Understanding Broad Absorption Line (BAL) Variability: Ionization Parameter vs. Transiting Clouds

The observed variability in broad absorption line (BAL) can usually be explained by changes in the ionization parameter, clouds passing in front of the line of sight, or a combination of both. The transiting clouds can affect the equivalent width (EW) of the BAL troughs, as well as the shape and intensity of the ionization continuum. A key difference between these two causes is that variations in the ionization parameter are likely to impact the entire spectral energy distribution, whereas transiting clouds primarily affect specific wavelength regions. In this study, a power-law spectrum fitted in various wavelength ranges without absorption lines is used to characterize the intensity of the continuum, reflecting the influence of fluctuations in the central source's energy rather than the local effects of transiting clouds (Lu et al., 2018).