Transiting Scenario: A Potential Explanation for Variations in Low-Ionization Broad Absorption Line (loBAL) Quasars

Certainly, we must consider the possibility of transiting as a cause for the variations observed in some of these loBALs. For example, Capellupo et al. (2014) argue that the variability seen in a highly saturated C IV BAL strongly supports the transiting scenario. This is further supported by the presence of a P V BAL at the corresponding velocities, indicating a saturated C IV BAL. The C IV/P V optical depth ratio, as shown by Hamann (1998), Leighly et al. (2009), Leighly, Dietrich & Barber (2011), and Borguet et al. (2012), is consistently larger than 100.

In cases where the variations occur in only a small portion of the BAL troughs, it suggests that at least some of these variations can be explained by the transiting scenario. This has been observed in studies by Hamann et al. (2008) and Capellupo (2011). If there are changes in the ionizing flux, it should result in simultaneous variability across a wide range of velocities in the outflow lines, as demonstrated by Hamann et al. (2011) and Capellupo et al. (2012).

Another example is when there is evidence of one BAL, or a portion of a BAL, strengthening while another weakens within the same quasar. This can be easily explained in the transiting scenario, as clouds at different velocities can enter or leave our line of sight at different times, as discussed by Capellupo (2012).