Results of Monitoring the Dramatically Variable C IV Mini-BAL System in the Quasar HS 1603+3820

1.4 MB·English

by  Toru Misawa

February5,2008
PreprinttypesetusingLATEXstyleemulateapjv.11/12/01
RESULTS OF MONITORING THE DRAMATICALLY VARIABLE C IV MINI-BAL SYSTEM IN
THE QUASAR HS 1603+38201
Toru Misawa2, Michael Eracleous2,3,4, Jane C. Charlton2, and Nobunari Kashikawa5,6
misawa,mce,[email protected],[email protected]
February 5, 2008
ABSTRACT
We presentsix new andtwopreviouslypublishedhigh-resolutionspectraofthe quasarHS 1603+3820
(z =2.542) taken over an interval of 4.2 years (1.2 years in the quasar rest frame). The observations
em
were made with the High-Dispersion Spectrograph on the Subaru telescope and Medium-Resolution
7 Spectrograph on the Hobby-Eberly Telescope. The purpose was to study the narrow absorption lines
0
(NALs). We use time variability as well as coverage fraction analysis to separate intrinsic absorption
0
lines, which are physically related to the quasar, from intervening absorption lines. By fitting models
2
to the line profiles, we derive the parameters of the respective absorbers as a function of time. Only

n the mini-BAL system at z ∼ 2.43 (v ∼ 9,500 km s−1) shows both partial coverage and time
abs shift
a variability, although two NAL systems possibly show evidence of partial coverage. We find that all the
J
troughs ofthe mini-BAL systemvaryin concertandits totalequivalentwidth variationsresemble those
3
ofthecoveragefraction. However,noothercorrelationsareseenbetweenthevariationsofdifferentmodel
2
parameters. Thus, the observed variations cannot be reproduced by a simple change of ionization state

norbymotionofahomogeneousparcelofgasacrossthe cylinderofsight. We proposethattheobserved
1
variationsarearesultofrapidcontinuumfluctuations,coupledwithcoveragefractionfluctuationscaused
v
1 by a clumpy screen of variable optical depth located between the continuum source and the mini-BAL
6 gas. Analternativeexplanationis thatthe observedpartialcoveragesignatureis the resultofscattering
6 of continuum photons around the absorber, thus the equivalent width of the mini-BAL can vary as the
1 intensity of the scattered continuum changes.
0
Subject headings: quasars: absorption lines – quasars: individual (HS 1603+3820)
7
0
/ 1. introduction 500km s−1)presentapowerfulwaytodeterminephysical
h
parameters of the accretion disk winds. Unlike the broad
-p theQguaasseaorusshpavheasbeeseonfuasevdaraiestbyaocfkgorbojuecntdsstohuartcaersetolosctautdedy absorption lines (BALs; FWHM ≥ 2,000 km s−1; Wey-
o mann et al. 1991) that are also associated with quasars
r along our sight-lines to them. These objects include not
outflows, NALs do not suffer from self-blending or from
t only intervening absorbers such as intervening galaxies,
s saturation, so that line parameters are more easily evalu-
a the intergalacticmedium (IGM), clouds in the halo of the
ated. BALsarethoughtto be associatedwithradiatively-
: Milky Way, and the host galaxies of the quasars them-
v driven outflows from accretion disks (e.g., Weymann et
selves, but also intrinsic absorbers that are physically as-
Xi sociated with the quasarcentralengines. One of the most al. 1991; Becker et al. 1997), while it is difficult to sep-
arate intrinsic NALs from intervening NALs. Mini-broad
r promising candidates for the intrinsic absorbers are out-
a absorptionlines(mini-BALs)areanintermediatesubclass
flowing winds from the quasars that could be accelerated
between NALs and BALs, which are typically wider than
by radiation pressure from the accretion disk (Murray et
thoseofNALs,butnarrowerthanBALs. Mini-BALshave
al. 1995; Arav et al. 1995; Proga et al. 2000) or by
the advantages of both BALs (i.e., high probability of be-
magnetocentrifugalforces(e.g.,Everett2005). Outflowing
ing intrinsic lines) and NALs (i.e., line profiles can be
windsareimportantcomponentsofquasarcentralengines
resolved into individual components), which make them
because they carry awayangular momentum from the ac-
useful targets (e.g., Churchill et al. 1999; Hamann et al.
