X-ray Spectroscopy of Cool Core Galaxy Clusters

In this thesis, I present the results of my PhD research on the cooling flow problem in galaxy clusters. The centre of relaxed galaxy clusters has a short radiative cooling time suggesting the presence of a massive cooling flow. However, early studies using high resolution X-ray spectroscopy indicated much lower levels of cooling rate. AGN feedback is thought to be the most likely energy source to balance radiative cooling, though the energy transport and dissipation mechanisms are still under debate. In this work, I study whether AGN feedback can actually balance radiative cooling in a large number of galaxy clusters using high resolution X-ray spectroscopy. The first chapter contains the necessary background of the cooling flow problem and AGN feedback. This is followed by a chapter describing data reduction of XMM-Newton observations. In the third chapter, I present a study of 45 nearby cool core galaxy clusters and groups, where I measure the radiative cooling rate in the softest X-ray band. Then I select a small sub sample of bright clusters to understand the mass temperature profile of the gas in Chapter 4. In Chapter 5, I present a deep study of recent XMM-Newton observations of two luminous clusters at intermediate redshift. Finally, I extend my research on 40 more clusters within a much larger range of distances corresponding to redshift up to 0.6

Superradiant Atomic Beam Laser

Steady-state superradiant lasers are a promising candidate for next-generation ultracoherent light sources. In this thesis, we propose a new type of superradiant laser based on a hot atomic beam traversing an optical cavity. We show that the theoretical minimum linewidth and maximum power are competitive with the best ultracoherent clock lasers. Also, our system operates naturally in a continuous wave modality, which has been elusive for superradiant lasers so far. Unlike many existing proposals for ultracoherent lasers, our design is simple and rugged. This makes it a potential candidate for the first widely accessible ultracoherent laser, as well as the first to realize sought-after applications of ultracoherent lasers in challenging environments.Aside from metrological usefulness, the superradiant atom beam laser system is of fundamental interest in terms of various superradiant phase transitions. To this end, we theoretically analyze the system for three different configurations: (i) For a thermal atomic beam interacting with a resonant cavity mode, we derive a semiclassical model and determine the onset of superradiant emission and its stability. We find two different superradiant phases; a steady-state superradiant phase and a multi-component superradiant phase. In the latter case we observe sidebands in the frequency spectrum that can be calculated using a stability analysis of the amplitude mode of the collective dipole. We show that both superradiant phases are robust against free-space spontaneous emission and $T_2$ dephasing processes. (ii) For a collimated atomic beam interacting with an off-resonant cavity mode, we derive an analytical formula for the cavity pulling coefficient. We find that the pulling is small if the cavity linewidth is much larger than the collective linewidth of the atomic beam. This regime is desired for building stable lasers because the emission frequency is robust against cavity length fluctuations. Furthermore, we find polychromatic emission regimes, where the spectrum has several frequency components while the light output is still superradiant. (iii) For a slanted collimated atomic beam passing through a cavity that is on resonance, we find that the atoms undergo superradiant emission when the collective linewidth exceeds the transit-time broadening. We find steady-state superradiance providing the tilt of the atomic beam is sufficiently small. However, if the atoms travel more than half a wavelength along the cavity axis during one transit time we predict a dynamical phase transition to a new bistable superradiant regime. In this phase the atoms undergo collective spontaneous emission with a frequency that can be either blue or red detuned from the free-space atomic resonance. We show that the linewidth of the emitted light exhibits features of a critical scaling close to the phase boundaries.</p

The Fast and the Private: Task-based Dataset Search

Modern dataset search platforms employ ML task-based utility metrics instead of relying on metadata-based keywords to comb through extensive dataset repositories. In this setup, requesters provide an initial dataset, and the platform identifies complementary datasets to augment (join or union) the requester's dataset such that the ML model (e.g., linear regression) performance is improved most. Although effective, current task-based data searches are stymied by (1) high latency which deters users, (2) privacy concerns for regulatory standards, and (3) low data quality which provides low utility. We introduce Mileena, a fast, private, and high-quality task-based dataset search platform. At its heart, Mileena is built on pre-computed semi-ring sketches for efficient ML training and evaluation. Based on semi-ring, we develop a novel Factorized Privacy Mechanism that makes the search differentially private and scales to arbitrary corpus sizes and numbers of requests without major quality degradation. We also demonstrate the early promise in using LLM-based agents for automatic data transformation and applying semi-rings to support causal discovery and treatment effect estimation