Professor Roger Davies

Abstract:

The light we observe from distant astrophysical objects including supernovae and quasars allows us to determine large distances in terms of a cosmological model. Despite the success of the standard cosmological model in fitting the data, there remains no underlying explanation for the accelerated expansion and dark matter. Furthermore, there is a current tension between early- and late-universe determinations of the Hubble constant. New techniques may offer the possibility of measuring out to larger distances, provide complementary information, or be able to side-step current limitations. After reviewing in detail the fundamentals of standard cosmology and gravitational lensing, including a derivation of the cosmological lens equation, this thesis investigates a novel method of cosmography based on combining the techniques of strong gravitational lensing time delay measurements and quasar reverberation mapping. The motivation for this method was the possibility of avoiding lens modelling challenges, such as the mass-sheet degeneracy, typically associated with time delay cosmography. It suggested that differential time delays originating from spatially separated signals in the Broad Line Region of a quasar could be distinguished and measured from the spectroscopy of the images, and utilised to provide a ratio of cosmological distances independent of the lensing potential. An analytic description of the effect of the differential lensing on the emission line spectral flux for axisymmetric Broad Line Region geometries is given, with the inclined ring or disk, spherical shell, and double cone as examples. This critical examination shows that the proposed method is unable to recover cosmological information, as the observed time delay and inferred line-of-sight velocity do not uniquely map to the three-dimensional position within the quasar.Comment: PhD Thesis, 191 page

Abstract:

We report the time-resolved spectral analysis of a bright near-infrared and moderate X-ray flare of Sgr A ⋆ . We obtained light curves in the M , K , and H bands in the mid- and near-infrared and in the 2 − 8 keV and 2 − 70 keV bands in the X-ray. The observed spectral slope in the near-infrared band is νL ν ∝ ν 0.5 ± 0.2 ; the spectral slope observed in the X-ray band is νL ν ∝ ν −0.7 ± 0.5 . Using a fast numerical implementation of a synchrotron sphere with a constant radius, magnetic field, and electron density (i.e., a one-zone model), we tested various synchrotron and synchrotron self-Compton scenarios. The observed near-infrared brightness and X-ray faintness, together with the observed spectral slopes, pose challenges for all models explored. We rule out a scenario in which the near-infrared emission is synchrotron emission and the X-ray emission is synchrotron self-Compton. Two realizations of the one-zone model can explain the observed flare and its temporal correlation: one-zone model in which the near-infrared and X-ray luminosity are produced by synchrotron self-Compton and a model in which the luminosity stems from a cooled synchrotron spectrum. Both models can describe the mean spectral energy distribution (SED) and temporal evolution similarly well. In order to describe the mean SED, both models require specific values of the maximum Lorentz factor γ max , which differ by roughly two orders of magnitude. The synchrotron self-Compton model suggests that electrons are accelerated to γ max ∼ 500, while cooled synchrotron model requires acceleration up to γ max ∼ 5 × 10 4 . The synchrotron self-Compton scenario requires electron densities of 10 10 cm −3 that are much larger than typical ambient densities in the accretion flow. Furthermore, it requires a variation of the particle density that is inconsistent with the average mass-flow rate inferred from polarization measurements and can therefore only be realized in an extraordinary accretion event. In contrast, assuming a source size of 1 R S , the cooled synchrotron scenario can be realized with densities and magnetic fields comparable with the ambient accretion flow. For both models, the temporal evolution is regulated through the maximum acceleration factor γ max , implying that sustained particle acceleration is required to explain at least a part of the temporal evolution of the flare.