Variability in quasar broad absorption line outflows – III. What happens on the shortest time-scales?

Abstract

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.

Broad absorption line disappearance and emergence using multiple-epoch spectroscopy from the Sloan Digital Sky Survey

“…Coordinated BAL variability over a large velocity interval (e.g., greater than ∼10 000 km s −1 ) is more easily explained by situations involving an ionization change; such behavior in the transverse-motion scenario would require coordinated motions of gas that would presumably exist at a large range of distances from the SMBH (see e.g., Hamann et al 2011;Capellupo et al 2013;Grier et al 2015;Wang et al 2015). Fig.…”

“…We constrain BAL-outflow distances using BAL disappearance and emergence events, and our results are consistent with previous nondisappearing/non-emerging BAL-variability studies that attribute BAL changes to absorbers traversing our LOS. Capellupo et al (2011Capellupo et al ( , 2012Capellupo et al ( , 2013) used short-term and long-term variable C iv/Si iv BAL time-scales to constrain outflows to exist between 0.001 and 0.02 pc and ∼10 pc from the SMBH, respectively. Capellupo, Hamann & Barlow (2014) placed an upper limit of 3.5 pc on the outflow distance using variable P v, Si iv, and C iv BALs.…”

“…BAL profiles can be altered either by a change in coverage of the background light source due to transverse motions of outflows across our line of sight (LOS), or by a change in optical depth within the absorbers due to fluctuations in the ionizing radiation field. Capellupo et al (2013) discussed other possible scenarios for BAL variations, and reasons why they are unlikely.…”

“…We estimate v trans by considering the 'crossing disk' model from Capellupo et al (2013), which consists of a circular outflow moving across a circular continuum source relative to our LOS (see their fig. 14; see also section 4.1 of Rogerson et al 2016). In this model v trans is expressed as √ ∆A D cont /∆t, where ∆A is the normalized flux difference between two spectra, D cont is the diameter of the continuum source, and ∆t is the measured variability timescale between two spectra.…”

Narrow absorption lines with two observations from the Sloan Digital Sky Survey

“…of two observations, distances of absorbers with respect to central regions of quasars, gaseous densities and sizes of absorbers (e.g., Sargent et al 1988;Steidel & Sargent 1991;Wild et al 2008;Capellupo et al 2013;Hacker et al 2013;Misawa et al 2014). Figure 6 displays the distribution of differences in Wrλ1548 versus time intervals between two observations at absorber rest frame for all narrow C IV absorption systems.…”