Broad Absorption Line Outflows Research Articles

BAL outflow in quasar B0254-3327B: analysis and comparison with other extreme UV outflows

ABSTRACT We have identified a broad absorption line (BAL) outflow in the HST/STIS spectrum of the quasar QSO B0254-3327B at velocity v = −3200 km s−1. The outflow has absorption troughs from ions such as Ne viii, Na ix, Si xii, and Ne v. We also report the first detection of S xiv absorption troughs, implying very high ionization. Via measurement of the ionic column densities, photoionization analysis, and determination of the electron number density of the outflow, we found the kinetic luminosity of the outflow system to be up to ∼1 per cent of the quasar’s Eddington luminosity, or ∼5 per cent of the bolometric luminosity, making it a potential contributor to AGN feedback. A solution with two ionization phases was needed, as a single phase was not sufficient to satisfy the constraints from the measured ionic column densities. We find that the ionization parameter of the very high-ionization phase of the outflow is within the expected range of an X-ray warm absorber. We also examined the physical properties of the outflow of Q0254-334 along with previously studied extreme UV outflows, with a total sample of 24 outflow systems, finding a weak negative correlation between outflow velocity and distance from the central source, with larger distances corresponding to slower velocities. The very high-ionization phase of the Q0254-334 outflow has one of the highest ionization parameters of UV absorption outflows to date, which we attribute to the presence of S xiv.

Extreme FeLoBAL outflow in the VLT/UVES spectrum of quasar SDSS J1321-0041

Quasar outflows are often analyzed to determine their ability to contribute to active galactic nucleus (AGN) feedback. We identified a broad absorption line (BAL) outflow in the VLT/UVES spectrum of the quasar SDSS J1321-0041. The outflow shows troughs from Fe ii and is thus categorized as an FeLoBAL. This outfow is unusual among the population of FeLoBAL outflows, as it displays C ii and Si ii BALs. Outflow systems require a kinetic luminosity above $ of the quasar's luminosity to contribute to AGN feedback. For this reason, we analyzed the spectrum of J1321-0041 to determine the outflow's kinetic luminosity, as well as the quasar's bolometric luminosity. We measured the ionic column densities from the absorption troughs in the spectrum and determined the Hydrogen column density and ionization parameter using those column densities as our constraints. We also determined the electron number density, $n_e$, based on the ratios between the excited-state and resonance-state column densities of Fe ii and Si ii . This allowed us to find the distance of the outflow from its central source, as well as its kinetic luminosity. We determined the kinetic luminosity of the outflow to be $8.4^ erg s $ and the quasar's bolometric luminosity to be $1.72 erg s $, resulting in a ratio of $ E _k/L_ Bol <!PCT!>$. We conclude that this outflow has a sufficiently high kinetic luminosity to contribute to AGN feedback.

Time variability of ultra-fast BAL outflows using SALT: C iv absorption depth based analysis

ABSTRACT We probe the small-scale absorption line variability using absorption depth based analysis of a sample of 64 ultra-fast outflow (UFO) C iv broad absorption line (BAL) quasars monitored using the Southern African Large Telescope. We confirm the strong monotonic increase in the strength of variability with increasing outflow velocity. We identify regions inside the BAL trough for each source where the normalized flux difference between two epochs is >0.1 for a velocity width ≥500 km s−1 (called ‘variable regions’). We find that the total number of variable regions increases with the time interval probed and the number of BALs showing variable regions almost doubles from short (<2 yr) to long (>2 yr) time-scales. We study the distributions of variable region properties such as its velocity width, depth, and location. These regions typically occupy a few-tenths of the entire width of the BAL. Their widths are found to increase with increasing time-scales having typical widths of ∼2000 km s−1 for Δt > 2 yr. However, their absolute velocity with respect to zem and their relative position within the BAL profile remain random irrespective of the time-scale probed. The equivalent width variations of the BALs are strongly dependent on the size and depth of the variable regions but are little dependent on their total number. Finally, we find that ∼17 per cent of the UFO BALs show uncorrelated variability within the BAL trough.

