In this work, a cryogenic electron beam ion trap (EBIT) for charge breeding of rare, short-lived isotopes at the ARIEL facility at TRIUMF was designed and assembled for future investigations of astrophysical processes. It was characterized at a maximum electron-beam current of 1 A. Furthermore, charge-exchange (CX) processes, which are relevant for the interface between hot plasmas and cold gases in astrophysical environments, were extensively studied. In these experiments, hydrogen-like and bare sulfur, argon, and oxygen ions capturing electrons from various neutral gas targets into highly-excited states with subsequent radiative relaxation via emission of X-rays were investigated. Thereby, a possible explanation for the recently observed emission line at 3.5 keV in galaxy clusters, with a proposed origin in the decay of sterile neutrinos, was given, in the form of CX induced K-shell emission from hydrogen-like sulfur. Additionally, measurements of extreme-ultraviolet L-shell transitions following CX into highly ionized oxygen in the range between 8 nm and 20 nm, utilizing a grating spectrometer, is presented. The results are compared with different CX models, mainly based on the multichannel Landau-Zener approach. Significant discrepancies between various experiments and models, as well as the models among each other, were ascertained and are extensively discussed.
Quantum electrodynamics in an intense plane-wave field is considered within the framework of light-cone quantization. In this context, high-energy vacuum birefringence and dichroism in an intense laser field are investigated. In particular, a setup is analyzed, in which probe gamma photons are generated via Compton backscattering, propagate through an intense laser pulse, and are subsequently detected via pair production in matter. The polarization of the photon beam is characterized by the Stokes parameters, and their change due to vacuum birefringence and dichroism is determined. The magnitude of the polarization effects is assessed for upcoming high-power laser facilities. Optimal parameters and regimes are identified, and the required statistics and the duration of the experiment in order to confirm the prediction of quantum electrodynamics are estimated. Furthermore, an approach for the treatment of the bispinor part of scattering amplitudes in a plane-wave field is introduced. Simplified expressions for the vertex functions, arising from the interaction terms of the lightfront Hamiltonian, are obtained. It is demonstrated that with the developed technique the evaluation of the gamma-matrix traces for scattering in an external plane-wave field can be performed in a relatively straightforward way and the final results can be written in a compact form.
Bright quasars are powerful sources of ionizing radiation and have profound impact on the Intergalactic Medium. In particular, they create regions with enhanced ionization and therefore reduced Lyman α forest absorption in their surroundings. Observing this so-called transverse proximity effect along background sightlines provides a view of the foreground quasar from different vantage points, and hence at different lookback times compared to the line-of-sight toward Earth. One can thus constrain the emission history (lifetime, age) and emission geometry (obscuration, opening angle) of the foreground quasar based purely on geometric and light travel time arguments. Both quantities are so far poorly constrained by observations but fundamental for the understanding of Active Galactic Nuclei. To investigate the HeII transverse proximity effect, we conducted an optical spectroscopic foreground quasar survey around 22 HST/COS sightlines, leading to a sample of 20 foreground quasars. We find statistical evidence for the the HeII transverse proximity effect and infer a constraint on the quasar lifetime of > 25 Myr. From a detailed modeling, based on cosmological hydrodynamical simulations and a dedicated photoionization model including quasar obscuration and finite quasar lifetime, we derive joint constraints on age and obscuration of individual objects, indicating that one quasar is old and unobscured (tage ≈ 25 Myr, Ωobsc < 30 %) while three other are either young (tage < 10 Myr) or highly obscured (Ωobsc > 70 %). However, the models also reveal that the large scatter intrinsic to the HeII Lyα forest prohibits further progress in the field. I therefore developed a novel method that uses large numbers of HI Lyα forest spectra to map the 3D light echo of individual quasars. An end-to-end test confirms that such tomographic observations can constrain the age of hyperluminous quasars to better than 20%, requiring only 1 – 2 nights on existing 8 – 10 m facilities. The method bears potential to also constrain the quasar emission geometry and the full lightcurve over the past 100 Myr, rendering it a viable tool to investigate quasar properties.