MODELING X-RAY DATA OF BLACK HOLE BINARIES

We test the truncated disc models using multiwavelength (optical/UV/X-ray) data from the 2005 hard state outburst of the black hole SWIFT J1753.5-0127. This system is both fairly bright and has fairly low interstellar absorption, so gives one of the best datasets to study the weak, cool disc emission in this state. We fit these data using models of an X-ray illuminated disc to constrain the inner disc radius throughout the outburst. Close to the peak, the observed soft X-ray component is consistent with being produced by the inner disc, with its intrinsic emission enhanced in temperature and luminosity by reprocessing of hard X-ray illumination in an overlap region between the disc and corona. This disc emission provides the seed photons for Compton scattering to produce the hard X-ray spectrum, and these hard X-rays also illuminate the outer disc, producing the optical emission by reprocessing. However, towards the end of the outburst, all these conclusions may change. The optical points clearly lie on an extrapolation of the hard X-ray flux, which may indicate that the seed photons for Compton scattering are now self-generated in the flow by Cyclo-Synchrotron radiation rather than being from the disc. The weak soft X-ray emission implies a small disc radius, unchanged from the outburst peak, in conflict with the expectations of the truncated disc model. However, this also requires that the energy to power the corona is advected vertically and radially in a dissipationless fashion from the disc. Thus it seems more likely that the soft X-ray component is not direct emission from the disc itself. We show that a similarly dim low/hard state spectrum from XTE J1118+480 puts similar constraints on the soft X-ray emission region, but here the very low interstellar absorption (an order of magnitude smaller than in SWIFT J1753.5-0127) allows detection of a much larger, cooler, UV component which is well fit by a truncated disc. Thus whatever the origin of the additional weak soft X-ray emission (irradiation of the inner face as opposed to the top surface of the inner edge of the disc, residual inner disc left from evaporation, ionised reflection, jet etc), its existence as a clearly separate component from the truncated disc in XTE J1118+480 shows that it does not trace the inner disc radius, so cannot be used to constrain the truncated disc models

COPPER OXIDE NANOARCHITECTURES FOR PHOTOELECTROCHEMICAL HYDROGEN GENERATION

Hydrogen is a high-quality energy carrier, similar to electricity, which can be used with high efficiency and near-zero emissions at the point of use. The most promising means of producing hydrogen using a renewable energy source and potentially reducing the generation of greenhouse gases production is through solar-hydrogen photoelectrochemical (PEC) water decomposition. In order to utilize the solar irradiation, small band gap material is essential. In this dissertation, I focus on the earth abundant, non-toxic and direct transit copper oxide (CuO) with band gap around 1.3-1.8 eV. In a PEC cell, the photo-excited charge carriers need to be separated as soon as they form in order to have a high photocurrent density. Thus, four approaches are studied: (1) decrease particle size to decrease the electron-hole recombination in the particles, (2) increase surface area to increase the active sites and decrease the distance for electrons travel to the surface to react with water (3) increase conductivity to decrease the resistance of the electrode, and (4) shorten charge carrier transport distance to decrease the chance of recombination of charge carriers. In the first part of this study, I describe the aerosol route, flame spray pyrolysis, for making CuO nanoparticles. By controlling the precursor concentration and flame conditions, the particle size can be tuned. Also, the simulation results of particle growth, based on collision/sintering theory with sintering by solid state diffusion, are in good agreement with the experimental results. Furthermore, the flame spray pyrolysis made CuO nanoparticles were spin coated on conducting ITO glass substrates for the PEC study. Here, the relatively uniform CuO nanoparticles showed much better photocurrent density compared to the commercial CuO nanoparticles with a broad size distribution. This demonstrates the importance of the size of material for PEC application. The second approach I introduced is to increase surface area to increase the active sites. Instead of changing the CuO suspension concentration to make films with different porosity, I present a new route for forming porous structures by spin coating the powder including CuO and its intermediate product, Cu2(NO3)(OH)3. During the post annealing process, the intermediate product transforms into CuO and leaves voids in the film, thus producing a porous film and increasing the active surface area for the water splitting reaction. In the third approach, lithium was incorporated as a dopant to increase conductivity and decrease the resistance of the electrode. With the lithium added, the conductivity increased by two orders of magnitude and thus highly decreased the film resistance and increased the photocurrent density. The final part of this dissertation focuses on three dimensional current collectors, used to decrease the charge carrier transport distance and thus decrease the chance of recombination. Here, the genetically modified tobacco mosaic virus (TMV1cys) served as a template for the three dimensional structure, made by sputter deposition of CuO. By varying the virus concentration, the distance between the current collectors can be tuned to optimize the charge carrier transport distance, light reflection as well as the CuO thickness for efficient absorption of solar energy

