Monitor of NAL/mini-BAL Quasars

Monitoring the Variability of Intrinsic Absorption Lines in Quasar Spectra11affiliationmark: 22affiliationmark: 33affiliationmark:

Toru Misawa44affiliation: School of General Education, Shinshu University, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan , Jane C. Charlton55affiliation: Department of Astronomy & Astrophysics, The Pennsylvania State University, University Park, PA 16802 , and Michael Eracleous55affiliation: Department of Astronomy & Astrophysics, The Pennsylvania State University, University Park, PA 16802 66affiliation: Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802 77affiliation: Center for Relativistic Astrophysics, School of Physics, Georgia Institute of Technology, Atlanta, GA 30332 88affiliation: Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195 misawatr@shinshu-u.ac.jp
Abstract

We have monitored 12 intrinsic narrow absorption lines (NALs) in five quasars and seven mini-broad absorption lines (mini-BALs) in six quasars for a period of 4–12 years (1–3.5 years in the quasar rest-frame). We present the observational data and the conclusions that follow immediately from them, as a prelude to a more detailed analysis. We found clear variability in the equivalent widths (EWs) of the mini-BAL systems but no easily discernible changes in their profiles. We did not detect any variability in the NAL systems nor in narrow components that are often located at the center of mini-BAL profiles. Variations in mini-BAL EWs are larger at longer time intervals, reminiscent of the trend seen in variable broad absorption lines. If we assume that the observed variations result from changes in the ionization state of the mini-BAL gas, we infer lower limits to the gas density cm and upper limits on the distance of the absorbers from the central engine of order a few kpc. Motivated by the observed variability properties, we suggest that mini-BALs can vary because of fluctuations of the ionizing continuum or changes in partial coverage while NALs can vary primarily because of changes in partial coverage.

quasars: absorption lines – quasars: individual (HE 0130-4021, HE 0940-1050, Q 1009+2956, HS 1700+6416, HS 1946+7658, Q 2343+125, UM 675, Q 1157+014, HE 1341-1020, HE 0151-4326, and HS 1603+3820)
slugcomment: August 3, 201911footnotetext: Based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.22footnotetext: Data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation.33footnotetext: Based on observations obtained at the European Southern Observatory at La Silla, Chile in programs 65.O-0063(B), 65.O-0474(A), 67.A-0078(A), 68.A-0461(A), 69.A-0204(A), 70.B-0522(A), 072.A-0346(A), 076.A-0860(A), 079.B-0469(A), 166.A-0106(A)

1 Introduction

Quasars are routinely used as background sources, allowing us to study the gaseous phases of intervening objects (e.g., galaxies, the intergalactic medium, clouds in the halo of the Milky Way, and the host galaxies of the quasars themselves) via absorption-line diagnostics. A fraction of these absorption lines are produced by gas ejected from the quasar central engine (i.e., an outflowing wind or gas from the host galaxy that is swept up by this wind) rather than by unrelated intervening structures. These absorbers that are intrinsic to the quasars themselves may be accelerated by magnetocentrifugal forces (e.g., Everett, 2005; de Kool & Begelman, 1995), radiation pressure in lines and continuum (Murray et al., 1995; Proga, Stone, & Kallman, 2000), radiation pressure acting on dust (e.g., Voit, Weymann, & Korista, 1993; Konigl & Kartje, 1994), or a combination of the above mechanisms. The outflowing wind is an important component of a quasar central engine: it can carry angular momentum away from the accretion disk, facilitating accretion onto the black hole. Moreover, it may be the origin of the broad emission lines that are the hallmark of quasar spectra. In fact, the absorption-line and emission-line regions may be different layers/phases of the wind (e.g., Shields, Ferland, & Peterson, 1995; Murray & Chiang, 1997; Flohic, Eracleous, & Bogdanović, 2012; Chajet & Hall, 2013). Therefore, it is necessary to understand the properties of the wind in order to understand how quasar central engines work. An outflowing wind also has the potential to influence the evolution of the quasar host galaxy and its environs (e.g., Silk & Rees, 1998; King, 2003) by (a) delivering energy and momentum to the interstellar medium (ISM) of host galaxy and to the intergalactic medium (IGM), thus inhibiting star formation (e.g., Springel, Di Matteo, & Hernquist, 2005), and (b) transporting heavy elements from the vicinity of the central engine to the IGM (e.g., Hamann et al., 1997b; Gabel, Arav, & Kim, 2006). Therefore, to fully understand the role that quasars play in galaxy evolution, we need to understand the broader impact of ouflows from their central engines.

In view of the importance of quasar outflows, we have been focusing our attention on rest-frame UV absorption lines from intrinsic absorbers, in order to probe their demographics and physical conditions. We have been primarily studying narrow absorption lines (NALs; FWHM  km s) from intrinsic absorbers since these allow us to probe wind properties in a unique way (e.g., Hamann et al., 1997a, b; Misawa et al., 2007b), unlike broad absorption lines (BALs; FWHM 2,000 km s; e.g., Weymann et al. 1991), whose UV resonance doublets are blended and may be saturated. Equally useful but more rare are mini-BALs, with intermediate widths between NALs and BALs. Intrinsic NALs and mini-BALs present a powerful way to determine the physical conditions of the outflowing gas. We are also interested in finding out how the gas that gives rise to NALs and mini-BALs is related to the gas producing the BALs.

The X-ray properties of quasars with intrinsic NALs suggest that the corresponding absorbers are viewed along different lines of sight to the central engine that those intercepting BAL gas (Misawa et al., 2008; Chartas et al., 2009, 2012; Hamann et al., 2013). In one possible picture emerging from the models and observations, the BAL gas has the form of a fast, dense, equatorial wind, following the models of Murray et al. (1995) and Proga, Stone, & Kallman (2000), while the mini-BAL and NAL makes up the lower density portion of the wind at high latitudes above the disk (as suggested by Ganguly et al., 2001). In this context, NALs and mini-BALs are valuable probes of different regions, different phases, and different acceleration mechanisms of the wind. As noted by Hamann et al. (2013) the absence of strong X-ray absorption in mini-BALs suggests very strongly that there is no thick shielding along those particular lines of sight analogous to what is invoked for lines of sight along BALs (the equivalent hydrogen column density in the shield can only be up to ). In the absence of a thick shield, then, implies that the intensity of the ionizing continuum must be lower so as to avoid overionization of the gas and preserve the strong SiIV, CIV, NV that are commonly observed in mini-BAL spectra. An important implication of this constraint is that the mini-BAL gas cannot be accelerated to high velocities (of order 0.1–0.3) by radiative driving alone.

In an alternative picture, motivated by the models of Kurosawa & Proga (2009), the NAL gas may be gas from the host galaxy, at substantial distances from the quasar central engine, swept up by an accretion disk wind. This picture is bolstered by determinations of distances for the NAL gas in a few quasars, that range from a few hundred pc to many kpc from the source of the ionizing continuum (e.g., Hamann et al., 2001; Arav et al., 2013; Borguet et al., 2012, 2013). These determinations rely on constraints on the density of the absorber, obtained from observations of metastable absorption lines. If this is the appropriate picture, then NALs give us a view of the evacuation of the gas from the host galaxy by a quasar wind.

The variability of absorption lines is a very useful source of information about the absorbing gas and can be used to test models for the formation and structure of quasar winds. Thus, the variability of BALs has been studied extensively. The probability that BALs will vary over a given rest-frame time interval, , has been found to increase with  and approach unity for a few years. (e.g., Gibson et al., 2008; Capellupo et al., 2011, 2013; Filiz Ak et al., 2013). Variability is usually seen only in portions of BAL troughs (e.g., Capellupo et al., 2011, 2012), and detected more often in shallower troughs at larger outflow velocities (e.g., Lundgren et al., 2007; Gibson et al., 2008; Filiz Ak et al., 2013). In a relatively small number of cases, BALs have transformed into mini-BALs by becoming narrower or have disappeared completely (e.g., Hamann et al., 2008; Leighly et al., 2009; Krongold et al., 2010; Vivek et al., 2012; Filiz Ak et al., 2012). Possible origins of this time variability include (a) motion of the absorbing gas parcels across our line of sight (e.g., Hamann et al., 2008; Gibson et al., 2008), (b) changes in the ionization state of the absorber (e.g., Hamann et al., 2011; Misawa et al., 2007b), and (c) redirection of photons around the parcels of gas that make up the absorber by scattering material of variable optical depth, resulting in time-variable dilution of the absorption troughs (e.g., Lamy & Hutsemékers, 2004). None of these mechanisms are applicable to intervening absorbers because they have much larger sizes and lower densities compared to intrinsic absorbers as discussed in Narayanan et al. (2004).

