The View through the Wind: X-ray Observations of Broad Absorption Line Quasi-Stellar Objects

Open Access
Gallagher, Sarah Connoran
Graduate Program:
Astronomy and Astrophysics
Doctor of Philosophy
Document Type:
Date of Defense:
February 28, 2002
Committee Members:
  • Michael Eracleous, Committee Member
  • Lee S Finn, Committee Member
  • Jane Camilla Charlton, Committee Member
  • Steinn Sigurdsson, Committee Member
  • W Nielsen Brandt, Committee Chair
  • Gordon Paul Garmire, Committee Member
  • X-ray astronomy
  • active galaxies
  • absorption lines
  • quasars
The 2-10 keV bandpasses and unprecedented sensitivity of modern X-ray observatories have enabled new insights into the immediate environments of Broad Absorption Line (BAL) QSOs. BAL QSOs, approximately 10% of the QSO population, exhibit deep, broad absorption lines from high ionization ultraviolet resonance transitions. These blueshifted absorption features are understood to arise along lines of sight which travel through radiatively driven winds with terminal velocities reaching 0.1-0.3<i>c</i>. These energetic outflows are an important component of QSO environments; mass ejection is apparently fundamentally linked to the process of active mass accretion onto supermassive black holes. X-rays, generated in the innermost region surrounding accreting black holes, travel through the nuclear environments to the observer. X-ray studies of BAL QSOs thus offer a privileged view through the wind. <p>In this thesis, I present results from X-ray surveys of BAL QSOs. This work includes both spectroscopic and exploratory X-ray observations designed to characterize the X-ray properties of specific objects as well as the population as a whole. Before this project, BAL QSOs were known to be X-ray weak relative to normal QSOs, however, the cause for this faintness had not been demonstrated. In general, I find that X-ray weakness in BAL QSOs results from intrinsic absorption of a typical QSO X-ray continuum. Fitting X-ray spectra with simple absorption models results in intrinsic column densities ranging from (1-50)x10<sup>22 </sup>cm<sup>-2</sup>. Even at the spectral resolution currently available for such observations, there is significant evidence for complexity in the absorber, perhaps arising from ionized gas with velocity structure that may partially cover the direct continuum. From the X-ray continua above ~5 keV, the X-ray to ultraviolet flux ratios are found to be consistent with normal QSOs, indicating that the spectral energy distributions of BAL QSOs are not anomalous. For those BAL QSOs exhibiting BALs in low-ionization lines (e.g., Mg II), the situation is qualitatively different. The direct line of sight to the primary X-ray continuum appears to be completely blocked by Compton-thick absorption. In this situation, only indirect X-rays, either scattered or reflected, are able to reach the observer. <p>In the future, deeper observations of the brightest BAL QSOs as well as expanded exploratory surveys will offer additional insight into the state and location of the X-ray absorbers. Higher spectral resolution will clarify the physical state of the X-ray absorber as well as elucidate the relationship between the ultraviolet and X-ray absorbing gas. This information is essential for attaining the ultimate goal of constraining the mass outflow rate of luminous QSOs.