VLT/UVES observation of the SDSS J2357−0048 outflow

ABSTRACT We found a broad absorption line (BAL) outflow in the VLT/UVES spectrum of the quasar SDSS J235702.54−004824.0, in which we identified four subcomponents. We measured the column densities of the ions in one of the subcomponents (v = −1600 km s−1), which include O i and Fe ii. We found the kinetic luminosity of this component to be at most $\sim 2.4{{\ \rm per\ cent}}$ of the quasar’s Eddington luminosity. This is near the amount required to contribute to active galactic nucleus feedback. We also examined the time variability of a C iv mini-BAL found at v = −8700 km s−1, which shows a shallower and narrower absorption feature attached to it in previous SDSS observations from 2000 to 2001, but not in the spectra from 2005 and onwards.

A broad absorption line outflow associated with the broad emission line region in the quasar SDSS J075133.35+134548.3

We report on the discovery of unusual broad absorption lines (BALs) in the bright quasar SDSS J075133.35+134548.3 at z ∼ 1, using archival and newly obtained optical and NIR spectroscopic data. The BALs are detected reliably in He i* λ3889, He i* λ10830 and tentatively in Al iii, Mg ii. These BALs show complex velocity structures consisting of two major components: a high-velocity component (HV), with a blueshifted velocity range of Δv HV ∼ −9300 − −3500 km s−1, which can be reliably detected in He i* λ10830, and tentatively in Al iii and Mg ii, whereas it is undetectable in He i* λ3889 and a low-velocity component (LV), with Δ v LV ∼ −3500 − −1800 km s−1, is only detected in He i* and He i* λ10830. With the BALs from different ions, the HV outflowing gas can be constrained to have a density of n H ∼1010.3–1011.4 cm−3, a column density of N H ∼ 1021 cm−2 and an ionization parameter of U ∼10−1.83–10−1.72; inferring a distance of R HV∼0.5 pc from the central continuum source with a monochromatic luminosity λ Lλ (5100) = 7.0×1045 erg s−1 at 5100 Å. This distance is remarkably similar to that of the normal broad line region (BLR) estimated from reverberation experiments, suggesting association of the BLR and the HV BAL outflowing gas. Interestingly, a blueshifted component is also detected in Al iii and Mg ii broad emission lines (BELs), and the Al iii/Mg ii of such a BEL component can be reproduced by the physical parameters inferred from the HV BAL gas. The LV BAL gas likely has a larger column density, a higher ionization level and hence a smaller distance than the HV BAL gas. Further spectroscopy with a high S/N ratio and broader wavelength coverage is needed to confirm this to shed new light on the possible connection between BALs and BELs.

Extreme High-velocity Outflows from High-redshift BOSS Quasars

Abstract It is common to assume that all narrow absorption lines (NALs) at extreme high-velocity shifts form in cosmologically intervening gas or galaxies unrelated to quasars. However, previous detailed studies of individual quasars have shown that some NALs at these large velocity shifts do form in high-speed quasar ejecta. We search for extreme high-velocity NAL outflows (with speeds ∼0.1c–0.2c) based on relationships with associated absorption lines (AALs) and broad absorption-line (BAL) outflows. We find that high-velocity NALs are strongly correlated with AALs, BALs, and radio loudness, indicating that a significant fraction of high-velocity systems are either ejected from the quasars or form in material swept up by the radio jets (and are not unrelated intervening gas). We also consider line-locked C iv doublets as another indicator of high-velocity NALs formed in outflows. The fact that line-locked NALs are highly ionized and correlated with BAL outflows and radio-loud quasars implies that physical line locking due to radiative forces is both common and real, which provides indirect evidence that a significant fraction of high-velocity NALs are intrinsic to quasars.

