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Optical transient surveys have ushered in the golden era of time-domain astronomy and exposed an unexpected breadth of transient phenomena, including nuclear transients: flares powered by accretion onto supermassive black holes (SMBHs). These transients provide a means to identify otherwise-quiescent SMBHs. More importantly, if their physics is well understood, they can be used to study the properties of SMBHs across a range of masses and redshifts. Here I detail my work on the major classes of nuclear transients through both case studies of well-observed objects—all discovered by UH-affiliated surveys—and population studies to investigate trends. Chapters 2–5 discuss various aspects of tidal disruption events (TDEs), Chapter 6 explains coronal line emitters (CLEs) as TDEs in gas-rich environments, Chapters 7–9 establish the growing group of ambiguous nuclear transients (ANTs) with observables similar to both TDEs and active galactic nuclei, and Chapter 10 introduces the class of extreme nuclear transients (ENTs) which are the most energetic transients known. Each class is powered by accretion, but it is clear that the nuclear environment shapes their observational signatures. I finally conclude by summarizing our current understanding of nuclear transients and promising future directions to leverage their utility as probes of SMBHs.

Alongside the recent increase in discoveries of tidal disruption events (TDEs) have come an increasing number of ambiguous nuclear transients (ANTs). These ANTs are characterized by hot blackbody-like UV/optical spectral energy distributions (SEDs) and smooth photometric evolution, often with hard powerlaw-like X-ray emission. ANTs are likely exotic TDEs or smooth flares originating in active galactic nuclei (AGNs). While their emission in the UV/optical and X-ray has been relatively well-explored, their infrared (IR) emission has not been studied in detail. Here we use the NEOWISE mission and its low-cadence mapping of the entire sky to study mid-infrared dust reprocessing echoes of ANTs. We study 19 ANTs, finding significant MIR flares in 18 objects for which we can estimate an IR luminosity and temperature evolution. The dust reprocessing echoes show a wide range in IR luminosities (\\(\\sim10^{42} - 10^{45}\\) erg s\\(^{-1}\\)) with blackbody temperatures largely consistent with sublimation temperature of graphite grains. Excluding the two sources possibly associated with luminous supernovae (ASASSN-15lh and ASASSN-17jz), the dust covering fractions (f\\(_c\\)) for detected IR flares lie between 0.05 and 0.91, with a mean of f\\(_c\\) = 0.29 for all ANTs (including limits) and f\\(_c\\) = \\(0.38 \\pm 0.04\\) for detections. These covering fractions are much higher than optically-selected TDEs and similar to AGNs. We interpret the high covering fractions in ANT host galaxies as evidence for the presence of a dusty torus.

While X-ray emission from active galactic nuclei (AGN) is common, the detailed physics behind this emission is not well understood. This is in part because high quality broadband spectra are required to precisely derive fundamental parameters of X-ray emission such as the photon index, folding energy, and reflection coefficient. Here we present values of such parameters for 33 AGN observed as part of the 105 month Swift/BAT campaign and with coordinated archival XMM-Newton and NuSTAR observations. We look for correlations between the various coronal parameters in addition to correlations between coronal parameters and physical properties such as black hole mass and Eddington ratio. Using our empirical model, we find good fits to almost all of our objects. The folding energy was constrained for 30 of our 33 objects. When comparing Seyfert 1 - 1.9 to Seyfert 2 galaxies, a K-S test indicates that Seyfert 2 AGN have lower Eddington ratios and photon indices than Seyfert 1 - 1.9 objects with p-values of \\(5.6 \\times 10^{-5}\\) and \\(7.5 \\times 10^{-3}\\) respectively. We recover a known correlation between photon index and reflection coefficient as well as the X-ray Baldwin effect. Finally, we find that the inclusion of the high energy Swift BAT data significantly reduces the uncertainties of spectral parameters as compared to fits without the BAT data.

Some galaxies show little to no sign of active galactic nucleus (AGN) activity, yet exhibit strong coronal emission lines (CLs) relative to common narrow emission lines. Many of these CLs have ionization potentials of \\(\\geq 100\\) eV, thus requiring strong extreme UV and/or soft X-ray flux. It has long been thought that such events are powered by tidal disruption events (TDEs), but owing to a lack of detailed multi-wavelength follow-up, such a connection has not been firmly made. Here we compare coronal line emitters (CLEs) and TDEs in terms of their host-galaxy and transient properties. We find that the mid-infrared (MIR) colors of CLE hosts in quiescence are similar to TDE hosts. Additionally, many CLEs show evidence of a large dust reprocessing echo in their mid-infrared colors, a sign of significant dust in the nucleus. The stellar masses and star formation rates of the CLE hosts are largely consistent with TDE hosts, with many CLEs residing within the green valley. The blackbody properties of CLEs and TDEs are similar, with some CLEs showing hot (T \\(\\geq 40,000\\) K) blackbody temperatures. Finally, the location of CLEs on the peak-luminosity/decline-rate parameter space is much closer to TDEs than many other major classes of nuclear transients. Combined, these provide strong evidence to confirm the previous claims that CLEs are indeed TDEs in gas-rich environments. We additionally propose a stricter threshold of CL flux \\(\\geq 1/3\\) \\(\\times\\) [O III] flux to better exclude AGNs from the sample of CLEs.