All of the above scenarios still appear to be plausible for BALs, judging from their variability properties (Capellupo et al., 2011, 2012, 2013) and based on spectropolarimetric observations (e.g., Goodrich & Miller, 1995; Lamy & Hutsemékers, 2004), although Filiz Ak et al. (2014) favor scenario (b) based on their results. In the case of mini-BALs scenarios (a) and (c) are disfavored for at least one objects HS1603+3820. Scenario (a) is disfavored by the observation that all the troughs of the CIV mini-BAL in this quasar vary in concert (i.e., the depths of all troughs increase or decrease together) requiring an unlikely coincidence between the motions of the individual parcels of gas (Misawa et al., 2007b). Scenario (c) is disfavored because the polarization level ( 0.6%) is not high enough throughout the spectrum to reproduce the variation of absorption strength (Misawa et al., 2010) and, more importantly, the polarization in the mini-BAL trough is the same as in the continuum. These results suggest that the most promising scenario for variability, which may be applicable to a wide variety of absorbers, is change of ionization conditions in the absorbing gas. Adopting this hypothesis, and associating the observed variability time scale with the recombination time in the absorbing gas, one can set a lower limit to the electron density of this gas and an upper limit to its distance from the source of ionizing radiation (see details in §3.1). A number of authors have carried out this exercise, obtaining lower limits on the electron density in the range 2,000–40,000 cm and upper limits on the distance from the source of ionizing radiation in the range 100–7,000 pc (e.g., Hamann et al., 1997b; Wise et al., 2004; Narayanan et al., 2004; Rodríguez Hidalgo et al., 2012). These values are consistent with those derived by applying photoionization models to explain the strengths of absorption lines from excited or metastable levels such as FeII, SiII (Korista et al., 2008; Dunn et al., 2010), and OIV (Arav et al., 2013). However, some of these lines could be arising not in the outflowing wind but in the interstellar medium of the host galaxies because their distances ( several kpc) are quite large compared to the region of an accretion disk from which an outflow may be launched ( pc for a black hole) and the radius of the broad-emission line region (BLR;  pc for a very luminous quasar of bolometric luminosity ; Kaspi et al., 2007).

The physical mechanism for changing the ionization state of the absorbing gas is not known. An obvious choice would be variability of the quasar UV continuum. However, Gibson et al. (2008) monitored 13 BAL quasars over 3–6 years (in the quasar rest-frame) and found no significant correlation between the change in flux density and the strength of BALs. Moreover, mini-BALs generally vary considerably faster than the expected variability time scale of the UV continuum of luminous quasars (e.g., Giveon et al., 1999; Hawkins, 2001; Kaspi et al., 2007). Nonetheless, the fast variability of mini-BALs can be explained if the variations of the ionizing continuum seen by the absorber is instead caused by a porous/clumpy screen of variable optical depth between it and the continuum source (Misawa et al., 2007b). This screen could be the inner part of the outflow, perhaps a “warm absorber” such as those detected in the X-ray spectra of BAL quasars (e.g., Gallagher et al., 2002, 2006). Since the ionization parameter and the total column density of warm absorbers are at least two orders of magnitude higher than those of UV absorbers (Arav et al., 2001, 2013), their variations should have a large impact on the continuum that is transmitted through them and illuminates UV absorbers. Indeed, significant variations in the warm absorbers and the soft X-ray flux that they transmit are detected (Chartas et al., 2007; Ballo et al., 2008; Giustini et al., 2010a, b). This mechanism also allows for the possibility that the effective size of the background source can fluctuate, which will also lead to fluctuations in the apparent EWs of the UV absorption lines due to a changing coverage fraction.

In this paper, we present results from a campaign to monitor the variability of NALs and mini-BALs, building on previous such efforts by other authors (Barlow & Sargent, 1997; Wampler et al., 1995; Hamann et al., 1997b; Wise et al., 2004; Narayanan et al., 2004; Misawa et al., 2005; Hacker et al., 2013). We present the data and measurements of changes in the equivalent widths (EWs) on time scales from months to years in the absorber’s rest frame. We also discuss the conclusions that follow immediately from these data. A more detailed analysis of coverage fractions, profile variability, and comparison with photoionization models is referred to future papers. Our goals are to (a) constrain the mechanism that causes the variability, and (b) place limits on the density of the absorbers, which will then lead to limits on the distance of the gas from the central engine. The distance of the gas from the central engine, combined with other constraints, may also allow us to distinguish between possible models for the NAL gas (e.g., filaments in the accretion-disk wind gas in the host galaxy that has been swept up by the wind). Thus, our campaign focuses on 12 NALs and 7 mini-BALs in 11 quasars and employs high resolution spectra (), taken with 10-m class telescopes (i.e., Subaru, Keck, and VLT), covering several detected transitions. At high resolution it is possible to resolve internal narrower components in NALs and mini-BALs and monitor variability of the line profiles, in addition to the variability of the line strengths. Ultimately, this information can be used to look for the underlying cause of the variability and understand which of the intrinsic absorption lines are due to the quasar wind, and which are due to the circumgalactic medium of the host galaxy.

In §2 we describe the target selection, observations, and data reduction. Our results concerning NAL and mini-BAL variability are given in §3. In §4 we discuss our results and in §5 we summarize our conclusions. We use a cosmology with H = 72 km s Mpc, = 0.3, and = 0.7.

2 Observations and Data Analysis

2.1 Sample Selection

We have selected quasars with intrinsic NALs based on the partial coverage indicator: the dilution of absorption troughs by unocculted light (e.g., Wampler et al., 1995; Barlow & Sargent, 1997) from the background sources. The basic test relies on the optical depth ratio of resonant, rest-frame UV doublets of Lithium-like species (e.g., CIV, NV). When this ratio is inconsistent with 2:1, as dictated by atomic physics (for fully resolved unsaturated absorption profiles), the deviation is interpreted as dilution of the absorption troughs by an unocculted portion of the background source. The optical depth ratio provides a means of computing the fraction of background light occulted by the absorber (the “coverage fraction”): , where and are the continuum-normalized intensities of the weaker and stronger components of the doublet. Recently, Misawa et al. (2007a) surveyed 37 quasars at redshift and found that at least 43% had one or more intrinsic NALs as evidenced by partial coverage in the CIV, NV, and/or SiIV doublets, which implies that a substantial fraction of the solid angle around a substantial fraction of quasars is covered by material related to the quasar. It is more difficult to determine with certainty the origin of this material, more specifically whether it originates in the host galaxy, or whether it is a part of the quasar wind itself.

Our systematic study is based on long-term monitoring observations of 11 quasars (at –3.08) with at least one intrinsic NAL or mini-BAL. We select sample quasars from three sources; 1) quasars with intrinsic NALs that were identified in Misawa et al. (2007a), 2) quasars with mini-BALs that we identified from the VLT/UVES archive sample of Narayanan et al. (2007), and 3) quasars with mini-BALs already published in the literature (Hamann et al., 1995, 1997b; Dobrzycki et al., 1999). These intrinsic NALs and mini-BALs were identified through their partial coverage (as described above) or their broad absorption profile. Thus, our sample quasars are all bright enough for high dispersion spectroscopy, and classified into a category of very luminous quasars. Another essential selection criterion is that at least two previous high-signal-to-noise ratio (), high-resolution (–45,000) spectra already exist, taken at different epochs with either the Keck/HIRES, the VLT/UVES and/or the Subaru/HDS. The sample quasars that satisfy both criteria are summarized in Table 1.

2.2 Keck/HIRES Observations

Four of our quasars come from a quasar sample that was originally selected and observed for measuring the deuterium-to-hydrogen abundance ratio (D/H) in the Ly forest (e.g., Tytler et al., 1996). The typical value of D/H is so small, 2– (O’Meara et al., 2001, and references therein), that we can detect only DI lines corresponding to HI lines with large column densities, (/cm) . Therefore the survey included 40 quasars, in which either damped Ly (DLA) systems or Lyman limit systems (LLSs) were detected. This target selection method does not directly bias our sample with respect to the properties of any intrinsic absorption-line systems that these spectra may contain. The observations were carried out by a group led by David Tytler, using Keck/HIRES with a 114 slit resulting in a velocity resolution of  km s (FWHM). The spectra were extracted using the automated program, MAKEE, written by Tom Barlow. Misawa et al. (2007a) used the spectra of 37 of these quasars, after removing spectra of three quasars that cover only the Ly forest region, and identified 39 candidate intrinsic systems (28 reliable and 11 possible systems) by partial coverage analysis. However, most spectra used in Misawa et al. (2007a) were produced by combining multi-epoch spectra in order to increase the . In this paper, we used 4 NAL quasars (HE01304021, Q10092956, HS17006416, and HS19467658) out of the 37 quasars above because their original spectra (i.e., before combining data from multiple epochs) were available to us. We also used a spectrum of the mini-BAL quasar (UM675) in one epoch. The spectrum was obtained by Fred Hamann, using Keck/HIRES with a 115 (Hamann et al., 1997a), who kindly provided it to us.

