Time variability of broad absorption-line QSO's

Details Time variability of broad absorption-line QSO's Time variability of broad absorption-line QSO's

Jan 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt........56b&link_type=abstract

PhD Dissertation, California Univ. San Diego, CA United States

Astronomy and Astrophysics

Astronomy

30

Absorption Spectra, Quasars, Stellar Spectra, Gas Ionization, Photoionization, Line Spectra, Stellar Spectrophotometry, Astronomical Spectroscopy, Variability

Scientific paper

In this thesis, we present the results of a three year project to monitor broad absorption-line QSO's (BALQSO's) with both broadband imaging (for continuum flux changes) and spectrophotometry (for broad absorption-line variations). Of the 54 BALQSO's monitored with broadband imaging, approximately 33% showed significant time variability with deviations of 0.1 to 0.3 magnitudes. Spectroscopic monitoring of 23 BALQSO's resulted in the detection of broad absorption-line (BAL) time variations in 15 BALQSO's. Six of these cases are small (marginal) variations, and four show large BAL changes. All of the BAL time variations appear to be changes in the residual intensity, or normalized flux, within the line, rather than variations in the velocity structure of the outflowing BAL gas. Our primary hypothesis is that a variable photoionizing continuum causes a change in the ionization levels, and thus a redistribution of the fractional abundances of the ions, resulting in the strengthening or weakening of the absorption lines. The BAL changes will appear nearly simultaneous with the continuum variations if the BAL variation mechanism travels in step with the continuum photons, and the mechanism induces rapid changes in the ionization of the gas. Thus, unlike studies of broad emission-line variability in active galactic nuclei, BAL variability does not (for the simplest models) yield direct information about the size of the region. Time-variability studies have suggested that the velocity of the BAL gas increases (or decreases) monotonically with distance from the continuum source. With this assumption, BAL time variability provides the opportunity for detailed study of the ionization properties, column density, and elemental abundance of the gas in an isolated region of space in the environment of moderate and high-redshift QSO's. In our studies, we have found cases of BAL variability that appear to be correlated with a change in continuum flux. However, we have also discovered a few cases where the BAL's varied, but no change was detected (to a limit of approximately 0.06 magnitudes) in the broadband flux. Our studies suggest that the ionizing flux (lambda approximately 100-400 A) varies with greater amplitude and possibly in a non-synchronous manner with the observed continuum flux (lambda approximately 2000 A).

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