cretion disk and allow the remaining gas in the disk to
1997a;Narayananetal. 2004). BALscouldprobethelow-
accrete onto the central black hole. Quasar outflows are
latitude, dense, fast portion of the wind, while the mini-
also important for cosmology since they deliver energy,
BALs and NALs may probe the lower-density portion of
momentum, and metals to the interstellar and intergalac-
thewindathighlatitudesabovethedisk. Thus,thestudy
tic media, thus significantly affecting star formation and
of mini-BALs and NALs complements the study of BALs
galaxy assembly (e.g., Granato et al. 2004; Scannapieco
becausethecorrespondingabsorbersresideindifferentre-
& Oh 2004; Springel, Di Matteo & Hernquist 2005).
gions of the outflow and allow us to sample different sets
Intrinsic narrow absorption lines (NALs; FWHM ≤
of physical conditions.
1 Basedondatacollected atSubaruTelescope,whichisoperated bytheNationalAstronomicalObservatoryofJapan.
2 DepartmentofAstronomy&Astrophysics,ThePennsylvaniaStateUniversity,UniversityPark,PA16802
3 DepartmentofPhysics&Astronomy,NorthwesternUniversity,2131TechDrive,Evanston,IL60208
4 CenterforGravitational WavePhysics,ThePennsylvaniaStateUniversity
5 NationalAstronomicalObservatory,2-21-1Osawa,Mitaka,Tokyo181-8588, Japan
6 DepartmentofAstronomicalScience,GraduateUniversityforAdvancedStudies,2-21-1Osawa,Mitaka,Tokyo181-8588, Japan
1
2 Misawa et al.
ToisolateintrinsicNALs,twotestsarecommonlyused: In the latter case, the crossing velocity is constrained to
(i) partial coverage, i.e., trough dilution by un-occulted be v ≥8,000 km s−1, and the distance from the con-
cross
light (e.g., Hamann et al. 1997b; Barlow & Sargent 1997; tinuum source, r ≤ 0.2 pc. This is larger than the size of
Gangulyetal. 1999),and(ii)variabilityoftheabsorption the continuum source, R ∼ 0.02 pc, but smaller than
cont
profiles within a few years in the quasar rest frames (e.g., that of the BLR, R ∼3 pc, estimated for this quasar.
BLR
Hamann et al. 1997a; Narayanan et al. 2004; Wise et On the other hand, the other C IV NALs found towards
al. 2004). These effects occur for intrinsic absorbers that the quasar show no sign of being intrinsic. Further mon-
are verycompactand verydense comparedto intervening itoring observations make it possible to (i) discriminate
absorbers. The fractionofquasarsthathostintrinsicCIV between changesin the ionizationstate and motionof the
associatedNALs(NALswithin5,000kms−1 ofthequasar absorbersacrossthe continuum source as the cause of the
emission redshifts) has been estimated to be ∼25–27%at time variability seen in the C IV mini-BAL, and (ii) to
z <2, using the time variability technique (Narayanan et confirm whether the other CIV NALs really show neither
al. 2004;Wiseetal. 2004),and∼23%atz ∼2.5,basedon partial coverage nor variability.
partial coverage analysis (Misawa et al. 2007). These sig- In this paper, we present the results of monitoring
natures are a sufficient, but not a necessary, condition to HS 1603+3820 over 4.2 years (1.2 years in the quasar
demonstratethe intrinsicnature ofanabsorber. Itis very rest frame) based on eight spectra. Six of these spectra
likely that some NALs without time variability or partial were taken with the High Dispersion Spectrograph(HDS;
coverage are also intrinsic. Noguchi et al. 2002) on the Subaru telescope, and two
Both of the above methods have been applied to BAL withtheMedium-ResolutionSpectrograph(MRS;Horner,
and mini-BAL systems. For example, partial coverage Engel, & Ramsey, 1998) on the Hobby-Eberly Telescope
analysis has been carried out by Petitjean & Srianand (HET). The first two spectra in this time series were pre-
(1999), Srianand & Petitjean (2000,) Yuan et al. (2002), sented and discussed by Misawa et al (2003, 2005). With
andGangulyetal. (2003),whiletime-variabilityhasbeen the new spectra we are able to sample the variations of
studied by Foltz et al. (1987), Turnshek et al. (1988), the absorption lines more densely and probe the internal
Smith & Penston (1988), Barlow et al. (1992), Barlow structure of the absorber.
