Time variability of ultra fast BAL outflows using SALT: C iv equivalent width analysis

Abstract

We study the time variability (over ≤7.3 yrs) of ultra fast outflows (UFOs) detected in a sample of 64 C iv broad absorption line (BAL) quasars (with 80 distinct BAL components) monitored using the Southern African Large Telescope. By comparing the properties of the quasar in our sample with those of a control sample of non-BAL quasars we show that the distributions of black hole mass are different and the bolometric luminosities and optical photometric variations of UFO BAL quasars are slightly smaller compared to that of non-BAL quasars. The detection fraction of C iv equivalent width (W) variability (∼95%), the fractional variability amplitude ($\frac{\Delta W}{W}$) and the fraction of “highly variable” BAL (i.e., $\big {|}\frac{\Delta W}{W}\big {|}$>0.67) components (∼ 33%) are higher in our sample compared to the general BAL population. The scatter in $\frac{\Delta W}{W}$ and the fraction of “highly variable” BALs increase with the time-scale probed. The $\frac{\Delta W}{W}$ distribution is asymmetric at large time scales. We attribute this to the BAL strengthening time scales being shorter than the weakening time scales. The BAL variability amplitude correlates strongly with the BAL properties compared to the quasar properties. BALs with low W, high-velocity, shallow profiles, and low-velocity width tend to show more variability. When multiple BAL components are present a correlated variability is seen between low- and high-velocity components with the latter showing a larger amplitude variations. We find an anti-correlation between the fractional variations in the continuum flux and W. While this suggests photoionization-induced variability, the scatter in continuum flux is much smaller than that of W.

Coordinated time variability of multi-phase ultra-fast outflows in J132216.25 + 052446.3

“…In Aromal et al (2023) we have seen the BAL components at higher ejection velocities show larger variability in C iv equivalent width compared to the components at low ejection velocities. Here we test whether this is true even when we perform velocity bin based analysis without identifying individual BAL components.…”

“…We identify 80 BALs in these sources according to the definition given in Weymann et al (1991). The list of sources, log of observations, and details of spectra obtained at different epochs are given in Table B1 in the online material of Aromal et al (2023).…”

“…Note that the equivalent width calculated for the entire BAL profile is denoted as W without any superscript. Other notations related to variations in W are same as above without the superscript as done in Aromal et al (2023).…”

“…We estimate ∆W v and | ∆W v W v | for each velocity bin between all possible combinations of epochs with the time separation between them (∆t) falling in a 2-3.5 yr time bin for all the UFO sources. We chose this time bin as it is the most well-sampled time bin in our sample (see Aromal et al 2023, for details). To avoid any bias in the ∆W v and | ∆W v W v | distributions due to the non-uniform number of available spectroscopic epochs for our sources, we randomly choose one measurement of ∆W v and | ∆W v W v | per source in each velocity bin and measure the mean for the corresponding ∆W v and | ∆W v W v | distribution.…”

“…In a previous paper (Aromal et al 2023) we have studied the time variability (over 7.3 yrs) of C iv ultra-fast outflows (UFOs) detected in a sample of 64 C iv broad absorption line quasars (with 80 distinct BAL components) monitored using the Southern African Large Telescope. These outflows are characterized by maximum outflow velocities greater than 15000 km s −1 .…”