In November 2020, the Swift team announced a major update to the calibration of the UltraViolet and Optical Telescope (UVOT) data to correct for the gradual loss of sensitivity over time. Beginning in roughly 2015, the correction affected observations in the three near ultraviolet (UV) filters, reaching levels of up to 0.3 mag immediately prior to the correction. Over the same time period, an increasing number of Type I superluminous supernovae (SLSNe-I) were discovered and studied. Many SLSNe-I are hot (T\\(_\\textrm{eff}\\) \\(\\approx 10,000\\) K) near peak, and therefore accurate UV data are imperative towards properly understanding their physical properties and energetics. We re-compute Swift UVOT photometry for SLSNe-I discovered between 2014 and 2021 with at least 5 Swift observations in 2015 or later. We calculate host-subtracted magnitudes for each SLSN and fit their spectral energy distributions with modified blackbodies to obtain the radius and temperature evolution. We also fit multi-band photometry using the Modular Open Source Fitter for Transients (MOSFiT) to obtain key parameters such as the spin period (P), magnetic field strength (B), ejecta mass (M\\(_\\textrm{ej}\\)), and kinetic energy (E\\(_\\textrm{kin}\\)). From our MOSFiT modeling, we also estimate the peak UV/optical luminosity (L\\(_\\textrm{peak}\\)) and total radiative energy (E\\(_\\textrm{rad}\\)). Under the assumption of magnetar-powered SLSNe we find several strong trends, including anti-correlations between P and both L\\(_\\textrm{peak}\\) and E\\(_\\textrm{rad}\\), a correlation between E\\(_\\textrm{kin}\\) and E\\(_\\textrm{rad}\\), and an anti-correlation between B and E\\(_\\textrm{rad}\\).

We introduce the Hawai'i Supernova Flows project and present summary statistics of the first 1,217 astronomical transients observed, 668 of which are spectroscopically classified Type Ia Supernovae (SNe Ia). Our project is designed to obtain systematics-limited distances to SNe Ia while consuming minimal dedicated observational resources. To date, we have performed almost 5,000 near-infrared (NIR) observations of astronomical transients and have obtained spectra for over 200 host galaxies lacking published spectroscopic redshifts. In this survey paper we describe the methodology used to select targets, collect/reduce data, calculate distances, and perform quality cuts. We compare our methods to those used in similar studies, finding general agreement or mild improvement. Our summary statistics include various parametrizations of dispersion in the Hubble diagrams produced using fits to several commonly used SN Ia models. We find the lowest dispersions using the \\texttt{SNooPy} package's EBV\\_model2, with a root mean square (RMS) deviation of 0.165 mag and a normalized median absolute deviation (NMAD) of 0.123 mag. The full utility of the Hawai'i Supernova Flows data set far exceeds the analyses presented in this paper. Our photometry will provide a valuable test bed for models of SN Ia incorporating NIR data. Differential cosmological studies comparing optical samples and combined optical and NIR samples will have increased leverage for constraining chromatic effects like dust extinction. We invite the community to explore our data by making the light curves, fits, and host galaxy redshifts publicly accessible.

We analyze high-cadence data from the Transiting Exoplanet Survey Satellite (TESS) of the ambiguous nuclear transient (ANT) ASASSN-18el. The optical changing-look phenomenon in ASASSN-18el has been argued to be due to either a drastic change in the accretion rate of the existing active galactic nucleus (AGN) or the result of a tidal disruption event (TDE). Throughout the TESS observations, short-timescale stochastic variability is seen, consistent with an AGN. We are able to fit the TESS light curve with a damped-random-walk (DRW) model and recover a rest-frame variability amplitude of \\(\\hat{\\sigma} = 0.93 \\pm 0.02\\) mJy and a rest-frame timescale of \\(\\tau_{DRW} = 20^{+15}_{-6}\\) days. We find that the estimated \\(\\tau_{DRW}\\) for ASASSN-18el is broadly consistent with an apparent relationship between the DRW timescale and central supermassive black hole mass. The large-amplitude stochastic variability of ASASSN-18el, particularly during late stages of the flare, suggests that the origin of this ANT is likely due to extreme AGN activity rather than a TDE.