2.3 VLT/UVES Observations

Spectra of seven of the quasars studied here were obtained from the VLT/UVES archive. Narayanan et al. (2007) retrieved all R 45000 spectra made available before 2006 June, and made a catalog of MgII absorption systems in 81 quasar spectra. Since most of these quasars were originally observed for studies of intervening absorbers, they should not have a particular bias toward or against intrinsic NALs and mini-BALs. In this catalog, we found four mini-BAL systems in the spectra of HE01514326, Q1157014, HE13411020, and Q2343125, as well as intrinsic NAL systems in two quasars (HE01304021 and HE09401050). Some of the mini-BAL quasars above were already discovered in past work (Hamann et al., 1997b; Menou et al., 2001). In addition to these archival data, we re-observed three NAL quasars (HE01304021, HE09401050, and Q10092956) and four mini-BAL quasars (HE01514326, Q1157014, HE13411020, and Q2343125) in 2007, using VLT/UVES with appropriate standard setups for each target to cover the wavelength from Ly to CIV 1548,1551 with only a few exceptions. We used a 10 slit (yielding ; 7.5 km s) and 22 CCD binning. Neither the ADC nor the image-slicer were used. We reduced the data following the procedure of Narayanan et al. (2007) using the ESO provided MIDAS pipeline. We also performed helio-centric velocity correction and air-to-vacuum wavelength correction.

2.4 Subaru/HDS Observations

We obtained high-resolution spectra of one NAL quasar (HS17006416) that was identified in Misawa et al. (2007a) and two mini-BAL quasars (UM675 and HS16033820) from the literature (Hamann et al., 1997a; Dobrzycki et al., 1999) using Subaru/HDS. We chose these quasars because of their good visibility from the Subaru telescope and their brightness, 17. We used a slit (yielding R36,000, 8.33km/s), the ADC, and the red-sensitive grating. The CCD binning was set to 21. We used non-standard setups to cover Ly, N V 1239,1243, Si IV 1394,1403, and C IV 1548,1551. We reduced the data in a standard manner with the IRAF software999IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.. Wavelength calibration was performed using the spectrum of a Th-Ar lamp. Because the blaze profile function of each echelle order changes with time, we could not perform flux calibration using the spectrum of a standard star. Therefore we directly fitted the continuum, which also includes substantial contributions from broad emission lines, with a third-order cubic spline function. Around heavily absorbed regions, in which direct continuum fitting was difficult, we used the interpolation technique introduced in Misawa et al. (2003). We have already confirmed the validity of this technique by applying it to a stellar spectrum.

2.5 Data Summary and Spectroscopic Analysis

To enhance the  of the spectra, all available observations of a particular target, taken within 2 weeks (14 days) of each other, were combined into a single spectrum, representing a single epoch. Although some BAL features show variability on very short time scales ( years), the variation probability is much smaller than that for longer time intervals (e.g., Capellupo et al., 2013). Therefore we prioritize increasing data quality above investigating short-term variability (i.e., combining as many spectra as possible to increase the ). In total, we have monitored 12 intrinsic NALs in five quasars and 7 mini-BAL in six quasars for 4–12 years (–3.5 years in the quasar rest-frame). Each was observed with one or more of the telescopes/spectrographs discussed above. The observations are summarized in Table 2.

For the data analyses in the following sections, we normalized all spectra as follows. We fit the VLT/UVES spectra (including both the continuum and the broad emission lines) with cubic spline functions and divided the original spectra by these functions. While fitting, we avoid absorbed regions so that the fit interpolates across them. An example of continuum fits around the CIV mini-BAL of the VLT/UVES spectrum of Q2343+125 (Epoch 1) is shown in Figure 1. Because the continuum level usually depends on the regions used for the fit, we experimented with three different fit regions. The uncertainty from the continuum placement will be discussed in §3.1. For Keck/HIRES and Subaru/HDS, we separately fit each echelle order of the spectra with a cubic spline function that includes the instrumental blaze function as well as the continuum and the broad emission lines. This was necessary because flux calibration was not applied to these spectra. (i.e., each echelle order is not smoothly connected to adjacent orders.)

The normalized profiles of Ly, N V 1239,1243, Si IV 1394,1403, and C IV 1548,1551, superimposed for all epochs of observation (listed in Table 2), of the 12 intrinsic NALs and 7 mini-BALs are displayed in Figures 213, if they are covered by the observed spectra. The different epochs are represented by curves of different colors, with the 1st to 6th epochs shown as black, red, green, blue, light-blue, and purple, respectively. The velocity scale is relative to the flux-weighted center of a line, calculated by equation (4) of Misawa et al. (2007a). For this display the mini-BAL spectra, only, were resampled every 0.3Å because their  are relatively low and their profiles are broad enough that no information is lost. The Ly and other metal absorption lines of mini-BALs are omitted from this plot if they are badly blended with the Ly forest and/or if they are observed with Subaru/HDS, because in that case the continuum fit is too uncertain to allow a variability analysis.

3 Measurements and Results

3.1 Methodology and General Results

Our initial inspection of the normalized spectra shown in Figures 213 did not reveal any substantial variations in the profiles of either the NALs or the mini-BALs i.e., no changes in the profile asymmetries or the velocities of the troughs were discernible. However, changes in the strengths of the mini-BALs were quite evident. To quantify changes in line strengths, we measured the EWs of the blue components of NALs and the entire doublet regions of mini-BALs in each of the observed epochs for each transition, i.e., CIV, SiIV, and NV. We list these measurements in Table 3 along with their uncertainties. For the mini-BALs, the doublet members often suffer self-blending and cannot be reliably separated. We calculated the 1 error bar on the measured EW, , as the sum of contributions from uncertainties in the intensities of individual pixels in the spectrum and from uncertainties in the placement of the continuum: . The first term was evaluated by summing in quadrature the EW error bars from the individual pixels making up the absorption trough (see horizontal arrows in Figures 213)

(1)

where is the error in the normalized intensity at pixel , and is the width of that pixel (in Å). To compute the uncertainty resulting from the continuum placement, we assumed that it is proportional to the product of the width of the line near the continuum level, (see horizontal arrows in Figures 213), and the characteristic uncertainty of the continuum next to the line, . In other words, we supposed that the fitted continuum level in the spectrum could fluctuate by an amount of order , sweeping out an area of order , which sets . The fluctuations in the normalized continuum level are , where is the observed flux density, which is the inverse of the signal-to-noise ratio, . Hence we may write

(2)

In the above expression we have included a “calibration parameter,” , whose value we found empirically as follows. We carried out several experiments in which we fitted the continuum around some NALs and mini-BALs multiple times, choosing different regions around them each time, so as to determine the amount by which the fitted level could fluctuate. We then compared the variation in EW measured after different continuum fits to that given by equation (2) and found that very uniformly (see Figure 1). Thus we adopted equation  (2) with as an estimate of the EW uncertainty resulting from continuum placement errors. We find that the continuum placement errors are the dominant source of uncertainty in the measured EWs.

To assess the significance of observed variations in the EW we computed the change in EW between two epochs as (where epoch 1 is earlier than epoch 2) and its uncertainty as and defined the variation significance as

(3)

Significant variability is seen preferentially in the mini-BAL systems (see Figures 1419), but not in NALs. The greatest changes for NALs are seen from E1 E2 of NV in the   452 km s system of Q1009+2956 and from E2 E4 of NV in the   767 km s system of HS1700+6416. In both of these cases the variation significance is between 1.5 and 2, and this is consistent with expectations in a sample of this size that does not experience variability. It is also consistent with variations in the EWs of intervening NALs caused by measurement errors. We tabulate the measured variation significance for mini-BALs in Table 4 for the shortest time intervals where a variation was detected (in the case of Q2343+125 we only detect variability at a significance level of but we include this object in the table for completeness). Distributions of absolute variation significance, , as a function of EW and time interval are shown in Figures 2023 for NALs and mini-BALs, respectively. Histograms of , measured for all unique pairs of observing epochs, are also shown in Figures 24 and 25.

Five of the six mini-BAL systems in our sample show significant variability at greater than in at least one transition (in the sixth quasar, Q2343+125, the CIV line varied at ; see Table 4). Out of 52 unique pairs of epochs plotted in the histogram of Figure 25, four are in the range while another 6 are in the range . For NALs, we compare our sample with intervening NALs that are detected in the same spectra but are classified as intervening NALs with coverage fractions of unity (Misawa et al., 2007a). In Figure 24 we compare the distribution of of intrinsic NALs to that of intervening NALs. This comparison shows the two distributions to be very similar and reinforces our conclusion that none of the intrinsic NALs in our sample have varied significantly.

In Figure 26, we plot the change in EW of mini-BALs for every unique pair of observing epochs, , as a function of the rest-frame time interval, . The EW changes are larger for longer time intervals for lines from all observed ions, CIV, NV, and SiIV. The trend persists if we normalize the EW changes by the average EW as shown in Figure 27. In other words, both the absolute and the fractional change in EW increase over longer time intervals (fractional variations of 50% or more occur on time intervals of a year or longer). Such a behavior has been noted for CIV BALs (Gibson et al., 2008; Capellupo et al., 2011, 2013; Filiz Ak et al., 2013). We find an analogous result namely, when we detect significant variability in two transitions at the same time in the same system the EWs change in the same direction. We illustrate this behavior in Figure 28 where we plot the change in the SiIV or NV EW against that of CIV (see caption for error bars and other specific information). It is noteworthy, however, that we have never observed these three strong lines vary at the same time because one of them is either saturated or not detected. By comparing the data points in this figure with the straight line of unit slope included for reference, we conclude that the fractional variations of EW of lines in the same system are not necessarily proportional to each other.