(1993),Vilkoviskij& Irwin(2001),Ma (2002),Narayanan In §2, we describe the observations and data reduction.
et al. (2004), and Lundgren et al. (2007). It is quite Themethodsusedformodelfittingareoutlinedin§3. The
difficult (oralmostimpossible)to deblendkinematic com- propertiesofthe mini-BALandofthe otherNAL systems
ponents in those heavily blending absorption features us- are examined in §4. The possible origins of the time vari-
ing low/intermediate resolution spectra. However, some ability seen in the mini-BAL system are discussed in §5,
authors successfully deblended narrower absorption com- and a summary is given in §6. We adopt z = 2.542 as
em
ponents from each other in such systems using high res- the systemic redshift of the quasar, which was estimated
olution spectra of R > 40,000 (e.g., Yuan et al. 2002; fromits narrowemissionlines (Misawaetal. 2003). Time
Gangulyetal. 2003). Systematicmonitoringofindividual intervalsbetweenobservationsaregiveninthequasarrest
mini-BALs using high-resolution spectra taken on several frame throughout the paper, unless otherwise noted. We
epochs canpotentially provideveryimportantconstraints useacosmologywithH =75kms−1Mpc−1,Ω =0.3,and
0 m
onthepropertiesoftheabsorbers. However,nosuchcam- Ω =0.7.
Λ
paign has been attempted so far, to our knowledge.
The opticallybrightquasarHS1603+3820(z =2.542,
em 2. observations and data reduction
B=15.9), first discovered in the Hamburg/CfA Bright
QuasarSurvey(Hagenetal. 1995;Dobrzyckietal. 1996), We observed HS 1603+3820 eight times over a period
is known to have a large number (13) of CIV doublets at of 4.2 years in the observed frame, from March 2002 to
1.965< z < 2.554 (Dobrzycki, Engels, & Hagen 1999). May 2006. We will call the dates of these observations
abs
Using high-resolution spectra (R = 45,000) taken with epoch 1 through epoch 8. Six of the observations were
the Subaru telescope, Misawa et al. (2003, 2005) clas- obtainedwith Subaru+HDS, using a 0′.′8 or 1′.′0 slit width
sified all C IV doublets into 8 C IV systems, and found (R = 45,000 or 36,000), and adopting 2×1 pixel binning
that only a mini-BAL system at z ∼ 2.43 (shift veloc- along the slit. The red grating, with a central wavelength
abs
ity7, vshift = 8,300–10,600 km s−1) shows both partial of 4900 ˚A, was used to cover as many C IV systems as
coverage and time variability in an interval of 0.36 years possible,exceptforthefirstandthelastobservationsthat
in the quasar rest frame. Based on these results, Mis- usedcentralwavelengthsof6450˚Aand5700˚A.Theother
awa et al. (2005) placed constraints (i) on the electron two spectra were taken with the HET+MRS. The MRS
density (n ≥ 3.2 × 104 cm−3) and the absorber’s dis- features a pair of fibers: one for the target and another
e
tance from the quasar (r ≤ 6 kpc), if a change in the for the sky, which enables us to perform sky subtraction
ionization state causes the variability, or (ii) on the time effectively. We used a 1′.′5 fiber to get R=9,200 spectra.
scale for the absorber to cross the continuum source and A setup with a central wavelength of 7,000 ˚A is required
the distance from the continuum source, if gas motion for the queue observations with MRS+HET.
across the background UV source causes the variability. We reduced both the Subaru and HET data in a stan-
dard manner with the IRAF software8. We used a Th-Ar
7 Theshiftvelocityisdefined aspositiveforabsorptionlinesthatareblueshiftedfromthequasar.
8 IRAFisdistributedbytheNationalOpticalAstronomyObservatories,whichareoperatedbytheAssociationofUniversitiesforResearchin
Astronomy,Inc.,undercooperativeagreementwiththeNationalScienceFoundation.