We present the discovery that ATLAS18mlw was a tidal disruption event (TDE) in the galaxy WISEA J073544.83+663717.3, at a luminosity distance of 334 Mpc. Initially discovered by the Asteroid Terrestrial Impact Last Alert System (ATLAS) on 2018 March 17.3, the TDE nature of the transient was uncovered only recently with the re-reduction of a SuperNova Integral Field Spectrograph (SNIFS) spectrum. This spectrum, taken by the Spectral Classification of Astronomical Transients (SCAT) survey, shows a strong blue continuum and a broad H\\(\\alpha\\) emission line. Here we present roughly six years of optical survey photometry beginning before the TDE to constrain AGN activity, optical spectroscopy of the transient, and a detailed study of the host galaxy properties through analysis of archival photometry and a host spectrum. ATLAS18mlw was detected in ground-based light curves for roughly two months. From a blackbody fit to the transient spectrum and bolometric correction of the optical light curve, we conclude that ATLAS18mlw is best explained by a low-luminosity TDE with a peak luminosity of log(L [erg s\\(^{-1}\\)]) = \\(43.5 \\pm 0.2\\). The TDE classification is further supported by the quiescent Balmer strong nature of the host galaxy. We also calculated the TDE decline rate from the bolometric light curve and find \\(\\Delta L_{40} = -0.7 \\pm 0.2\\) dex, making ATLAS18mlw a member of the growing class of ``faint and fast'' TDEs with low peak luminosities and fast decline rates.

We present observations of ASASSN-20hx, a nearby ambiguous nuclear transient (ANT) discovered in NGC 6297 by the All-Sky Automated Survey for Supernovae (ASAS-SN). We observed ASASSN-20hx from \\(-\\)30 to 275 days relative to peak UV/optical emission using high-cadence, multi-wavelength spectroscopy and photometry. From Transiting Exoplanet Survey Satellite (TESS) data, we determine that the ANT began to brighten on 2020 June 22.8 with a linear rise in flux for at least the first week. ASASSN-20hx peaked in the UV/optical 30 days later on 2020 July 22.8 (MJD = 59052.8) at a bolometric luminosity of \\(L = (3.15 \\pm 0.04) \\times 10^{43}\\) erg s\\(^{-1}\\). The subsequent decline is slower than any TDE observed to date and consistent with many other ANTs. Compared to an archival X-ray detection, the X-ray luminosity of ASASSN-20hx increased by an order of magnitude to \\(L_{x} \\sim 1.5 \\times 10^{42}\\) erg s\\(^{-1}\\) and then slowly declined over time. The X-ray emission is well-fit by a power law with a photon index of \\(\\Gamma \\sim 2.3 - 2.6\\). Both the optical and near infrared spectra of ASASSN-20hx lack emission lines, unusual for any known class of nuclear transient. While ASASSN-20hx has some characteristics seen in both tidal disruption events (TDEs) and active galactic nuclei (AGNs), it cannot be definitively classified with current data.

The tidal disruption event (TDE) AT2018dyk/ASASSN-18UL showed a rapid dimming event 500 days after discovery, followed by a re-brightening roughly 700 days later. It has been hypothesized that this behavior results from a repeating partial TDE (rpTDE), such that prompt dimmings/shutoffs are coincident with the return of the star to pericenter and rebrightenings generated by the renewed supply of tidally stripped debris. This model predicted that the emission should shut off again around August of 2023. We report AT2018fyk's continued X-ray and UV monitoring, which shows an X-ray (UV) drop in flux by a factor of 10 (5) over a span of two months, starting 14 Aug 2023. This sudden change can be interpreted as the second emission shutoff, which 1) strengthens the rpTDE scenario for AT2018fyk, 2) allows us to constrain the orbital period to a more precise value of 1306\\(\\pm\\)47 days, and 3) establishes that X-ray and UV/optical emission track the fallback rate onto this SMBH -- an often-made assumption that otherwise lacks observational verification -- and therefore the UV/optical lightcurve is powered predominantly by processes tied to X-rays. The second cutoff implies that another rebrightening should happen between May-Aug 2025, and if the star survived the second encounter, a third shutoff is predicted to occur between Jan-July 2027. Finally, low-level accretion from the less bound debris tail (which is completely unbound/does not contribute to accretion in a non-repeating TDE) can result in a faint X-ray plateau that could be detectable until the next rebrightening.