If we assume that the observed changes in EW result from changes in the ionization state of the gas (see discussion in §1), we can set a lower limit to the electron density by taking the variability time scale, , to be an upper limit to the recombination time of the relevant ion, i.e., , where is the electron density and is the recombination coefficient of the transition in question. We use the recombination coefficients for a gas temperature of 20,000 K (Arnaud & Rothenflug, 1985; Hamann et al., 1995). It is not straightforward to infer a density from recombination to/from SiIV (Arnaud & Rothenflug, 1985), therefore we perform the calculations only for CIV and NV. If the EWs have increased, we assume that CV CIV and NVI NV, while if the EWs have decreased, we assume that CIV CIII and NV NIV, based on the following argument. In all cases (with a single exception; see previous paragraph) of variable mini-BAL systems absorption lines from all ions (i.e., SiIV, CIV, and NV, with ionization potentials of 45.1, 64.5, and 97.9 eV, respectively) get stronger and weaker together. In order for all three lines to increase and decrease together, the ionization parameter should be either or (Hamann, 1997). The regime of low can be rejected because we do not detect any absorption lines from low ionization species with ionization potentials between 14 and 33 eV corresponding to any mini-BAL systems (e.g., OI 1302, FeII 2344, FeII 2383, SiII 1260, SiII 1527, AlII 1671, CII 1335, AlIII 1855, AlIII 1863, and SiIII). This suggests that and that as the ionization parameter varies, the main ionization states of Silicon, Carbon, and Nitrogen vary as follows: SiIV SiV, CIV CV, and NV NVI. Under these conditions, we estimate lower limits on the electron density in the range cm. We are only able to derive such limits for mini-BALs; since the NALs in our sample did not show significant variability, we are not able to apply this method to them. We list the results for individual objects in Table 4 and discuss them in more detail in §3.2

From the lower limits on the density of the mini-BAL gas we then estimate upper limits to the distance of the absorber from the source of ionizing radiation by making use of the definition of the ionization parameter as the ratio of ionizing photon density to hydrogen number density at the illuminated face of the cloud:

(4)

where is the emission rate of hydrogen-ionizing photons by the central engine and is the distance of the absorber from that source. We assume that so that CIV is the dominant ionization stage of Carbon (Hamann et al., 1995, 1997a), and to satisfy the constraint that all the observed absorption lines increase and decrease together. We also assume that . Thus, we obtain by scaling the bolometric luminosity, which we compute via at Å, based on the SED of Richards et al. (2006). This SED is a segmented power law of the form with from 1,000 Å to 10 m, from 10 to 1,000 Å, and from 0.1 to 10 Å, following Zheng et al. (1997) and Telfer et al. (2002). By integrating this SED, we find that for a quasar with a bolometric luminosity of . Inserting the expression for and limits on and into equation (4) and rearranging we arrive at the following expression for the distance of the absorber from the continuum source

(5)

where is the bolometric luminosity in units of and is the electron density in units of . Using equation (5) we estimate upper limits on the distance of the absorber to be in the range 3–6 kpc (see Table 4 and §3.2). These values decrease by a factor of 4 if we use the SED of a typical radio-loud quasar (Mathews & Ferland, 1987) or a high luminosity radio-quiet quasar (Dunn et al., 2010).

3.2 Notes on Individual Mini-BAL Systems

Here we discuss the variable mini-BALs in more detail before moving on to a general discussion about their properties and the possible causes of variability.

UM675 (), mini-BAL system at km s ():

Time variability of the CIV and NV doublets over 2.9 yrs in the quasar rest frame was discovered in the mini-BAL absorber at   2.13 (i.e.,  1900 km s) in this radio-quiet quasar (Hamann et al., 1995). Additional observations indicated that the different ions can have different coverage fractions:   0.45 for CIV,   0.83 for Ly, and   0.6 for OVI (Hamann et al., 1997a).

We collected two spectra taken with Keck/HIRES and Subaru/HDS in 1994 and 2005, respectively. As shown in Figure 8, NV and CIV got weaker at a 5 significance over 3.5 years in the quasar rest frame. Ly also showed the same trend. The variation occurs across the entire profile, though perhaps fractionally less in the central narrower component, as seen in the spectrum of Q2343+125 (Hamann et al., 1997b). We cannot distinguish between a change in ionization and a change in coverage fraction as the cause of the variability.

Summary: The CIV and NV profiles have weakened on timescales of several years in the quasar rest frame. This places constraints on density ( 3.310 cm), and thus distance (  4.0 kpc). The detection of NeVIII (Beaver et al., 1991) also suggests proximity to the quasar, although this line is not covered by our optical spectra.

HE01514326 (), mini-BAL system at and 8,900 km s ( and 2.67):

This system has not yet been studied in detail as an intrinsic absorption system. There are two distinct absorption systems at   8900 km s (corresponding to 0 – 1000 km s in Figure 9), with CIV and SiIV detected, and at   11300 km s ( 2800 to 1800 km s), detected in CIV. In addition to these two systems, there is likely to be a weaker system at   6400 km s ( 1500 – 3000 km s) with detected CIV.

This mini-BAL system spans more than 5000 km s in velocity space, although it does not satisfy the criterion for being a BAL (which requires that the observed spectrum falls at least 10% below the model of continuum plus emission lines over a contiguous velocity interval of at least 2000 km s).

The first three epochs of observation are within one month, and neither CIV nor SiIV showed variation in EW at 5 level, except for E2 E3 of CIV for the   8900 km s system. The last epoch is separated from those first three by  years in the quasar rest frame. The absorption is significantly stronger in the last epoch, again over the entire velocity range of observation. The SiIV is either not detected or very weak in the first three epochs, but then has appeared by the fourth epoch of observation in the component at 800 km s (see Figure 9). This suggests that there has been an ionization change in this component at least, and not just a change in coverage fraction. To adequately test this hypothesis we would need high quality spectra covering some higher ionization ions such as NV and OVI.

Summary: The CIV profile for this absorption complex, which spans 5000 km s, has increased in strength between observations separate by 2 years in the quasar rest frame. SiIV has also appeared in one component. This is consistent with a lower degree of ionization at the later time, and places constraints on densities ( 3.910 cm), and thus distances ( 6.3 kpc) for the   11300 km s and 8900 km s systems.

Q1157+014 (), mini-BAL system at km s ():

Wright et al. (1979) first studied the spectrum of this quasar in detail, and noted that it has CIV and CIII] emission lines (although the Ly emission line is absent or extremely weak) and several broad absorption lines such as Ly, NV, and CIV. This system is classified as a BAL by the traditional definition because of the broad absorption profiles of CIV and NV. However, we regard it as a mini-BAL in this study because SiIV is not so strong as in typical BALs.

We collected 5 epochs of observations with VLT/UVES from 2000 – 2007, covering CIV, SiIV, and NV at multiple epochs. The profiles are shown in Figure 10. The SiIV varies significantly on a timescale of 223 days (80 days in the quasar rest frame). The NV appears to get a bit weaker and the CIV to get a bit stronger over the interval, but the change is at less than a 5 level except for E1 E4 for CIV. The CIV and NV are clearly saturated over most of the profile, and so are not subject to variations in column density. A variation in coverage fraction would lead to a significant change in those EWs, since non-unity coverage fraction would cause these saturated profiles not to be black. Thus the lack of variability in the saturated CIV and NV suggests that an ionization change is responsible for the variability of the SiIV profile in this system.

Summary: This system shows variability, but mostly in the weaker SiIV absorption, and not in the saturated CIV and NV. The most plausible explanation for this is that the coverage fraction remains similar, but that the ionization state changes, because a change in the CIV and NV column density won’t make a significant difference in the profile if it is saturated (e.g., Capellupo et al., 2012). This would be consistent with these profiles having changed only in their unsaturated parts. Any strict constraints cannot be placed on density and distance because recombination from/to SiIV is not simple.

HE13411020 (), mini-BAL system at km s ():

We found this mini-BAL system with broad absorption features of CIV and NV in the catalog of Narayanan et al. (2007). As far as we know, this system has not been previously studied in detail as an intrinsic absorption system.

We collected two epochs of observations with VLT/UVES in 2001 and 2007, which are shown in Figure 11. With the two available epochs we find that both lines show time variability on a timescale of 2 yrs in the quasar rest frame. Both the CIV and the NV are stronger at the later epoch at more than 5 significance level, and they are stronger over the entire profile. SiIV is not detected at either epoch. The profiles again have the appearance of narrower profiles superimposed on broad ones.

Summary: The CIV and NV profiles have varied on a timescale of  years in the quasar rest frame, with both ions becoming stronger over the full range in velocity. This is certainly consistent with a change in coverage fraction, but an ionization change also cannot be ruled out with only two ions available as constraints. If an ionization change is the cause of variability, we can place constraints on density ( 3.010 cm), and distance ( 3.5 kpc). The absorption profiles show narrow components superimposed on broader ones.