9 The broad emission lines probably do not vary between observations, because bright quasars like HS 1603+3820 do not show significant
variabilityinmagnitudeonatime-scaleasshortas∼1year(e.g,Giveonetal. 1999, Hawkins2001,Kaspietal. 2007).
Dramatically Variable Mini-BAL in HS 1603+3820 3
spectrumforwavelengthcalibration. Wedirectlyfittedthe at z = 2.5114. This system was probably a false de-
abs
continuum, which also includes substantial contributions tection(Dobrzycki2005;privatecommunication). Among
frombroademissionlines9,withathird-ordercubicspline the 9 CIV systems, only the system at zabs =2.42–2.45is
function. Aroundheavilyabsorbedregions,inwhichdirect classified as a mini-BAL system, because its line width is
continuum fitting is difficult, we used different techniques larger than 500 km s−1.
for the Subaruand HET spectra. For the Subaru spectra, WecarriedoutmodelfitstoallCIVsystemsinthesame
we adopted the interpolation technique introduced in Mi- mannerasinourpreviouspaper(Misawaetal. 2005),ex-
sawa et al. (2003). Since the echelle blaze profile in HDS cept for the mini-BAL. Misawa et al. (2005) performed
shows time variability during a night, we cannot directly a manual Voigt profile fit to the mini-BAL, because self-
usetheinstrumentalblazefunctionasacontinuumprofile. blending (blending of the blue and red members of the
In this scheme, we obtain the continuum shape by inter- doublet) occurs in this system. In its original form, the
polating between the two echelle orders adjacent to the minfit code was not able to handle such self-blended re-
orderofinterest10,afterweightingflux levelsbasedonthe gions. The code was originally written for narrowabsorp-
flat-frame spectrum [see eqn. (1) of Misawa et al. 2003]. tion lines, thus it fitted the models to the blue and red
We then divide the quasar spectrum by this interpolated members of doublets separately. This procedure overpro-
continuum shape to obtain the normalized spectrum. We duced the residual intensity at the self-blending regions
have verified the validity of this technique by applying it because it added contributions from two components. To
to a stellar spectrum; the continuum error is always less avoidthis problem,werevisedthecodesoastofitmodels
than 3.3%. For HET spectra, we used a blaze profile con- to the blue and red members of doublets simultaneously,
structedfromtheflatfieldspectrumasacontinuummodel i.e.,bymultiplyingthecontributionsfromthetwodoublet
after multiplying it by a scaling factor to adjust its count members. In this paper, we fit the observed mini-BALs
level to that of the quasar spectrum (the scaling factor is automatically using this improved minfit.
a ratio of counts in the quasar spectrum to those in a flat As in Misawa et al. (2005), we fitted only the first
spectrum for the same unabsorbed region). (bluest) two C IV kinematic components in the system,
An observation log is given in Table 1, in which we list atλ =5284–5318˚A, because the other componentsare
obs
the observation epoch, date of observation, relative time soheavilyblendedwitheachotherthatwecannotseparate
in the quasar rest frame, instrument, wavelength cover- them, even with the improved minfit (i.e., minfit gives
age, total exposure time, spectral resolving power, and multiplesolutions). Atfirst,weremovethecontamination
signal-to-noise ratio (S/N) per pixel (after rebinning to from Si II λ1527 in System B, at λ = 5303–5318 ˚A. The
pixel scales of 0.03 ˚A and 0.15 ˚A for Subaru and HET SiII λ1527line in the systemdid not show any detectable
data, respectively). The S/N is evaluated around 5370 ˚A, variability between the first two observations. The C IV
close to the C IV mini-BAL. In Figure 1, we show a nor- doublets for System B show neither partial coverage nor
malized spectrum, after combining 6 spectra taken with time variability, and are consistent with it being an inter-
Subaru+HDS in the region redward of the Lyα emission vening system. Therefore, the Si II λ1527 line is also not
line of the quasar (λ>4300 ˚A). likely to vary. Thus, we removed the contamination from
3. fitting procedure Si II λ1527 by dividing the observed spectra by a model
profilesynthesizedwiththe line parametersofSiIIλ1527,
We used the line-fitting software package minfit determined from an average of the first two observations
(Churchill 1997; Churchill et al. 2003), with which we presented in Misawa et al. (2005).
