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We present a systematic X-ray analysis of the narrow-line Type 1 quasar PHL 1811, which has long been regarded as the prototype of intrinsically X-ray weak quasars. A critical breakthrough came with the first detection of a bright X-ray flare from this source by the Einstein Probe (EP) in 2024. We utilize archival X-ray observations spanning 2001─2024, including the post-flare EP and Swift data. We confirm that PHL 1811 shows X-ray weakness factors f<SUB>weak</SUB> ≍ 23─179 across all epochs before 2024. The 2024 EP flare marks the first detection of an X-ray nominal state with f<SUB>weak</SUB> ≍ 0.63, followed by a rapid flux decline. We identify three key observational signatures that strongly support heavy obscuration: (1) a significant hard X-ray excess above ≍5 keV in the 2015 XMM-Newton spectrum; (2) relatively flat spectral shapes in two Swift observations; and (3) transitions between X-ray nominal and multiple X-ray weak states without corresponding optical/infrared variability, consistent with expectations from obscuration by a clumpy dust-free absorber. Fitting with a partial-covering obscuration model reproduces all multiepoch spectra well. The observed steep spectra are dominated by a small leaked/scattered fraction of the intrinsic continuum, and variability is driven by changes in the leakage fraction and column density. Our results strongly favor the scenario where PHL 1811 is obscured by a radiatively driven accretion-disk wind from super-Eddington accretion, unifying PHL 1811 with the broader population of super-Eddington accreting active galactic nuclei under a single obscuration framework.

Sterile neutrinos offer a minimal and testable explanation for dark matter (DM), with their radiative decay actively searched for in X-ray observations. We show that cold sterile neutrino DM can be efficiently produced during reheating from inflaton decays with a small branching ratio, BR ≲ 10<SUP>-4</SUP>. This production mechanism opens regions of parameter space where the active-sterile mixing is small enough to evade current X-ray constraints while reproducing the observed DM abundance. We systematically map the viable parameter space in terms of the sterile neutrino mass, mixing angle, inflaton mass, reheating temperature, and branching ratio. We further demonstrate that sterile neutrino DM can serve as a probe of inflationary reheating, with future X-ray observations capable of yielding information on the inflaton mass and the reheating-temperature ratio m<SUB>ϕ</SUB> /T <SUB>rh</SUB>. For a given inflationary model in which the inflaton mass is known, this leads to a lower bound on the reheating temperature that is several orders of magnitude stronger than the existing bound from Big Bang Nucleosynthesis.

In order to shed light on the quantum-to-classical transition of the primordial perturbations in single field inflation, we investigate the decoherence of large-scale (superhorizon) scalar curvature perturbations. The latter are considered as an open quantum system which undergoes quantum decoherence induced by a time-dependent environment of deep subhorizon tensorial modes through the trilinear interactions predicted by General Relativity. A complete calculation is a complex one and still to be fully performed. We advance in that direction, and we elaborate on various aspects that have been overlooked in the literature. We first prove that, in full generality, a time dependent subhorizon environment can be relevant for decoherence during inflation, by considering derivativeless interactions, which, in our case, give the most important results. For the first time, the time dependence of the environment is properly taken into account by modifying the quantum master equation. Important non-Markovian effects pop up, instead, when dealing with derivative interactions. We adopt a possible way to treat them which has been recently proposed and seems well suited for our case. Our results show that when considering the interplay between derivativeless and derivative interactions, decoherence is slowed down. This underlines the importance of accounting for all the interactions in open quantum-system calculations in an inflationary setting. We finally compute the quantum corrections to cosmological correlation functions, by solving the transport equations induced by the quantum master equation. We also compare the results to the solutions obtained by an alternative method previously used in the literature. We observe a resummation of the quantum corrections to the power-spectrum of the Sasaki Mukhanov variable v, which is a general property of quantum master equations. We extend these results to the bispectrum of v.