HS1603+3820 (), mini-BAL system at km s ():

This system was first discovered by Dobrzycki et al. (1999), and then monitored with high-resolution spectra for 1.2 yrs in the quasar rest frame (Misawa et al., 2005, 2007b). The CIV mini-BAL shows both partial coverage (Misawa et al., 2003) and time variability (Misawa et al., 2005, 2007b). The CIV profile changes across the entire range of velocity, 2000 km s, at once (see Figure 12). Among three possible scenarios for the variation: i) a gas motion, ii) a variable scattering material, and iii) a change of ionization condition, the first two cannot be applicable for this quasar (see §1). The preferred explanation of changing ionization parameter is likely to be caused by variable shielding material between the continuum source and the absorber, since a quasar of this luminosity does not vary enough to explain the change. Warm absorbers are a candidate for the shielding material as evidenced by X-ray observations of this quasar (Misawa et al., 2010; Rozanska et al., 2013).

Summary: There is significant variability of CIV over the full velocity range on timescales as short as months in the quasar rest frame. The variability is likely to be caused by changing ionization parameter due to variable shielding material that modulates the quasar flux. We place constraints on the electron density ( 1.610 cm) and the absorber’s distance from the flux source ( 4.0 kpc)

Q2343+125 (), mini-BAL system at km s ():

Previously, Hamann et al. (1997b) detected CIV, NV, and SiIV mini-BALs at   2.24 (corresponding to   24400 km s) in this radio-quiet quasar spectrum. These lines showed both time variability (the lines weakened and then strengthened again by a factor of 4 on a timescale of 0.3 yrs in the quasar rest frame) and partial coverage (  0.2 for CIV).

We collected additional spectra with VLT/UVES at three different epochs from 2002 to 2007. Because of the different wavelength coverage of the different observations, only CIV has multiple observations (Figure 13). Ly and NV are blended with the Ly forest. The EW measurements for CIV 1548 were  Å,  Å, and  Å for observations 1, 2, and 3, respectively, indicating only negligible variability at less than the 2 level. We do not detect any SiIV features, while Hamann et al. (1997b) found a weak SiIV mini-BAL. Several narrow components inside the broader absorption profiles of CIV (Hamann et al., 1997b) are not apparent in our lower- spectra.

Summary: In this   24,400 km s mini-BAL, CIV gets slightly weaker, and then stronger, with changes taking place in only 124 and 403 days in the quasar rest frame, but in 2 level. More detailed analysis is not possible due to there being multiple epoch observations only of CIV.

4 Discussion

4.1 Comparison of BAL and mini-BAL Variability and Assessment of Physical Mechanisms

The observational results of our monitoring campaign, combined with earlier case studies (e.g., Misawa et al., 2007b; Hamann et al., 2011; Rodríguez Hidalgo et al., 2012, and references therein), suggest some similarities between the variability patterns observed in mini-BALs and BALs. In particular, we have found that the EWs of all mini-BALs in our sample vary on rest-frame time scales of months to years and have observed the following patterns (a) when different transitions in a mini-BAL system vary together, their EWs change in the same direction, (b) absolute and relative changes in EWs are larger over longer rest-frame time intervals, and (c) variations in the EWs of the lines are not accompanied by discernible changes in the profiles, i.e., different troughs in the profile vary together. In comparison (a) more than 50% of BALs vary over time scales of order a year in the quasar rest-frame with the probability of variation approaching unity on multi-year time scales (e.g., Capellupo et al., 2011, 2013), and (b) the EWs of the UV absorption lines from CIV, SiIV, and NV in BAL systems vary together and in the same direction, although not necessarily in proportion to each other (e.g., Filiz Ak et al., 2013). However, BAL profiles do vary as the EWs vary (e.g., Lundgren et al., 2007; Gibson et al., 2008; Capellupo et al., 2011; Filiz Ak et al., 2012, 2013) in contrast to what is observed for mini-BAL troughs. In view of the similarities, we consider the two variability mechanisms often discussed for BALs as explanations for the observed mini-BAL variability, gas motion across our sightline and change of ionization conditions (see §1 for references and more details)

To evaluate the likelihood that the mini-BAL variations result from motion of the absorbing gas across the line of sight, we estimate the time scale for a parcel of gas to cross the cylinder of sight that encompasses either the continuum source or the BLR. We take the transverse speed of the absorber to be of the same order as the observed outflow speed (both are related to the free-fall speed, the only velocity scale in the problem), i.e., . We assume that the observed UV continuum is emitted from the inner parts of the accretion disk within a radius from the center, where is the gravitational radius and is the mass of the black hole, i.e., cm, where . Using the bolometric luminosities from Table 4 and assuming that the accretion rates are close to the Eddington limit, we estimate the black hole masses to be in the range: –15, which leads to continuum source radii in the range cm and crossing times –0.7 years. For the radius of the BLR, we adopt the relation between it and the continuum luminosity given by Kaspi et al. (2007), which leads to –0.63 pc and crossing times of –60 years. The long crossing times of the BLR lead us to disfavor gas motion across the BLR cylinder of sight as the cause of variability. Moreover, a substantial fraction of the mini-BAL troughs in our sample are found at large outflow velocities, , so that they do not overlap with the broad emission lines (i.e., the mini-BAL gas absorbs only continuum photons and the continuum source crossing time is the more relevant time scale). These considerations make this particular version of the scenario very unlikely. The crossing times of the continuum source for the more luminous quasars in our sample are on a par with the observed variability time scales and the amplitude of EW variations is largest for time scales just over a year, which is comparable to the time required to traverse the continuum source completely. However, for the less luminous quasars in our sample, we would have expected the EW variations to be faster. Moreover, all troughs in a mini-BAL system vary together when the EW varies, which is difficult to orchestrate unless the parcels of gas producing the mini-BAL troughs in a given system all cross the cylinder of sight to the continuum source at the same time. Thus, we disfavor this version of the scenario as the cause of variability, although we cannot decisively rule it out (see also the discussion in Rodríguez Hidalgo et al., 2011).

As argued by Misawa et al. (2007b), the fact that all troughs in a mini-BAL system vary together strongly suggests that the variability is the result of changes in the ionization state of the absorber, presumably because of a variable ionizing continuum. Filiz Ak et al. (2014) suggest that this mechanism may also operate in BALs. But since the continuum variability time scales of such luminous quasars are considerably longer than the observed variability of mini-BALs (e.g., Giveon et al., 1999; Hawkins, 2001; Kaspi et al., 2007), one is led to invoke variable obscuration of the ionizing continuum seen by the absorbers by shielding material of variable column density. This screen could be the X-ray warm absorber (Gallagher et al., 2002, 2006), and indeed some X-ray absorbers toward mini-BAL quasars show this particular behavior (e.g., Giustini et al., 2011). In this scenario, an X-ray warm absorber lies along sight lines toward mini-BAL and BAL systems but not toward NAL absorbers. This conclusion is based on the measurement of high column densities from the X-ray spectra of quasars with BALs and mini-BALs but not from the spectra of quasars with intrinsic NALs (e.g., Chartas et al., 2009). Recent radiation-MHD simulations by Takeuchi et al. (2013) reproduce variable clumpy structures in warm absorbers with typical sizes or order and typical optical depth of order unity. They appear above heights of from the plane of the accretion disk but, interestingly, they are not found at very high latitudes, where sight lines towards intrinsic NAL absorbers have been suggested to be by Ganguly et al. (2001) and Misawa et al. (2008). The fact that these clumps have sizes comparable to the size of the continuum source, suggests that they can modulate the apparent size of the continuum source and, by extension, the coverage fraction of the absorbers. Thus, such clumpy structures provide a plausible explanation for the frequent variability of mini-BALs and the lack of frequent variability in intrinsic NALs.

4.2 Variability of NALs and Narrow Components in mini-BAL Systems

It is noteworthy that the intrinsic NALs we have monitored in this program did not show any variability. These were identified as intrinsic systems based on the fact that their CIV, NV, or SiIV doublets show the signature of partial coverage of the background source. Although the same signature is seen on some rare occasions in low ionization lines from intervening absorbers (presumably arising in cold and compact parcels of gas; see Kobayashi et al., 2002; Jones et al., 2010; Chen & Qin, 2013), it has never been seen in high-ionization lines, such as CIV and NV, from intervening absorbers. Therefore we are confident that the systems we have targeted are intrinsic. Moreover, variability has been observed in NALs in other monitoring campaigns, both at high and low redshifts, by Narayanan et al. (2004) and Wise et al. (2004). The NAL variability time scales and resulting lower limits on electron densities from those two studies are comparable to what we have found here for mini-BALs.

Narrow kinematic components are sometimes also found near the centers of mini-BALs. Examples in our sample can be seen at  km s in the CIV mini-BAL of HE13411020, at km s in the CIV and NV mini-BALs of UM675, and at km s in the SiIV mini-BAL of Q1157+014. Among these, we can monitor the narrow components in UM675 without complications from noise and line blending. The narrow CIV components of UM675 in Epochs 1 and 2 are shown in Figure 29, in which broader absorption troughs are fitted out by high order spline functions. The total EWs (including both CIV 1548 and CIV 1551) of Epochs 1 and 2 are measured to be and . Thus, they show no variability over a time interval of   years. Hamann et al. (1997b) noted a similar behavior for the narrow core of the CIV mini-BAL in Q2343+125 and suggested the narrow components may not be physically associated to the quasar itself, but arising in the quasar host galaxy or foreground galaxy (although this quasar is one of our targets, we cannot monitor the variability of the narrow components because our spectra have lower  than those of Hamann et al. (1997b)). However, narrow components appear to always coincide with the centers of mini-BAL profiles, which suggests a physical association between the absorbing media. One possible explanation for the behavior of the narrow cores of mini-BALs is that they originate in dense clumps of high optical depth, embedded in the mini-BAL flow. When the ionizing continuum varies, the broad mini-BAL troughs show an easily discernible response but the highly saturated, narrow cores do not.