can fit absorption profiles with four free parameters: red- After removing the narrow Si II λ1527 components, we
shift (z), column density (log N), Doppler parameter (b), applied the improved minfit to the spectrum using two
and coverage fraction (Cf). Here, the coverage fraction is C IV components, one narrow and one broad, as in Mis-
the fraction of the background continuum source and the awa et al. (2005). During the fitting process, we initially
broad-emission line region that is covered by the intrin- forcedthesamecoveragefractionforthenarrowandbroad
sic absorber. The coverage fraction can be systematically components, as in Misawa et al. (2005). In that paper we
evaluated in an unbiased manner by considering the op- showed that, during the first two epochs of observation,
tical depth ratio of resonant, rest-frame UV doublets of thissolutionproducedanequallygoodfitassolutionsthat
Lithium-like species (e.g., CIV, NV, and SiIV), namely, allowedthese coveragefractions to differ. We also consid-
(R −1)2 ered the possibility of differing coverage fractions for the
r
Cf = 1+R −2R , (1) narrow and broad components in later epochs of observa-
b r tion.
where R and R are the residual fluxes of the blue and
b r Figure 2 shows 6 spectra taken with Subaru+HDS, for
red members of the doublets in the continuum normal-
which we find good model fits. The fits themselves are
ized spectrum (Hamann et al. 1997b; Barlow & Sargent
shown in Figure 3. We cannot fit spectra obtained with
1997; Crenshaw et al. 1999). A C value less than unity
f HET+MRS in the same manner, because of their lower
signifies that a portion of the background source is not
resolutionandS/N.Therefore,ontheHET+MRSspectra,
occulted by the absorber. This, in turn, means the dou-
weoverplottedmodelprofilessynthesizedwithparameters
blet is probably produced by an intrinsic absorber (e.g.,
that are linearly interpolated between the two adjacent
Wampler et al. 1995; Barlow & Sargent 1997). We found
epochs observed with Subaru. These models provide rea-
9 C IV systems at zabs = 1.88–2.55, as identified in Do- sonably good fits to the HET+MRS profiles. The fitting
brzyckiet al. (1999),but we did not detect a CIV system
10Iftheorderofinterestism,weuseordersm−1andm+1asreferenceorders. However,inourdatathoseadjacentordersarealsosomewhat
affected bybroadabsorptionfeatures. Therefore,weusedordersm−3andm+3asreferenceorders.
4 Misawa et al.
parameters for the narrow and broad C IV components, identified because of heavy contamination from the Lyα
at different epochs, are summarized in Table 2. Columns forest. SiIV was not detected, even in our final, high-S/N
(1)–(3) give the epoch number, observation date and the spectrum. Figure 5 presents graphically the evolution of
relative time in the quasar rest frame. Columns (4)–(8) the fit parameters of the C IV mini-BAL, including (from
list,forthenarrowcomponent,theshiftvelocity,CIVcol- top to bottom) the total rest frame equivalent width at
umn density, C IV Doppler parameter, coverage fraction, λ = 5288–5346 ˚A, the C IV column density, the Doppler
and observed-frame equivalent width evaluated from the parameter, the coverage fraction, and the shift velocity.
syntheticmodelspectrum. Columns(9)–(13)arethesame We discuss each in detail below.
ascolumns(4)–(8),butforthebroadcomponent. Wecon- Thetotalrestframeequivalentwidthincreasedbyabout
firmed that the fit parameters for the first two epochs are afactoroftwooverarestframetimeintervalof0.36years,
consistentwithin1σerrorswiththosepresentedinMisawa and then decreased over an interval of 0.6 years. During
et al. (2005), based on a manual fitting procedure. this entire period, the general appearance of the line pro-
We also applied minfit to all metal lines in the other file did not change significantly. Based on the results of
NAL systems. Since, as described later, we did not see just the first two epochs, Misawa et al. (2005) specu-
anyremarkablevariabilityintheseNALsystems,wecom- lated that this mini-BAL may soon evolve into a BAL,
bined all 6 Subaru spectra to produce a final spectrum with more than 10% of the flux absorbed over at least a
with higher S/N (∼96 per pixel) before applying minfit 2,000kms−1range. Thisideaisnotconfirmedbythesub-
to the NALs11. The best fit parameters are summarized sequentepochs,since the mini-BAL was foundto weaken.