Absorption-line Environments of High-redshift BOSS Quasars

Abstract The early stage of massive galaxy evolution often involves outflows driven by a starburst or a central quasar plus cold mode accretion (infall), which adds to the mass build-up in the galaxies. To study the nature of these infall and outflows in the quasar environments, we have examined the correlation of narrow absorption lines (NALs) at positive and negative velocity shifts to other quasar properties, such as their broad absorption-line (BAL) outflows and radio-loudness, using spectral data from SDSS-BOSS DR12. Our results show that the incidence of associated absorption lines (AALs) and outflow AALs is strongly correlated with BALs, which indicates most AALs form in quasar-driven outflows. Multiple AALs are also strongly correlated with BALs, demonstrating quasar outflows tend to be highly structured and can create multiple gas components with different velocity shifts along our line of sight. Infall AALs appear less often in quasars with BALs than quasars without BALs. This suggests that BAL outflows act on large scale in host galaxies and inhibit the infall of gas from the IGM, supporting theoretical models in which quasar outflow plays an important role in the feedback to host galaxies. Despite having larger distances, infall AALs are more highly ionized than outflow AALs, which can be attributed to the lower densities in the infall absorbers.

Evidence that emission and absorption outflows in quasars are related

ABSTRACT We analyse VLT/X-shooter data for seven quasars, where we study the relationships between their broad absorption line (BAL) and emission line outflows. We find (1) the luminosity of the [O iii] λ5007 emission profile decreases with increasing electron number density ($n_\mathrm{e}$) derived from the BAL outflow in the same quasar, (2) the measured velocity widths from the [O iii] emission features and C iv absorption troughs in the same object are similar, and (3) the mean radial velocity derived from the BAL outflow is moderately larger than the one from the [O iii] emission outflow. These findings are consistent with [O iii] and BAL outflows being different manifestations of the same wind, and the observed relationships are likely a reflection of the outflow density distribution. When we have outflows with smaller distances to the central source, their $n_\mathrm{e}$ is higher. Therefore, the [O iii] emission is collisionally de-excited and the [O iii] luminosity is then suppressed. Comparisons to previous studies show that the objects in our sample exhibit broad [O iii] emission features similar to the ones in extremely red quasars (ERQs). This might imply that BAL quasars and ERQs have the same geometry of outflows or are at a similar evolutionary stage. We find that the physical parameters derived from the BAL outflows can explain the amount of observed [O iii] luminosity, which strengthens our claim that both BAL and [O iii] outflows are from the same wind. These estimates can be tested with upcoming James Webb Space Telescope, which will be able to spatially resolve some of the ionized gas outflows.

A Kinematic Shift of the C iv Broad Absorption Line in Quasar SDSS J120819.29+035559.4

Abstract We report the kinematic shift of the C iv broad absorption line (BAL) in quasar SDSS J120819.29+035559.4 (hereafter J1208+0355). This quasar shows two BAL systems, including a blue component of system A at ∼23,500 km s−1 that shows a kinematic shift of 1166 ± 65 km s−1, and a red component of system B at ∼7000 km s−1 that can be decomposed into several narrow absorption lines (NALs). First, we confirm that the most likely cause for the equivalent width variations of the absorption lines (at least for system B) in J1208+0355 is the ionization change scenario as a response to the changes in the ionization continuum according to the following observational factors: (1) coordinated multiple absorption lines strengthening; (2) the continuum flux shows an obvious weakening. Second, we find line-locking phenomena from the blended NALs within system B, indicating that these outflow clouds are driven by a radiative force caused by resonance lines. The above two research aspects convincingly reveal that the BAL outflows of J1208+0355 are affected by the background radiation energy. Therefore, we infer that the kinematic shift shown in system A may be produced by actual line-of-sight acceleration of the outflow clouds, which is driven by radiation pressure from the background light source.