4.3 Possible Geometry and Locations of mini-BAL and NAL Gas

The variability properties of mini-BALs and NALs reported here in combination with previous reports in the literature (see references above), lead us to suggest the following possibilities for locations of the absorbing gas. These are inspired by numerical simulations of quasar outflows (see references in §1) and are a refinement of the picture suggested by Ganguly et al. (2001).

In the first scenario, both types of absorbers are located in an accretion-disk wind, as illustrated in Figure 30. The mini-BAL gas is in the form of filaments rising from the fast component of the outflow. They make up the interface between the dense BAL wind at low latitudes above the disk and a hotter, much more tenuous medium in the polar direction. As such, the mini-BAL filaments have a large outflow speed, comparable to that of the BAL gas and a substantial velocity gradient. Within those filaments there may exist small, dense clumps that are sometimes observed as narrow cores in mini-BAL troughs. Variability of the ionizing continuum incident on the mini-BAL filaments can change the optical depth of the mini-BAL troughs arising in the filaments but not those of their narrow cores which arise in the dense clumps with CIV optical depths of (assuming their predicted hydrogen column densities of , , and solar abundances). In this scenario, the NALs and the narrow cores of mini-BALs arise in higher-density, smaller filaments in the same region of the wind or at higher latitudes. It is possible that the NAL filaments are denser than the mini-BAL filaments thus the NALs are less susceptible to fluctuations in the ionizing continuum. However, NALs could vary because of changes in the coverage fraction, which could come about as the apparent size of the background source changes because of fluctuations in the structure of the inner screen (see discussion at the end of §4.3).

Alternatively, the NAL gas may be located in the host galaxy, at large distances from the center, as we noted in §1. In this scenario, small NAL filaments may form from the interaction of the wind’s blast wave with cold clouds in the host galaxy, as proposed by Faucher-Giguère et al. (2012). Radiative shocks resulting from this interaction can create thin, dense filaments whose properties are consistent with what is inferred from photoionization models for the NAL absorbers (e.g., Wu et al., 2010; Borguet et al., 2013) and are likely to have . Thus the variability of the NALs arising in these filaments is more likely to result from fluctuations in the coverage fraction than fluctuations in the ionizing continuum. This scenario is more likely to apply to associated NALs, with , as this range of blueshifts better matches the prediction of numerical models for the interaction of quasar outflows with the ISM of the host galaxy (Kurosawa & Proga, 2009). NALs found at (e.g., 20,000–65,000 km s; see Misawa et al., 2007a) are better explained by the previous scenario.

The underlying hypotheses in the above scenarios are that (a) NAL variations are more rare than those of mini-BALs (although when NALs do vary they do so on the same time scales as mini-BALs), and (b) that NAL variability is caused primarily by fluctuations in the coverage fraction while the variability of mini-BALs can be caused by fluctuations in both the coverage fraction and the ionizing continuum. Both of these hypotheses warrant further testing, which can be done by a more detailed analysis of the data we have presented in this paper as well as data collected from denser monitoring campaigns on select targets.

5 Summary

We have monitored 12 intrinsic NALs (selected by their partial coverage signature) and 7 mini-BALs in quasars of –3 for –12 years (–3.5 years in the quasar rest-frame). Using high-dispersion spectra, we have measured the line EWs as a function of time and have looked for substantial variations in the line profiles. Our main results are as follows.

  • We detect significant variability in the EWs of mini-BALs without any discernible changes in their profiles. However, we do not detect any significant variability in the NALs nor in the narrow cores that are sometimes present in mini-BAL profiles.

  • The EW variability of mini-BALs is larger on longer time scales, a behavior that has also been seen in BALs.

  • When we are able to measure variations in the EW of more than one transition at the same time (CIV, SiIV, NV) the changes occur in the same direction. This pattern has also been observed in BALs and is consistent with a scenario in which the variations result from changes in the ionizing continuum.

  • If we assume that the variability timescale is the recombination time, we can place lower limits on the electron density of the mini-BAL gas of cm. These imply upper limits on the distances of the absorbers in the range 3–6 kpc, which could be reduced by a factor of for a different choice of quasar SED.

  • The observational results lead us to suggest two possible scenarios for the structure and location of the mini-BAL and NAL gas. In the first scenario both types of absorber are associated with the high-latitude potion of an accretion-disk wind while in the second, the NAL gas is at distances pc from the quasar central engine.

The next stage of this work will entail a more detailed analysis of the variability data to study the behavior of the coverage fractions and the relative strengths of different lines, and interpret these with the help of photoionization models. Further progress in characterizing and interpreting the variability properties of mini-BALs and NALs can be made via dense monitoring campaigns of select targets in which we can observe changes on shorter time scales than we were able to probe here and associate them with changes in the ionizing continuum (via X-ray and UV observations). The data from such campaigns can also help us place better constraints on the scenario in which variability is cause by parcels of gas crossing the cylinder of sight to the continuum source.

It would also be extremely useful to quantify the properties that the variable shield we have invoked must have in order to produce the desired behavior. There are a number of constraints that such a shield must satisfy: it must reproduce the low column densities observed in the X-ray spectra, the continuum it transmits must lead to the observed ionization state of the UV absorber, and its structure must allow for variations in far-UV transparency and apparent source size to vary in order to reproduce the observed mini-BAL variability. It is also worth considering further different geometric arrangements of the shield, the absorber, and the background source(s) relative to the cylinder of sight, specifically whether the source of ionizing photons, the shield, and the mini-BAL gas must all be aligned.

We would like to thank David Tytler and Fred Hamann for providing us with their Keck/HIRES data. We also thank the anonymous referee for a number of comments that helped us improve the paper. The research was supported by JGC-S Scholarship Foundation. This work was also supported by NASA grant NAG5-10817 and by NSF grants AST-0807993 and AST-1312686 at Penn State. M.E. would like to thank the Center for Relativistic Astrophysics at Georgia Tech and the Department of Astronomy at the University of Washington for their warm hospitality during the writing of this manuscript.

