the calorimeter. The mission is based on an advanced space-proven detector technology, with unprecedented sensitivity, angular and energy resolution, combined with remarkable polarimetric capability. Thanks to its performance in the MeV-GeV domain, substantially improving its predecessors, e-ASTROGAM will open a new window on the non-thermal Universe, making pioneering observations of the most powerful Galactic and extragalactic sources, elucidating the nature of their relativistic outflows and their effects on the surroundings. With a line sensitivity in the MeV energy range one to two orders of magnitude better than previous and current generation instruments, e-ASTROGAM will determine the origin of key isotopes fundamental for the understanding of supernova explosion and the chemical evolution of our Galaxy. The mission will be a major player of the multiwavelength, multimessenger time-domain astronomy of the 2030s, and provide unique data of significant interest to a broad astronomical community, complementary to powerful observatories such as LISA, LIGO, Virgo, KAGRA, the Einstein Telescope and the Cosmic Explorer, IceCube-Gen2 and KM3NeT, SKA, ALMA, JWST, E-ELT, LSST, Athena, and the Cherenkov Telescope Array., QC 20181105

e-ASTROGAM is a concept for a breakthrough observatory space mission carrying a γ-ray telescope dedicated to the study of the non-thermal Universe in the photon energy range from 0.15 MeV to 3 GeV. The lower energy limit can be pushed down to energies as low as 30 keV for gamma-ray burst detection with the calorimeter. The mission is based on an advanced space-proven detector technology, with unprecedented sensitivity, angular and energy resolution, combined with remarkable polarimetric capability. Thanks to its performance in the MeV-GeV domain, substantially improving its predecessors, e-ASTROGAM will open a new window on the non-thermal Universe, making pioneering observations of the most powerful Galactic and extragalactic sources, elucidating the nature of their relativistic outflows and their effects on the surroundings. With a line sensitivity in the MeV energy range one to two orders of magnitude better than previous and current generation instruments, e-ASTROGAM will determine the origin of key isotopes fundamental for the understanding of supernova explosion and the chemical evolution of our Galaxy. The mission will be a major player of the multiwavelength, multimessenger time-domain astronomy of the 2030s, and provide unique data of significant interest to a broad astronomical community, complementary to powerful observatories such as LISA, LIGO, Virgo, KAGRA, the Einstein Telescope and the Cosmic Explorer, IceCube-Gen2 and KM3NeT, SKA, ALMA, JWST, E-ELT, LSST, Athena, and the Cherenkov Telescope Array.

Reverberation mapping is a time-domain technique used to resolve the supermassive black hole’s sphere of influence in active galactic nuclei. We carried out a nine-month reverberation mapping campaign to measure the broad-line region size and estimate the mass of the black hole in KA1858+4850, a narrow-line Seyfert 1 galaxy at redshift 0.078 and among the brightest active galaxies monitored by the Kepler mission. We obtained spectroscopic data using the Kast Spectrograph at the Lick 3-m telescope and complementary V-band images from five other ground-based telescopes. We measured the H light curve lag with respect to the V -band continuum light curve, and combined this lag with the H root-mean-square line profile width to obtain a virial estimate of MBH = 8.06 (+1.59, −1.72) × 10^6 M⊙ for the mass of the central black hole and an Eddington ratio of L/LEdd ≈ 0.2.I also used reverberation mapping to study in detail the broad line region in NGC 5548, a Seyfert 1 galaxy at redshift 0.017. Optical spectroscopic data targeting NGC 5548 were taken in 2014 as part of a larger multi-wavelength reverberation mapping campaign. The ground-based spectra spanned six months and achieved almost daily cadence with observations from five telescopes. We computed the H and He II 4686 lags relative to both the optical continuum and the UV continuum measured by the Hubble Space Telescope, and found the H–UV lag to be ∼50% longer than the H–optical lag. This suggests that the true broad-line region size is 50% larger than the size that would be inferred from optical data alone. We also measured velocity-resolved lags for H and found a complex velocity-lag structure with shorter lags in the line wings. The responsivity of both the H and He II lines decreased halfway through the campaign, an anomalous phenomenon also observed for the UV emission lines during the same monitoring period. Finally, we showed that, given the optical luminosity of NGC 5548 during our campaign, the measured H lag