Broad Emission and Absorption Line Outflows in the Quasar SDSS J163345.22+512748.4

Abstract We present a detailed study of the optical and near-infrared (NIR) emission and absorption line spectrum of the quasar SDSS J163345.22+512748.4. We discovered on the newly acquired NIR spectrum a highly metastable neutral helium broad absorption line (BAL) He i* λ10830 with a width of ∼2000 km s−1 and a blueshift of ∼7000 km s−1 in the velocity space. The BAL system is also significantly detected in Mg ii and He i* λ3889. We estimate a column density of (5.0 ± 1.7) × 1014 cm−2 for the He i*(2 3S) level and infer an ionization parameter of U A = 10−1.9±0.2 for the BAL outflow, assuming that the BAL region is thick enough for a full development of an ionization front. The total column density of the BAL outflow is constrained in the range N H ∼ 1021–1021.4 cm−2. We also found that the bulk of both Mg ii and UV Fe ii, as well as of Hα broad emission lines (BELs), are blueshifted with a velocity of ∼2200 km s−1 with respect to the quasar systemic redshift. We constrain the blueshifted BEL region to have a covering factor C f ≈ 16%, density n H ∼ 1010.6–1011.3 cm−3, column density N H ≳ 1023 cm−2, and ionization parameter U E ∼ 10−2.1–10−1.5. The outflow gas is located at ∼0.1 pc away from the central ionization source, at a scale comparable to the broad-line region. A toy kinetic model has been proposed to reproduce the profile of Mg ii BEL well if a partial obscured axisymmetric geometry of the outflow with a radial velocity as observed from the BALs is assumed.

VLT/X-Shooter Survey of BAL Quasars: Large Distance Scale and AGN Feedback

Abstract We conducted a survey of quasar outflows using the Very Large Telescope/X-Shooter spectrograph. When choosing the 14 broad absorption line (BAL) and mini-BALs comprising this sample, the data did not cover the S iv and S iv* troughs, whose ratio can be used to determine the distance of the outflows from the central source (R). Therefore, this “Blind Survey” is unbiased toward a particular distance scale. Out of the eight outflows where R can be measured, six have R > 100 pc (spanning the range 100–4500 pc), one has R > 10 pc, and only one (at R < 40 pc) is compatible with a much smaller R scale. At least two of the outflows have a kinetic luminosity greater than 0.5% of their Eddington luminosity, implying that they are able to provide significant active galactic nucleus (AGN) feedback. The outflows span a range of 0 to −10,000 km s−1 in velocity, total column density between 1020.1 and 1022.6 cm−2, ionization parameter ( ) in the range 0.01–1, and electron number density between 103.2 and 105.4 cm−3, with one upper and one lower limit. The results of this survey can be extrapolated to the majority of BAL outflows, implying that most of these outflows are situated far away from the AGN accretion disk, and that a significant portion of them can contribute to AGN feedback processes.

The properties of broad absorption line outflows based on a large sample of quasars

Quasar outflows carry mass, momentum and energy into the surrounding environment, and have long been considered a potential key factor in regulating the growth of supermassive black holes and the evolution of their host galaxies. A crucial parameter for understanding the origin of these outflows and measuring their influence on their host galaxies is the distance (R) between the outflow gas and the galaxy center. While R has been measured in a number of individual galaxies, its distribution remains unknown. Here we report the distributions of R and the kinetic luminosities of quasars outflows, using the statistical properties of broad absorption line variability in a sample of 915 quasars from the Sloan Digital Sky Surveys. The mean and standard deviation of the distribution of R are 10^{1.4+/-0.5} parsecs. The typical outflow distance in this sample is tens of parsec, which is beyond the theoretically predicted location (0.01 ~ 0.1 parsecs) where the accretion disc line-driven wind is launched, but is smaller than the scales of most outflows that are derived using the excited state absorption lines. The typical value of the mass-flow rate is of tens to a hundred solar masses per year, or several times the accretion rate. The typical kinetic-to-bolometric luminosity ratio is a few per cent, indicating that outflows are energetic enough to influence the evolution of their host galaxies.