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QSO RA Dec Class variability reference
[hh:mm:ss] [dd:mm:ss] [mag.] [km s]
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
NAL
HE01304021 01:33:01.9 40:06:28 17.02 3.030 2.5597 37037 A N 1
2.9749 4129 A N 1
2.2316 65181 B N 1
HE09401050 09:42:53.4 11:04:25 16.90 3.080 2.8347 18578 B N 1
Q1009+2956 10:11:55.6 29:41:42 16.05 2.644 2.2533 33879 A N 1
2.6495 452 A N 1
HS1700+6416 17:01:00.6 64:12:09 16.17 2.722 2.7125 767 A N 1
2.7164 452 A N 1
2.4330 24195 B N 1
2.4394 23640 B N 1
HS1946+7658 19:44:55.0 77:05:52 16.20 3.051 3.0385 927 A N 1
3.0497 96 A N 1
mini-BAL
UM675 01:52:27.3 20:01:07 17.40 2.15 2.13 1,900 A Y 2
HE01514326 01:53:27.2 43:11:38 16.80 2.78 2.64 11,300 A Y 3
Q1157+014 11:59:44.8 01:12:07 17.52 1.9997 1.97 3,000 A Y 4
HE13411020 13:44:27.1 10:35:42 17.80 2.135 2.12 1,300 A Y 3
2.67 8,900 A Y 3
HS16033820 16:04:55.4 38:12:02 15.99 2.542 2.43 9,500 A Y 5
Q2343+125 23:46:28.2 12:49:00 17.00 2.515 2.24 24,400 A N 6
aafootnotetext: Coordinates (J2000) from NASA/IPAC Extragalactic Database (NED).bbfootnotetext: V magnitude from Véron-Cetty & Véron (2010).ccfootnotetext: Quasar emission redshift and absorption redshift of NAL/mini-BAL system that we monitored. These are from the reference listed in (10).ddfootnotetext: Ejection velocity from the quasar emission redshift, calculated via , where and are the emission redshift of the quasar and the absorption redshift of the NAL/mini-BAL, respectively. A positive value indicates a blueshifted line.eefootnotetext: Reliability class: A and B mean reliable and possibly intrinsic absorption lines, respectively (see Misawa et al., 2007a).fffootnotetext: NAL/mini-BAL varies or not.ggfootnotetext: References: (1) Misawa et al. (2007a), (2) Hamann et al. (1995), (3) Narayanan et al. (2007), (4) Wright et al. (1979), (5) Dobrzycki et al. (1999), (6) Hamann et al. (1997b),.hhfootnotetext: The emission redshift is listed as in Narayanan et al. (2007), but we adopt 2.78 because the Ly forest starts to appear at 4600Å.iifootnotetext: From Misawa et al. (2007b).
Table 1: List of Monitored Quasars
QSO Type Epoch Obs. Date Instrument -coverage Exp. Time Note
[Å] [sec]
(1) (2) (3) (4) (5) (6) (7) (8) (9)
HE01304021 NAL 1 1995 Dec 28 Keck+HIRES 3700 – 6062 14400 36,000
2 2003 Jan 13 VLT+UVES 3523 – 4519 3065 40,000 70.B-0522(A)
4782 – 5751
5839 – 6264
3 2007 Sep 5 VLT+UVES 3508 – 4507 6000 40,000 079.B-0469(A)
4629 – 5593
5680 – 6349
HE09401050 NAL 1 2000 Apr 03 VLT+UVES 3601 – 3870 3600 40,000 65.O-0474(A)
4783 – 5756
5837 – 6428
2 2001 Feb 02–14 VLT+UVES 3605 – 5750 28800 40,000 166.A-0106(A)
5847 – 6428
3 2007 Jun 6,7 VLT+UVES 3588 – 4505 6000 40,000 079.B-0469(A)
4630 – 5591
5683 – 6428
Q1009+2956 NAL 1 1995 Dec 29 Keck+HIRES 4303 – 5517 12200 36,000
2 1998 Dec 15 Keck+HIRES 4350 – 4612 14260 36,000
3 1999 Mar 09 Keck+HIRES 4300 – 4642 14200 36,000
4 2007 Jun 5 VLT+UVES 4169 – 5158 3000 40,000 079.B-0469(A)
5238 – 5741
HS1700+6416 NAL 1 1994 Apr 07 Keck+HIRES 4490 – 5850 6750 36,000
2 1995 May 10 Keck+HIRES 4460 – 5864 11500 36,000
3 2005 Jul 07 Subaru+HDS 3547 – 4900 3600 36,000 S05A-041
4991 – 5864
4 2005 Aug 19 Subaru+HDS 3547 – 4897 5400 36,000 S05A-041
4992 – 5864
HS1946+7658 NAL 1 1994 Jul 31 Keck+HIRES 4860 – 6296 25200 36,000
2 1998 Sep 25 Keck+HIRES 4800 – 5047 8000 36,000
3 1998 Oct 26–28 Keck+HIRES 4850 – 5020 24000 36,000
UM675 mini-BAL 1 1994 Sep 24,25 Keck+HIRES 3750 – 4963 18000 34,000
2 2005 Aug 19,20 Subaru+HDS 3600 – 4963 23000 36,000 S05A-041
HE01514326 mini-BAL 1 2001 Sep 18–22 VLT+UVES 3160 – 5752 18000 40,000 166.A-0106(A)
5836 – 5893
2 2001 Oct 09,10 VLT+UVES 3161 – 3869 14400 40,000 166.A-0106(A)
4785 – 5755
5842 – 5893
3 2001 Nov 16–19 VLT+UVES 3161 – 5752 28800 40,000 166.A-0106(A)
5846 – 5893
4 2007 Sep 5 VLT+UVES 3486 – 4504 3000 40,000 079.B-0469(A)
4789 – 5753
5855 – 5893
Q1157+014 mini-BAL 1 2000 Apr 30 VLT+UVES 3640 – 3870 7200 40,000 65.O-0063(B)
4542 – 4727
2 2001 Jun 14,15 VLT+UVES 3636 – 4411 10800 40,000 67.A-0078(A)
3 2002 Jan 23 VLT+UVES 3715 – 4507 1800 40,000 68.A-0461(A)
4 2006 Jan 23 VLT+UVES 3643 – 4727 1960 40,000 076.A-0860(A)
5 2007 Jun 6,7 VLT+UVES 3762 – 4727 6000 40,000 079.B-0469(A)
HE13411020 mini-BAL 1 2001 May 02–12 VLT+UVES 3251 – 4938 43200 40,000 166.A-0106(A)
2 2007 Aug 4,5 VLT+UVES 3490 – 4510 6000 40,000 079.A-0469(A)
4629 – 4938
HS1603+3820 mini-BAL 1 2002 Mar 23 Subaru+HDS 5080 – 5581 2700 45,000 S01B-057
2 2003 Jul 7 Subaru+HDS 3520 – 4850 6000 45,000 Eng. Time.
4930 – 5581
3 2005 Feb 26 Subaru+HDS 3520 – 4855 7100 36,000 S04B-003
4925 – 5581
4 2005 Jun 29 Subaru+HDS 3520 – 4855 3600 45,000 S05A-041
4925 – 5581
5 2005 Aug 19 Subaru+HDS 3520 – 4850 3600 36,000 S05A-041
4920 – 5581
6 2006 May 31–Jun 1 Subaru+HDS 4320 – 5581 9000 45,000 S06A-153S
Q2343+125 mini-BAL 1 2002 Aug 18 VLT+UVES 3409 – 3872 7200 40,000 69.A-0204(A)
4785 – 5538
2 2003 Oct 29,30 VLT+UVES 3526 – 4507 7200 40,000 072.A-0346(A)
4801 – 5538
3 2007 Sep 16 VLT+UVES 4192 – 5157 3000 40,000 079.B-0469(A)
5240 – 5538
aafootnotetext: Wavelength range covered by the observed spectrum. Small gaps of 20 Å that are sometimes seen in Keck/HIRES spectra are ignored.bbfootnotetext: Proposal ID of the relevant observing program.ccfootnotetext: Provided by David Tytler.ddfootnotetext: Resolving power-slit products (i.e., [arcsec]) are 41,400 at 4500Å on Blue Arm and 38,700 at 6000Å on Red Arm. If we use the 10 slit, the resolving power is 40,000.eefootnotetext: Maximum effective wavelength, which corresponds to 5000 km s redward from CIV emission line. At higher wavelength than this limit, no CIV, SiIV, and NV NALs/mini-BALs are expected to exist.fffootnotetext: Provided by Fred Hamann.
Table 2: Log of Monitoring Observations
QSO Epoch NV SiIV CIV
[km s] [days] [Å] [Å] [Å]
(1) (2) (3) (4) (5) (6) (7)
HE01304021 65,181 1 0 0.990.05
2 2573 0.990.07
3 4269 1.0170.04
37,037 1 0 0.0570.008 1.490.08
2 2573 0.0590.013 1.460.12
3 4269 0.0540.007 1.450.07
4,129 1 0 1.870.14 0.23 -
2 2573 1.820.18 0.28 1.170.25
3 4269 1.730.11 0.17 1.150.15
HE09401050 18,578 1 0 - 0.5380.028 1.620.06
2 311 0.5330.014 1.650.03
3 2620 0.5420.021 1.610.05
Q1009+2956 33,879 1 0 - 0.9000.030 1.730.03
2 1082 - 0.9080.003 -
3 1166 - 0.8950.014 -
4 4176 - 0.9260.048 1.720.06
452 1 0 0.4340.008 0.0190.014 -
2 1082 0.3920.005 - -
3 1166 0.4050.004 - -
4 4176 0.3990.016 0.0180.009 0.4300.015
HS1700+6416 24,195 1 0 - 0.1020.019 0.4060.033
2 398 - 0.1040.006 0.4240.014
3 4109 0.0840.015 0.4300.036
4 4152 0.1060.01 0.4170.027
23,640 1 0 - 0.0870.017 0.1960.019
2 398 - 0.1000.007 0.2160.007
3 4109 0.0850.015 0.1890.018
4 4152 0.0890.014 0.1970.012
767 1 0 0.5010.027 0.02 0.2910.016
2 398 0.4650.009 0.007 0.2950.007
3 4109 0.5000.031 0.02 0.3280.021
4 4152 0.5040.022 0.01 0.3180.015
452 1 0 0.1080.015 0.015 0.011
2 398 0.1010.006 0.006 0.005
3 4109 0.0840.015 0.02 0.02
4 4152 0.1090.012 0.01 0.01
HS1946+7658 927 1 0 0.01 0.01 0.2860.007
2 1517 0.01 - -
3 1549 0.01 - -
96 1 0 0.3660.008 2.3180.01 4.1640.02
2 1517 0.3840.014 - -
3 1549 0.3710.012 - -
UM675 1,900 1 0 6.151.01 0.4 5.380.24
2 3982 3.630.33 0.3 3.840.21
HE01514326 11,300 1 0 0.07 1.960.26
2 19 - 0.10 2.120.37
3 58 0.10 2.220.36
4 2176 0.240.16 3.690.53
8,900 1 0 0.090.04 4.070.21
2 19 - 0.100.06 4.220.30
3 58 0.080.06 3.920.32
4 2176 - 0.590.09 10.420.43
Q1157+014 3,000 1 0 38.381.28 - 37.121.23
2 410 38.271.17 9.160.52 -
3 633 - 10.311.23 -
4 2094 37.74.5 9.771.12 38.172.33
5 2593 - 11.260.71 38.531.05
HE13411020 1,300 1 0 15.390.47 0.3 6.860.24
2 2280 21.630.79 0.7 12.400.45
HS1603+3820 9,500 1 0 - - 10.121.15
2 471 17.951.84
3 1071 0.8 13.620.79
4 1194 13.031.43
5 1245 12.690.84
6 1530 - 0.5 13.340.61
Q2343+125 24,400 1 0 - - 3.290.69
2 437 - 2.040.44
3 1855 - 0.76 2.690.63
aafootnotetext: Time delay from the first epoch in the observed frame.bbfootnotetext: This line is blended with the Ly forest.ccfootnotetext: Parameters are measured using only blue component, because doublet lines are not self-blended and can be fitted separately.ddfootnotetext: This line is significantly affected by bad columns.eefootnotetext: Spectral region is partially truncated.fffootnotetext: This line is affected by a data defect that could slightly alter the EW measurement.--footnotetext: This line is not covered by the observed spectrum.
Table 3: Observed Frame Equivalent Width
QSO ion Ionization
[km s] [days] Change [erg s] [cm] [kpc]
(1) (2) (3) (4) (5) (6) (7) (8) (9)