Modelling the Extreme X-ray Spectrum of IRAS 13224-3809

The extreme NLS1 galaxy IRAS 13224-3809 shows significant variability, frequency depended time lags, and strong Fe K line and Fe L features in the long 2011 XMM-Newton observation. In this work we study the spectral properties of IRAS 13224-3809 in detail, and carry out a series of analyses to probe the nature of the source, focusing in particular on the spectral variability exhibited. The RGS spectrum shows no obvious signatures of absorption by partially ionised material (warm absorbers). We fit the 0.3-10.0 keV spectra with a model that includes relativistic reflection from the inner accretion disc, a standard powerlaw AGN continuum, and a low-temperature (~0.1 keV) blackbody, which may originate in the accretion disc, either as direct or reprocessed thermal emission. We find that the reflection model explains the time-averaged spectrum well, and we also undertake flux-resolved and time-resolved spectral analyses, which provide evidence of gravitational light-bending effects. Additionally, the temperature and flux of the blackbody component are found to follow the $L\propto T^{4}$ relation expected for simple thermal blackbody emission from a constant emitting area, indicating a physical origin for this component.Comment: 12 pages, 7 figures, accepted for publication in MNRA

On the evolution of the inner disk radius with flux in the neutron star low-mass X-ray binary Serpens X-1

We analyze the latest \emph{Suzaku} observation of the bright neutron star low-mass X-ray binary Serpens X-1 taken in 2013 October and 2014 April. The observation was taken using the burst mode and only suffered mild pile-up effects. A broad iron line is clearly detected in the X-ray spectrum. We test different models and find that the iron line is asymmetric and best interpreted by relativistic reflection. The relativistically broadened iron line is generally believed to originate from the innermost regions of the accretion disk, where strong gravity causes a series of special and general relativistic effects. The iron line profile indicates an inner radius of $\sim8$ $R_{\rm G}$, which gives an upper limit on the size of the neutron star. The asymmetric iron line has been observed in a number of previous observations, which gives several inner radius measurements at different flux states. We find that the inner radius of Serpens X-1 does not evolve significantly over the range of $L/L_{\rm Edd}\sim0.4-0.6$, and the lack of flux dependence of the inner radius implies that the accretion disk may be truncated outside the innermost stable circular orbit by the boundary layer rather than the stellar magnetic field.Comment: 8 pages, 6 figures, accepted for publication in Ap

Confirmation of the nature of the absorber in IRAS 09104+4109

We present the first long Suzaku observation of the hyperluminous infrared galaxy IRAS 09104+4109 which is dominated by a Type 2 AGN. The infrared to X-ray spectral energy distribution (SED) indicates that the source is an obscured quasar with a Compton-thin absorber. However, the 3σ hard X-ray detection of the source with the BeppoSAX PDS suggested a reflection-dominated, Compton-thick view. The high-energy detection was later found to be possibly contaminated by another Type 2 AGN, NGC 2785, which is only 17 arcmin away. Our new Suzaku observation offers simultaneous soft and hard X-ray coverage and excludes contamination from NGC 2785. We find that the hard X-ray component is not detected by the Suzaku Hard X-ray Detector/PIN (effective energy band 14-45 keV). Both reflection and transmission models have been tested on the latest Suzaku and Chandra data. The 0.5-10 keV spectrum can be well modelled by the two scenarios. In addition, our analysis implied that the absorption column required in both models is NH ˜ 5 × 1023 cm-2. Unless IRAS 09104+4109 is a `changing-look\u27 quasar, we confirm that it is a Compton-thin AGN. Although the lack of detection of X-ray emission above 10 keV seems to favour the transmission scenario, we found that the two models offer fairly similar flux predictions over the X-ray band below ˜40 keV. We also found that the strong iron line shown in the Suzaku spectrum is in fact a blend of two emission lines, in which the 6.4 keV one is mostly contributed from the AGN and the 6.7 keV from the hot cluster gas. This implies that the neutral line is perhaps caused by disc reflection, and the reflection-dominated model is more likely the explanation. The transmission model should not be completely ruled out, but a deeper hard X-ray spectrum observation is needed to discriminate between the two scenarios

X-ray Lags in PDS 456 Revealed by Suzaku Observations

X-ray reverberation lags from the vicinity of supermassive black holes have been detected in almost 30 AGN. The soft lag, which is the time delay between the hard and soft X-ray light curves, is usually interpreted as the time difference between the direct and reflected emission, but is alternatively suggested to arise from the direct and scattering emission from distant clouds. By analysing the archival Suzaku observations totalling an exposure time of ~ 770 ks, we discover a soft lag of $10\pm3.4$ ks at $9.58\times10^{-6}$ Hz in the luminous quasar PDS 456, which is the longest soft lag and lowest Fourier frequency reported to date. In this study, we use the maximum likelihood method to deal with non-continuous nature of the Suzaku light curves. The result follows the mass-scaling relation for soft lags, which further supports that soft lags originate from the innermost areas of AGN and hence are best interpreted by the reflection scenario. Spectral analysis has been performed in this work and we find no evidence of clumpy partial-covering absorbers. The spectrum can be explained by a self-consistent relativistic reflection model with warm absorbers, and spectral variations over epochs can be accounted for by the change of the continuum, and of column density and ionization states of the warm absorbers.Comment: accepted for publication in MNRA