Evidence that 50% of BALQSO Outflows Are Situated at Least 100 pc from the Central Source

Abstract The most robust way for determining the distance of quasar absorption outflows is the use of troughs from ionic excited states. The column density ratio between the excited and resonance states yields the outflow number density. Combined with a knowledge of the outflow’s ionization parameter, a distance from the central source (R) can be determined. Here we report results from two surveys targeting outflows that show troughs from S iv. One survey includes 1091 SDSS and BOSS quasar spectra, and the other includes higher-quality spectra of 13 quasars observed with the Very Large Telescope. Our S iv samples include 38 broad absorption line (BAL) outflows and four mini-BAL outflows. The S iv is formed in the same physical region of the outflow as the canonical outflow-identifying species C iv. Our results show that S iv absorption is only detected in 25% of C iv BAL outflows. The smaller detection fraction is due to the higher total column density (N H) needed to detect S iv absorption. Since R empirically anticorrelates with N H, the results of these surveys can be extrapolated to C iv quasar outflows with lower N H as well. We find that at least 50% of quasar outflows are at distances larger than 100 pc from the central source, and at least 12% are at distances larger than 1000 pc. These results have profound implications for the study of the origin and acceleration mechanism of quasar outflows and their effects on the host galaxy.

Mid-infrared Flare of TDE Candidate PS16dtm: Dust Echo and Implications for the Spectral Evolution

Abstract PS16dtm was classified as a candidate tidal disruption event in a dwarf Seyfert 1 galaxy with a low-mass black hole ( ) and has presented various intriguing photometric and spectra characteristics. Using the archival Wide-field Infrared Survey Explorer and the newly released NEOWISE data, we found that PS16dtm is experiencing a mid-infrared (MIR) flare that started ∼11 days before the first optical detection. Interpreting the MIR flare as a dust echo requires close pre-existing dust with a high covering factor and suggests that the optical flare may have brightened slowly for some time before it became bright detectable from the ground. More evidence is given at the later epochs. At the peak of the optical light curve, the new inner radius of the dust torus has grown to a much larger size (i.e., a factor of seven of the initial radius) due to the strong radiation field. At ∼150 days after the first optical detection, the dust temperature has dropped well below the sublimation temperature. Other peculiar spectral features shown by PS16dtm are the transient, prominent Fe ii emission lines and outflows indicated by broad absorption lines detected during the optical flare. Our model explains the enhanced Fe ii emission from iron that is newly released from the evaporated dust. The observed broad absorption line outflow could be explained by accelerated gas in the dust torus due to the radiation pressure.

Quantifying the AGN-driven outflows in ULIRGs (QUADROS) – I: VLT/Xshooter observations of nine nearby objects

Although now routinely incorporated into hydrodynamic simulations of galaxy evolution, the true importance of the feedback effect of the outflows driven by active galactic nuclei (AGN) remains uncertain from an observational perspective. This is due to a lack of accurate information on the densities, radial scales and level of dust extinction of the outflow regions. Here we use the unique capabilities of VLT/Xshooter to investigate the warm outflows in a representative sample of 9 local (0.06 $<$ z $<$ 0.15) ULIRGs with AGN nuclei and, for the first time, accurately quantify the key outflow properties. We find that the outflows are compact (0.05 $<$ R$_{[OIII]}$ $<$ 1.2 kpc), significantly reddened (median E(B-V)$\sim$0.5 magnitudes), and have relatively high electron densities (3.4 $<$ log$_{10}$ n$_e$ (cm$^{-3}$) $<$ 4.8). It is notable that the latter densities -- obtained using trans-auroral [SII] and [OII] emission-line ratios -- exceed those typically assumed for the warm, emission-line outflows in active galaxies, but are similar to those estimated for broad and narrow absorption line outflow systems detected in some type 1 AGN. Even if we make the most optimistic assumptions about the true (deprojected) outflow velocities, we find relatively modest mass outflow rates ($0.07 < \dot{M} < 11$ M$_{sol}$ yr$^{-1}$) and kinetic powers measured as a fraction of the AGN bolometric luminosities ($4\times10^{-4} < \dot{E}/L_{BOL} <1$%). Therefore, although warm, AGN-driven outflows have the potential to strongly affect the star formation histories in the inner bulge regions ($r \sim$ 1kpc) of nearby ULIRGs, we lack evidence that they have a significant impact on the evolution of these rapidly evolving systems on larger scales.