UM675 1900 3982 (E1 E2) NV NV NIV
CIV CIV CIII
HE01514326 11300 2118 (E3 E4) CIV CV CIV
8900 2118 (E3 E4) SiIV SiV SiIV
8900 2118 (E3 E4) CIV CV CIV
Q1157+014 3000 2183 (E2 E5) SiIV SiV SiIV
HE13411020 1300 2280 (E1 E2) NV NVI NV
CIV CV CIV
HS16033820 9500 471 (E1 E2) CIV CV CIV
Q2343+125 24400 1855 (E1 E3) CIV CIV
aafootnotetext: Time interval in the observed frame in units of days. We list only the shortest time intervals on which variability was detected since these yield the most stringent limit on the density of the absorbing gas (see §3.1).bbfootnotetext: Variation significance, defined as ; see equation (3). We have included a low-significance measurement for Q2343+125 for completeness since we have not detected more significant variability in this object.ccfootnotetext: Bolometric luminosity, calculated as .ddfootnotetext: Electron density, calculated by assuming that the variation time scale is the recombination time.eefootnotetext: Absorber’s distance from the flux source, calculated by assuming the quasi-static photoionization change. These numbers become smaller by a factor of 4 if we use the SED of typical radio-loud quasar (Mathews & Ferland, 1987) or high luminosity radio-quiet quasars (Dunn et al., 2010).ggfootnotetext: No constraints can be placed because recombination processes to/from SiIV do not lead to constraints on the density in straightforward way (Arnaud & Rothenflug, 1985).hhfootnotetext: No constraints can be placed because of no time variation.
Table 4: Variability and Physical Properties of mini-BAL Absorbers
Figure 1: An example of a continuum fit to the VLT/UVES spectrum (gray histogram) with a cubic spline function. This spectrum covers the CIV mini-BAL at   24,400 km s in Q2343+125 and was obtained on Aug 18, 2002 (Epoch 1). Best-fitting continua are plotted with red, green, and blue curves whose fit regions are, 5015 – 5035 Å, 5010 – 5040 Å, and 5000 – 5050 Å, respectively as shown with horizontal lines. Similar fits are applied for all VLT/UVES spectra, but the exact methodology differ for Keck/HIRES and Subaru/UVES spectra as discussed in §2 of the text.
Figure 2: Normalized spectra around the Ly, CIV, NV, and SiIV absorption lines of intrinsic systems at  = 37,037 km s (left), 4,129 km s (middle), and 65,181 km s (right) in the spectrum of HE01304021. The velocity scale is relative to the flux-weighted center of a line, calculated by equation (4) of Misawa et al. (2007a). The 1st to 6th epoch spectra are shown with black, red, green, blue, light-blue, and purple histograms, respectively. Wavelength regions used for EW measurements are marked with solid arrows. If lines are not detected with 5 level, upper limits of EWs are measured for the regions with dashed arrows. False patterns such as cosmic rays, bad columns, and order gaps, are marked with asterisks on the spectra.
Figure 3: Same as Figure 2, but for the system at  = 18,578 km s in the HE09401050 spectrum.
Figure 4: Same as Figure 2, but for the systems at  = 33,879 km s (left) and 452 km s (right) in the Q10092,956 spectrum.
Figure 5: Same as Figure 2, but for the systems at  = 767 km s (left) and 452 km s (right) in the HS17006416 spectrum.
Figure 6: Same as Figure 2, but for the systems at  = 24,195 km s (left) and 23640 km s (right) in the HS17006,416 spectrum.
Figure 7: Same as Figure 2, but for the systems at  = 927 km s (left) and 96 km s (right) in the HS19467658 spectrum.
Figure 8: Same as Figure 2, but for the system at   1,900 km s in the UM675 spectrum. The spectrum is resampled every 0.3Å.
Figure 9: Same as Figure 2, but for the system at   11,300 km s and 8,900 km s in the HE01514326 spectrum. The spectrum is resampled every 0.3Å.
Figure 10: Same as Figure 2, but for the system in the   3,000 km s of Q1157014 spectrum. The spectrum is resampled every 0.3Å.
Figure 11: Same as Figure 2, but for the system in the   1,300 km s of HE13411020 spectrum. The spectrum is resampled every 0.3Å.
Figure 12: Same as Figure 2, but for the system at   9,500 km s in the HS1603+3820 spectrum. The spectrum is resampled every 0.3Å.
Figure 13: Same as Figure 2, but for the system at   24,400 km s in the Q2343125 spectrum. The spectrum is resampled every 0.3Å.
Figure 14: Monitoring results for EWs of CIV (red), NV (green), and SiIV (blue) lines in the mini-BAL at   1,900 km s in the UM675 spectra. The horizontal axis is the time delay from the first epoch in the quasar-rest frame (in years). The vertical axis is the observed EW (with 1 errors) normalized to that of the first observed epoch. If the observed spectrum does not cover a particular line, that epoch is not plotted for that line. If lines are covered but not detected, upper limits are marked with downward arrows.
Figure 15: Same as Figure 14, but for a mini-BAL at   11300 km s (top) and 8900 km s (bottom) in the spectra of HE01514326.
Figure 16: Same as Figure 14, but for a mini-BAL at   3000 km s in the spectra of Q1157014.
Figure 17: Same as Figure 14, but for a mini-BAL at   1300 km s in the spectra of HE13411020.
Figure 18: Same as Figure 14, but for a mini-BAL at   9500 km s in the spectra of HS16033820.
Figure 19: Same as Figure 14, but for a mini-BAL at   24,400 km s in the spectra of Q2343125.
Figure 20: Variation significance, (see equation 3), as a function of the average, rest-frame EW for NALs. Intrinsic CIV, NV, and SiIV NALs are marked with red, green, and blue stars, while intervening NALs with full coverage are marked with crosses and included for comparison.
Figure 21: Same as Figure 20, but for mini-BALs. The = 3 and 5 thresholds are marked with horizontal dotted lines.
Figure 22: Variation significance, (see equation 3), as a function of time interval for NALs. Intrinsic CIV, NV, and SiIV NALs are marked with red, green, and blue stars, while intervening NALs with full coverage are marked with crosses for comparison.
Figure 23: Same as Figure 22, but for mini-BALs. The = 3 and 5 thresholds are marked with horizontal dotted lines.
Figure 24: The distribution of variation significance, (see equation 3) for intrinsic NALs (red histogram) and intervening NALs (blue histogram). We include all unique pairs of observing epochs.
Figure 25: Same as Figure 24, but for mini-BALs. The and 5 thresholds are marked with vertical dotted lines.
Figure 26: Change in the absolute rest-frame EW of mini-BALs as a function of rest-frame time interval for every unique pair of observing epochs. CIV, NV, and SiIV data are shown with red, green, and blue stars with 1 errors, respectively. The dashed and solid curves denote upper envelopes of the distribution with and without a outlying point at Å. They are included only as a guide to the eye. The outlying point corresponds to the system at  9,500 km s in HS16033820 (E1 E2). Note that the horizontal axis is logarithmic.
Figure 27: Change in the fractional equivalent width () as a function of rest-frame time interval. The vertical axis is the observed EW (with 1 errors) normalized to the average EW in the observed-frame. The solid curve denotes an upper envelope of the distribution and is included only as a guide to the eye. The symbols have the same meaning as in Figure 26 and the horizontal axis is logarithmic.
Figure 28: Fractional variations of the observed EWs of the SIV and NV mini-BALs plotted against the corresponding fractional variations of the CIV mini-BALs in the same systems. The EW error bars are not plotted as they are smaller than the size of the symbols (see Figure 26). In all but one case different lines from the same system vary together, in the same direction. The mini-BAL of Q1157+014, marked by the red star closest to the origin, appears to be an exception (the EW of NV appears to decrease while the other two lines appear to get stronger) but the statistical significance is very low. We also plot a straight solid line with unit slope for reference.
Figure 29: Absorption profiles of the kinematic narrow components at   1,900 km s of the mini-BAL of UM675. The red curve corresponds to Epoch 1 and the blue curve to Epoch 2. The latter is shifted downwards by 0.1 for clarity. The broader, mini-BAL trough is removed by fitting it high order spline function. Both members of the CV1548,1551 doublet are shown.
Figure 30: Possible geometry of the locations of NAL, mini-BAL, and BAL absorbers in the accretion-disk wind scenario (see text in detail).
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