inTable 3. Column(1)isanidentificationnumber forthe We also monitored the equivalent widths of the narrow
line. Columns (2) and (3) give the observed wavelength and broad components, separately. At first, both compo-
and redshift. Column (4) gives the shift velocity from the nents have almost the same equivalent width. But after
quasar systemic redshift. Columns (5) and (6) give the epoch3,theequivalentwidthofthebroadcomponentwas
column density and the Doppler parameter with 1σ er- much higher than that of narrow component.
rors. Column(7)reportsthe coveragefractionwith its 1σ The variability of the column density in the mini-BAL
error(whichincludes the errordue tocontinuumlevelun- is complex. This is especially true for the broad com-
certainty as described below). During a fitting trial, min- ponent, which showed a large increase in column density
fit sometimes gives unphysical coveragefractions such as within 0.13 years, from epoch 3 to epoch 7, as shown in
Cf < 0 or Cf ≥ 1. When this happens, the other fit pa- Figure 5. This increase in column density was concurrent
rameterssuchasthecolumndensityandDopplerparame- with the increase of equivalent width, as shownin the top
ter havenophysicalmeaning. These unphysicalfitresults window of the figure. One possible explanation for this
could be causedby errorsin the continuumfit. Misawaet dramaticincreaseisachangeofthe ionizationstateofthe
al. (2005)foundthatthederivedcoveragefractionscanbe gas (Hamann et al. 1997b; Narayananet al. 2004),which
significantly affected by continuum level errors, especially we discuss further in §5.
for very weak components whose Cf values are close to 1. An interesting trend is seen in Doppler parameter vari-
Therefore, if minfit gives unphysical Cf values for some ability. Fromepoch3to epoch5 (anintervalof0.09years
components, we carry out the fit again for the entire line, in the quasar rest frame), the Doppler parameter of the
assuming Cf =1 for those components, following Misawa broad component increased suddenly from ∼ 350 km s−1
et al. (2007). In this case, we do not evaluate the error to ∼ 860 km s−1, and then it decreased gradually. This
in Cf. The best fit models are overlaidwith the observed abrupt jump is correlated with the sudden increase of
spectra in Figure 4(a–i). the column density in the broad component, as described
Therearealsoothererrorsourcesforthefitparameters, above. Onthe other hand, the narrowcomponentdid not
such as blending with other lines, and convolution of the show a significant change in Doppler parameter.
true spectrum with the instrumental line spread function The coverage fraction (assuming it is the same for the
(LSF). Both of these are negligible if (1) the doublet is narrowandbroadcomponents)showsavariabilitysimilar
strong enough, i.e., log (N/cm−2) ≥ 14.0 in the case of to that of the total equivalent width. After increasing to
C IV doublets, (2) the normalized residual flux is smaller 0.45,thecoveragefractiondecreasedto0.24. Inspectionof
than ∼0.5,and (3) the line profile is much wider than the the separate contributions of the broad and narrow com-
instrumentalLSF(Misawaetal. 2005). Mostcomponents ponents to the total equivalent width shows that the cov-
in our sample are broad enough compared with the LSF erage fraction evolves similarly to the equivalent width of
(correspondingtob∼4km s−1),andnotseverelyblended the narrow component. If the change of coverage fraction
with each other, as described in section 4.2. is caused by the absorber’s motion across the background
flux source, the absorber size is constrained to be smaller
4. variability of fit parameters
thanthebackgroundsource(abouthalfofthebackground
4.1. The Mini-BAL System sourcebecauseofthe maximumC valueof∼0.5). Atthe
f
same time, the column density increased, suggesting an
We previously reported that the CIV mini-BAL of Sys-
inhomogeneous absorber. A more detailed discussion will
tem A shows trough dilution (Misawa et al. 2002), and
be presented in §5.
variability within 0.36 years (Misawa et al. 2005). With
The column density and equivalent width of the broad
theadditionalspectraofthispaper,wefollowthevariabil-
component increased over the same period (epoch 3 to
ity of this system for 1.2 years in the quasar rest frame,
epoch8)asitscoveragefractiondecreased. However,since
sampling it at eight epochs. N V and Lyα could not be
11 For system G, we combined only spectra in epochs 3, 7, and 8, because the other spectra are affected by a data defect near the blue/red
CCDgap.