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

We investigate broad absorption line (BAL) disappearance and emergence using a 470 BAL-quasar sample over < 0.10-5.25 rest-frame years with at least three spectroscopic epochs for each quasar from the Sloan Digital Sky Survey. We identify 14 disappearing BALs over < 1.73-4.62 rest-frame years and 18 emerging BALs over < 1.46-3.66 rest-frame years associated with the CIV 1548,1550 and/or SiIV 1393,1402 doublets, and report on their variability behavior. BAL quasars in our dataset exhibit disappearing/emerging CIV BALs at a rate of 2.3 and 3.0 per cent, respectively, and the frequency for BAL to non-BAL quasar transitions is 1.7 per cent. We detect four re-emerging BALs over < 3.88 rest-frame years on average and three re-disappearing BALs over < 4.15 rest-frame years on average, the first reported cases of these types. We infer BAL lifetimes along the line of sight to be nominally < 100-1000 yr using disappearing CIV BALs in our sample. Interpretations of (re-)emerging and (re-)disappearing BALs reveal evidence that collectively supports both transverse-motion and ionization-change scenarios to explain BAL variations. We constrain a nominal CIV/SiIV BAL outflow location of < 100 pc from the central source and a radial size of > 1x10^-7 pc (0.02 au) using the ionization-change scenario, and constrain a nominal outflow location of < 0.5 pc and a transverse size of ~0.01 pc using the transverse-motion scenario. Our findings are consistent with previous work, and provide evidence in support of BALs tracing compact flow geometries with small filling factors.

Leaked Lyα Emission: An Indicator of the Size of Quasar Absorption Outflows

Abstract The galactocentric distance of quasar absorption outflows are conventionally determined using absorption troughs from excited states, a method hindered by severely saturated or self-blended absorption troughs. We propose a novel method to estimate the size of a broad absorption line (BAL) region, which partly obscures an emission line region by assuming virialized gas in the emission region surrounding a supermassive black hole with known mass. When a spiky Lyα line emission is present at the flat bottom of the deep N v absorption trough, the size of the BAL region can be estimated. We have found three BAL quasars in the SDSS database showing such Lyα lines. The scale of their BAL outflows are found to be 3–26 pc, moderately larger than the theoretical scale (0.01–0.1 pc) of trough forming regions for winds originating from accretion disks, but significantly smaller than most outflow sizes derived using the absorption troughs of the excited states of ions. For these three outflows, the lower limits of the ratio of kinetic luminosity to Eddington luminosity are 0.02%–0.07%. These lower limits are substantially smaller than that required to have a significant feedback effect on their host galaxies.