Reverberation mapping is a time-domain technique used to resolve the supermassive black hole’s sphere of influence in active galactic nuclei. We carried out a nine-month reverberation mapping campaign to measure the broad-line region size and estimate the mass of the black hole in KA1858+4850, a narrow-line Seyfert 1 galaxy at redshift 0.078 and among the brightest active galaxies monitored by the Kepler mission. We obtained spectroscopic data using the Kast Spectrograph at the Lick 3-m telescope and complementary V-band images from five other ground-based telescopes. We measured the H light curve lag with respect to the V -band continuum light curve, and combined this lag with the H root-mean-square line profile width to obtain a virial estimate of MBH = 8.06 (+1.59, −1.72) × 10^6 M⊙ for the mass of the central black hole and an Eddington ratio of L/LEdd ≈ 0.2.I also used reverberation mapping to study in detail the broad line region in NGC 5548, a Seyfert 1 galaxy at redshift 0.017. Optical spectroscopic data targeting NGC 5548 were taken in 2014 as part of a larger multi-wavelength reverberation mapping campaign. The ground-based spectra spanned six months and achieved almost daily cadence with observations from five telescopes. We computed the H and He II 4686 lags relative to both the optical continuum and the UV continuum measured by the Hubble Space Telescope, and found the H–UV lag to be ∼50% longer than the H–optical lag. This suggests that the true broad-line region size is 50% larger than the size that would be inferred from optical data alone. We also measured velocity-resolved lags for H and found a complex velocity-lag structure with shorter lags in the line wings. The responsivity of both the H and He II lines decreased halfway through the campaign, an anomalous phenomenon also observed for the UV emission lines during the same monitoring period. Finally, we showed that, given the optical luminosity of NGC 5548 during our campaign, the measured H lag

Stellar evolution proceeds on timescales of millions to billions of years, however, most stars in both the birthing and dying processes will punctuate their lives with short-lived (few days to a year) eruptions or explosions that lead to dramatic changes in the brightness of the star. Observational studies of these events are the foundation upon which time-domain astronomy is built. While these events are short lived, they often provide some of our most significant insights into stellar evolution. This dissertation focuses on the use of time-domain techniques to discover and characterize these rare astrophysical gems, while also addressing some gaps in our understanding of the earliest and latest stages of stellar evolution. The observational studies presented herein can be grouped into three parts: (i) the study of stellar death (supernovae); (ii) the study of stellar birth; and (iii) the use of modern machine-learning algorithms to discover and classify variable sources. I present observations of supernova (SN) 2006gy, the most luminous SN ever at the time of discovery, and the even-more luminous SN 2008es. Together, these two supernovae (SNe) demonstrate that core-collapse SNe can be significantly more luminous than thermonuclear type Ia SNe, and that there are multiple channels for producing these brilliant core-collapse explosions. For SN 2006gy I show that the progenitor star experienced violent, eruptive mass loss on multiple occasions during the centuries prior to explosion, a scenario that was completely unexpected within the cannon of massive-star evolution theory. I also present observations of SN 2008iy, one of the most unusual SNe ever discovered. Typical SNe take ≤3 weeks to reach peak luminosity; SN 2008iy exhibited a slow and steady rise for ~400 days before reaching maximum brightness. The best explanation for such behavior is that the progenitor of SN 2008iy experienced an episodic phase of mass loss ~100 yr prior to explosion. The three SNe detail

Stellar evolution proceeds on timescales of millions to billions of years, however, most stars in both the birthing and dying processes will punctuate their lives with short-lived (few days to a year) eruptions or explosions that lead to dramatic changes in the brightness of the star. Observational studies of these events are the foundation upon which time-domain astronomy is built. While these events are short lived, they often provide some of our most significant insights into stellar evolution. This dissertation focuses on the use of time-domain techniques to discover and characterize these rare astrophysical gems, while also addressing some gaps in our understanding of the earliest and latest stages of stellar evolution. The observational studies presented herein can be grouped into three parts: (i) the study of stellar death (supernovae); (ii) the study of stellar birth; and (iii) the use of modern machine-learning algorithms to discover and classify variable sources. I present observations of supernova (SN) 2006gy, the most luminous SN ever at the time of discovery, and the even-more luminous SN 2008es. Together, these two supernovae (SNe) demonstrate that core-collapse SNe can be significantly more luminous than thermonuclear type Ia SNe, and that there are multiple channels for producing these brilliant core-collapse explosions. For SN 2006gy I show that the progenitor star experienced violent, eruptive mass loss on multiple occasions during the centuries prior to explosion, a scenario that was completely unexpected within the cannon of massive-star evolution theory. I also present observations of SN 2008iy, one of the most unusual SNe ever discovered. Typical SNe take ≤3 weeks to reach peak luminosity; SN 2008iy exhibited a slow and steady rise for ~400 days before reaching maximum brightness. The best explanation for such behavior is that the progenitor of SN 2008iy experienced an episodic phase of mass loss ~100 yr prior to explosion. The three SNe detail