Quasar Club: BAL Variability on Short Time-Scales

Title: Variability in quasar broad absorption line outflows III: What happens on the shortest time-scales?
Authors: Capellup, Hamann, Shields, Halpern, Barlow
Abs: Broad absorption lines (BALs) in quasar spectra are prominent signatures of high-velocity 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 CIV 1549A 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. 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 Р84 000 km/s 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.
arXiv: 1211.4868

Notes and Thoughts:
This is a 3 paper series: see Paper I and Paper II for more info.
Paper on a spectroscopic monitoring campaign for 24 BAL quasars over 1.2<z<2.9, monitoring specifically Civ 1549. Time-scales covered in this paper are 0.02 – 8.7 yr rest frame.
Results: typical variability of order 1 year suggests orbital crossing speed of ~1000 km/s at 1pc, typical variability of order 8-10 days suggests orbital crossing speed of ~10000 to 100000 km/s at 0.001-0.02pc.
Paper II covers the two major scenarios of variability. change in ionization vs. clouds moving across our line of sight. they cannot rule out either scenario. Need to write up solid comparison of these two scenarios.
Figure 1 – amplitude of variability gets larger with longer timescales. This is in keeping with other results (Gibson for instance). i.e., you have to wait longer time scales to get bigger changes in absorption.
Out of the 17 quasars for which Capellup has data on these shortest timescales, only 2 exhibited Civ BAL variability.
object 1 – 1246-0542 – secure variability over 8 days rest frame
Civ is obviously variable in two places, they shifted this to see if Nv and Siv are also. Nv does. Siv does not. They note that over their observations the BAL returned to its previous state from 25 days earlier. They also have further epochs on this target, which note variability in the same velocity bins. Further confirmation of variability on short timescales
object 2 – 0842+3431 – secure variability over 10 days rest frame
match the Civ to where the Siv should be. they find potential variability. previous data from Lick (early 1991, and late 1991) show short term variability in same velocity bin for Siv, but not Civ.
Method by which to examine Civ variability over all timescales (see Figure 9)
To examine the relationship of Civ BAL variability with time-scale across the full measured range from 0.02 to 8.7 yr, authors compare the BALs in each pair of observations at all velocities in every quasar then count the occurrences of Civ BAL variability [Note: this uses the definition of BAL variability in Paper 1]. Create a probability: by dividing number of occurrences of variability by the number of measurements, where a pair is one measurement. Plot measured probability of detecting Civ BAL variability against delta T in fig. 9. [Note: these are probabilities for detecting variability between two measurements separated by deltaT, NOT for detecting variability at any time in that deltaT time-frame. In other words, if a quasar varied then returned to its initial state within deltaT, would not contribute to the plot].
Authors did much analysis on removing possible biases from Figure 9. removed bias of those quasars that were observed most (which are known to be variable), not much changes. removed bias of taken spectra are clustered, not spread over all time lines. they do indeed have a slight bias towards greater variability fractions at longer time-scales
Summary of Results:
Paper I describes general trends in Civ BAL variability and finds that variability occurs more often at higher outflows and in shallower troughs.
Paper I and II note that variability typically occurs in just portions of troughs
Paper II also directly compares variability in Civ to Siv. Siv BALs are more likely to vary than Civ BALs. perhaps this is due to the tendency for weaker lines to vary more.
Paper III detects a strong trend towards greater variability fractions over longer time-scales. Both incidence and amplitude of variability decreases with deltaT.
Paper III detects variability to 8 days in deltaT.

Discussion:
causes of variability
time scales of variability help constrain location of outflowing gas, but heavilty pitted on the cause
1. change of ionization in far UV
2. outflow cloud moving across line of sight
Changes in ionizing flux incident on the entire outflow should cause global changes in the ionization of the flow. A change of covering fraction due to moving clouds is less likely when absorption regions at diff vels vary in concert, because this would require coordinate movements among outflow strucutres at different velocities. However, changes in narrow portions of the BAL fits more naturally in crossing clouds. Variability in saturated Civ saturated absorption lines strongly favours the corssing cloud scenario. Other possibilities: change in size of continuum source, instabilities within the flows themselves.
Implications of variability
Ionization:
important: ionization changes require significant variations in the quasar’s incident (ionizing) flux. seems unlikely because this study’s sample consists mostly of luiminous quasars, which have a smaller amplitude of continuum variability, and the amplitude shrinks at shorter timescales [reference?]
Interesting from Misawa et al. (2007). [detect variability in mini-BAL over 16 days, also too short for continuum variability] they propose a screen of gas with varying optical depth located between continuum source and absorbing gas. screen is in co-rotation with disk, and if screen is clump, then as it rotates will allow less/more continuum light through, and thus change the ionization parameters of the absorbing gas quickly. Another way of creating ionization changes on short timescales is hotspots on an accretion disk. However, ALL of these scenarios predict global changes across BAL troughs, which we typically do not see.
Crossing Clouds:
constant ionization and density, variability due to component moving across continuum source. use time scale to estimate speed given a geometry for the emission and absorption regions. estimate a characteristic diameter of the continuum source at 1500 Ang using observed fluxes at this wavelength and standard bolometric correction fator, they find D1500 ~ 0.008 pc, and the BEL is Dciv ~ 0.3pc.
results from disk/knife = 17000km/s to 84000km/s TRANSVERSE and distances of 0.001 pc. which is smaller than the UV continuum estimate of 0.004pc! Interesting
These results provide much smaller distance constraints than the changing ioinization scenario. There is indirect evidence for crossing cloud scenario because its variations at -17 200 km/s appear to be in an optically thick trough.
Follow Up:
Read Papers I and II for understanding of what exactly is called variability, and comparison on Ionization vs. Absorption.