HST and ground-based spectroscopy of quasar outflows: from mini-BALs to BALs

Quasar outflows have been posited as a mechanism to couple supermassive black holes to evolution in their host galaxies. We use multi-epoch spectra from the Hubble Space Telescope (HST) and ground-based observatories to study the outflows in seven quasars that have C iv outflow lines ranging from a classic broad absorption line (BAL) to weaker/narrower ‘mini-BALs’ across rest wavelengths of at least 850–1650 Å. The C iv outflow lines all varied within a time frame of ≤1.9 yr (rest). This includes equal occurrences of strengthening and weakening plus the emergence of a new BAL system at −38 800 km s−1 accompanied by dramatic strengthening in a mini-BAL at −22 800 km s−1. We infer from ∼1:1 doublet ratios in P v and other lines that the BAL system is highly saturated with line-of-sight covering fractions ranging from 0.27 to 0.80 in the highest to lowest column density regions, respectively. Three of the mini-BALs also provide evidence for saturation and partial covering based on ∼1:1 doublet ratios. We speculate that the BALs and mini-BALs form in similar clumpy/filamentary outflows, with mini-BALs identifying smaller or fewer clumps along our lines of sight. If we attribute the line variabilities to clumps crossing our lines of sight at roughly Keplerian speeds, then a typical variability time in our study, ∼1.1 yr, corresponds to a distance ∼2 pc from the central black hole. Combining this with the speed and minimum total column density inferred from the P v BAL, NH ≳ 2.5 × 1022 cm−2, suggests that the BAL outflow kinetic energy is in the range believed to be sufficient for feedback to galaxy evolution.

Transient C iv broad absorption lines in radio-detected QSOs

We study the transient (i.e. emerging or disappearing) C IV broad absorption line (BAL) components in 50 radio detected QSOs using multi-epoch spectra available in Sloan Digital Sky Survey DR10. We report the detectionof 6 BALQSOs having at least one distinct transient C IV absorption component. Based on the structure function analysis of optical light curves, we suggest that the transient absorption is unlikely to be triggered by continuum variations. Transient absorption components usually have low C IV equivalent widths (< 8 \AA), high ejection velocities (> 10000 \kms) and typically occur over rest-frame timescales > 800 days. The detection rate of transient C IV absorption seen in our sample is higher than that reported in the literature. Using a control sample of QSOs, we show that this difference is most likely due to the longer monitoring time-scale of sources in our sample while the effect of small number statistics cannot be ignored. Thus, in order to establish the role played by radio jets in driving the BAL outflows, we need a larger sample of radio detected BALs monitored over more than 3 years in the QSO's rest frame. We also find that the transient phenomenon in radio detected and radio quiet BALs does not depend on any of the QSO properties i.e. the Eddington ratio, black hole mass, bolometric luminosity and optical-to-IR colours. All this suggests that transient BAL phenomenon is simply the extreme case of BAL variability.

On the origins of C iv absorption profile diversity in broad absorption line quasars

There is a large diversity in the C IV broad absorption line (BAL) profile among BAL quasars (BALQs). We quantify this diversity by exploring the distribution of the C IV BAL properties, FWHM, maximum depth of absorption and its velocity shift ($v_{\rm md}$), using the SDSS DR7 quasar catalogue. We find the following: (i) Although the median C IV BAL profile in the quasar rest-frame becomes broader and shallower as the UV continuum slope ($\alpha_{\rm UV}$ at 1700-3000 A) gets bluer, the median individual profile in the absorber rest-frame remains identical, and is narrow (FWHM = 3500 km/s) and deep. Only 4 per cent of BALs have FWHM > 10,000 km/s. (ii) As the He II emission equivalent-width (EW) decreases, the distributions of FWHM and $v_{\rm md}$ extend to larger values, and the median maximum depth increases. These trends are consistent with theoretical models in which softer ionizing continua reduce overionization, and allow radiative acceleration of faster BAL outflows. (iii) As $\alpha_{\rm UV}$ becomes bluer, the distribution of $v_{\rm md}$ extends to larger values. This trend may imply faster outflows at higher latitudes above the accretion disc plane. (iv) For non-BALQs, the C IV emission line decreases with decreasing He II EW, and becomes more asymmetric and blueshifted. This suggests an increasing relative contribution of emission from the BAL outflow to the C IV emission line as the ionizing spectral energy distribution (SED) gets softer, which is consistent with the increasing fraction of BALQs as the ionizing SED gets softer.