Dramatically Variable Mini-BAL in HS 1603+3820 5
we assumed that the broad and narrow components had fractions of two components, 11 and 14, deviate
the same coverage fractions, we must assess whether this from unity by more than 1σ, but both are heav-
assumption could lead to the observed trends. Specifi- ily blended with other components and their C
f
cally, we consider the effect ofassuming a constantC for values are not very reliable. All components in the
f
the broad component (adopting Cf = 0.36 for epoch 3 N V NAL are consistent with full coverage. Only
through epoch 8, and allowing the narrow component Cf the epoch 8 spectrum covered the Si IV doublet of
to change as before). This model, which also producedan this system. For the Si IV doublet, only compo-
adequatefittothedata,isillustratedwiththedottedlines nent 7 suggests partial coverage, but the model fit
in the 2nd, 3rd, and 4th panels from the top of Figure 5. isnotgoodaroundthe leftwingofthis component.
For this case, the column density of the broadcomponent We concludethat this systemdoes notshowstrong
doesnotincreaseasrapidly,butitstilldoesincrease. The evidence for partial coverage, and is probably an
same applies for the equivalent width. Thus our conclu- interveningsystem, most likely a galaxybecause of
sions regarding the evolution of the column density and the strength of the low-ionization absorption lines.
equivalent width are not qualitatively changed by our as-
sumption about the C value of the broad component. System C —TheCIVNALinthissystemwithasimple
f
absorptionprofile(seeFigure4c)isfittedwithonly
We have looked for correlated changes in all of the
one component, as in Misawa et al. (2005). Both
aboveparametersofthemini-BALprofilebyplottingthem
fits are consistent with full coverage. We also did
against each other. In Figures 6 and 7, respectively, we
not find any evidence for time variability. Except
show the relevant plots for the narrow and broad com-
for its small shift velocity, ∼ 430 km s−1, there is
ponents of the mini-BAL. Other than a tentative trend
no evidence to suggestthat this system is intrinsic.
between C and b in Figure 6 and between C and N
f f CIV
in Figure 7, we see no convincing correlation. This lack
System D — Misawa et al. (2005) fitted this system’s
of correlated variability between model parameters is an
C IV and Si IV profiles with 4 and 2 components,
important clue, which we take into consideration in our
respectively, and did not find any partial coverage.
later discussion.
However, we found an additional C IV component
The observed shift velocity represents the ejection ve- at v = −160 km s−1 from the system center
locity of the absorber from the central engine projected shift
that shows partial coverage, and we have no rea-
alongthe lineofsight. While thelinecenterofthenarrow
son to suspect systematic error. The line profiles
componentremainsnearlyconstant,thebroadcomponent
shifts by about 300 km s−1 (v ∼ 10,100 km s−1 → are shown in Figure 4d. This component was not
shift studied by Misawa et al. (2005) because of a data
10,400 km s−1) from epoch 1 to epoch 7. If this veloc- defect. We also now find that component 1 in the
ity shift is really due to the radiative pressure from the SiIVNALpossiblyshowspartialcoverage,although
continuum source, the acceleration of the gas can be cal- it was consistent with full coverage in the shorter
culated to be ∼ 0.01 m s−2. However, the measurement exposure presented by Misawa et al. (2005). We
of the center of the broad component is highly uncertain, also note that it is surprising that the C IV λ1548
because the observed velocity shift is much smaller than is black at the position of this Si IV component if
the total line width because of self-blending of the blue the partial coverageis true. However,the fact that
and red members of the CIV doublet. Moreover,the cen- partial coverage is apparent for both a Si IV and
terofthebroadcomponentmovedinbothdirections(i.e., a C IV component suggests that this system may
bluewardandredward)duringourobservations. Although be an intrinsic system. The fact that this system
it is still not clear if there is a truly systematic increase is redshifted from the quasar systemic redshift also
in this parameter, we could regard the above value as an supports this idea. Although this system is located
upper limit on the acceleration of the absorber. near the top of the C IV emission line, there is no
residualflux at the position of the CIV doublet for
4.2. NAL Systems components4and5. Thesecomponentsmustthere-
Inadditiontothemini-BALsystem,wedetected8NAL fore cover both the continuum source and broad
systems in the HS 1603+3820 spectra. For these NALs, emission line region. We do not see any variability
we do not detect significant variability. Model fit results over the course of our observations.
