Broad absorption lines (BALs) in quasar spectra identify high‐velocity outflows that likely exist in all quasars and could play a major role in feedback to galaxy evolution. The variability of BALs can help us understand the structure, evolution and basic physical properties of the outflows. Here we report on our first results from an ongoing BAL monitoring campaign of a sample of 24 luminous quasars at redshifts 1.2 < z < 2.9, focusing on C ivλ1549 BAL variability in two different time intervals: 4–9 months (short term) and 3.8–7.7 yr (long term) in the quasar rest frame. We find that 39 per cent (7/18) of the quasars varied in the short‐term data, whereas 65 per cent (15/23) varied in the long‐term data, with a larger typical change in strength in the long‐term data. The variability occurs typically in only portions of the BAL troughs. The components at higher outflow velocities are more likely to vary than those at lower velocities, and weaker BALs are more likely to vary than stronger BALs. The fractional change in BAL strength correlates inversely with the strength of the BAL feature, but does not correlate with the outflow velocity. Both the short‐term and long‐term data indicate the same trends. The observed behaviour is most readily understood as a result of the movement of clouds across the continuum source. If the crossing speeds do not exceed the local Keplerian velocity, then the observed short‐term variations imply that the absorbers are <6 pc from the central quasar.

Broad absorption lines (BALs) in quasar spectra indicate high‐velocity outflows that may be present in all quasars and could be an important contributor to feedback to their host galaxies. Variability studies of BALs help illuminate the structure, evolution and basic physical properties of the outflows. Here we present further results from an ongoing BAL monitoring campaign of a sample of 24 luminous quasars at redshifts 1.2 < z < 2.9. We directly compare the variabilities in the C ivλ1549 and Si ivλ1400 absorption to try to ascertain the cause(s) of the variability. We find that Si iv BALs are more likely to vary than C iv BALs. When looking at flow speeds >−20 000 km s−1, 47 per cent of quasars exhibited Si iv variability while 31 per cent exhibited C iv variability. Furthermore, ∼50 per cent of the variable Si iv regions did not have corresponding C iv variability at the same velocities, while nearly all occurrences of C iv variability had corresponding changes in Si iv. We do not find any correlation between the absolute change in strength in C iv and in Si iv, but the fractional change in strength tends to be greater in Si iv than in C iv. When both C iv and Si iv varied, those changes always occurred in the same sense (either getting weaker or stronger). We also include our full data set so far in this paper, which includes up to 10 epochs of data per quasar. The multi‐epoch data show that the BAL changes were not generally monotonic across the full ∼5–8 yr time span of our observations, suggesting that the characteristic time‐scale for significant line variations, and (perhaps) for structural changes in the outflows, is less than a few years. Coordinated variabilities between absorption regions at different velocities in individual quasars seem to favour changing ionization of the outflowing gas as the cause of the observed BAL variability. However, variability in limited portions of broad troughs fits naturally in a scenario where movements of individual clouds, or substructures in the flow, across our lines of sight cause the absorption to vary. The actual situation may be a complex mixture of changing ionization and cloud movements. Further discussion of the implications of variability, e.g. in terms of the size and location of the outflowing gas, will be presented in a forthcoming paper.

Broad absorption lines (BALs) in quasar spectra are prominent signatures of highvelocity outflows, which might be present in all quasars and could be a major contributor to feedback to galaxy evolution. Studying the variability in these BALs allows us to further our understanding of the structure, evolution, and basic physical properties of the outflows. This is the third paper in a series on a monitoring programme of 24 luminous BAL quasars at redshifts 1.2 < z < 2.9. We focus here on the time-scales of variability in C iv λ1549 BALs in our full multi-epoch sample, which covers time-scales from 0.02−8.7 yr in the quasar rest-frame. Our sample contains up to 13 epochs of data per quasar, with an average of 7 epochs per quasar. We find that both the incidence and the amplitude of variability are greater across longer time-scales. Part of our monitoring programme specifically targeted half of these BAL quasars at rest-frame time-scales 2 months. This revealed variability down to the shortest time-scales we probe (8−10 days). Observed variations in only portions of BAL troughs or in lines that are optically thick suggest that at least some of these changes are caused by clouds (or some type of outflow substructures) moving across our lines of sight. In this crossing cloud scenario, the variability times constrain both the crossing speeds and the absorber locations. Specific results also depend on the emission and absorption geometries. We consider a range of geometries and use Keplerian rotational speeds to derive a general relationship between the variability times, crossing speeds, and outflow locations. Typical variability times of order ∼1 year indicate crossing speeds of a few thousand km/s and radial distances near ∼1 pc from the central black hole. However, the most rapid BAL changes occurring in 8-10 days require crossing speeds of 17 000 − 84 000 km s −1 and radial distances of only 0.001−0.02 pc. These speeds are similar to or greater than the observed radial outflow speeds, and the inferred locations are within the nominal radius of the broad emission line region.

This is the second in a series of papers aiming to test how the mass (M BH ), accretion rate (Ṁ ) and spin (a * ) of super massive black holes (SMBHs) determine the observed properties of type-I active galactic nuclei (AGN). Our project utilizes a sample of 39 unobscured AGN at z 1.55 observed by VLT/X-shooter, selected to map a large range in M BH and L/L Edd and covers the most prominent UV-optical (broad) emission lines, including Hα, Hβ, Mg II λ2798, and C IV λ1549. This paper focuses on single-epoch, "virial" M BH determinations from broad emission lines and examines the implications of different continuum modeling approaches in line width measurements. We find that using a local power-law continuum instead of a physically-motivated thin disk continuum leads to only slight underestimation of the FWHM of the lines and the associated M BH (FWHM). However, the line dispersion σ line and associated M BH (σ line ) are strongly affected by the continuum placement and provides less reliable mass estimates than FWHM-based methods. Our analysis shows that Hα, Hβ and Mg II can be safely used for virial M BH estimation. The C IV line, on the other hand, is not reliable in the majority of the cases, this may indicate that the gas emitting this line is not virialized. While Hα and Hβ show very similar line widths, the mean FWHM(Mg II) is about 30% narrower than FWHM(Hβ). We confirm several recent suggestions to improve the accuracy in C IV-based mass estimates, relying on other UV emission lines. Such improvements do not reduce the scatter between C IV-based and Balmer-line-based mass estimates.