to the combined spectrum of the 6 Subaru observations
System E — Misawa et al. (2005) fitted the C IV NAL
(with a total exposure time of 8.9 hours and a S/N per
withonlytwocomponents,andoneofthemshowed
0.03 ˚A pixel of about 91 at λ ∼ 5370 ˚A) are consistent
evidence of partial coverage. However, our higher-
with the results based on only the epoch 2 spectrum (a
S/Nspectrum(showninFigure4e)requires5com-
1.7 hour exposure with S/N ∼ 49 per pixel; Misawa et
ponents to fit the C IV NAL, and three of them
al. 2005), with only a few small exceptions as described
are consistent with full coverage. Although com-
below. Inthissectionwediscussindividualsystems,other
ponents 3 and 5 deviate from full coverage, they
than system A (the mini-BAL). The profiles of the lines
are heavily blended with the other components, so
of these systems are shown on a common velocity scale in
this result is quite uncertain. The shift velocity of
Figure 4(a–i). thesystemisalsoextremelylarge,∼60,000kms−1.
There is no compelling reason to believe that this
System B — We fit this strong C IV NAL, shown in system is intrinsic to the quasar.
Figure 4a,b,with15components,althoughMisawa
et al. (2005) used 19 components. The coverage System F —Ourbest-fitting modelis almostconsistent
6 Misawa et al.
with the previous one, having the same number of appear to be driven largely by changes in the coverage
components at almost the same positions (see Fig- fraction,as shownin Figure 5. Other physicalparameters
ure 4f). All components are consistent with full (N andb)arevaryingaswell,buttheredoesnotseem
CIV
coverage except for component 1 that is blended to be an obvious correlation between changes of different
with the strongcomponent2. This systemalso has profile parameters, at least in the bluest troughs of the
a very large shift velocity, ∼ 50,000 km s−1. We mini-BAL (see Figures 6 and 7).
consider it probable that this is an intervening sys-
5.1. Transverse Motion of a Homogeneous Absorber
tem.
In Misawa et al. (2005) we favored an interpretation in
System G — Because the C IV NAL in this system was which motion of the (homogeneous) absorber across the
positionedatthe edge ofthe blue CCDof the HDS
line (or cylinder) of sight causes the observed variability
(around which bad pixels are common) in some of
(see also Hamann et al. 1997a). The assumed scenario is
our observing setups, the absorption profiles are
depicted in the cartoon of Figure 2a of Hamann & Sabra
slightlyorseverelydifferentinsomespectra. There-
(2004). With time, an increasing fraction of the cylinder
fore, we combined only spectra from epochs 3, 7,
of sight was covered by the absorber, leading to changes
and 8 (these are not affected by the data defect)
inthe coveragefraction. Inthis contextandbasedontwo
to produce a final spectrum for this system (Fig-
snapshots of the spectrum, we derived a transverse veloc-
ure 4g). We fit the C IV NAL with 6 components, ity of 8,000 km s−1 and a constraint of r < 0.2 pc on the
and all of them are consistent with full coverage.
distance of the absorber from the continuum source (see
Component5wasnotfittedbyMisawaetal. (2005)
details in Misawa et al. 2005).
because of the unphysicalline ratio of the blue and
That model was motivated largely by the observation
red component, due to line blending. This system
that the coverage fraction was the only quantity that
is probably an intervening system.
changed substantially between the first two observations.
The subsequent observationsshowthat scenarioto be too
System H — Two components are necessary to fit this
simple,namelythecolumndensityoftheabsorberchanges
C IV NAL (see Figure 4h). The shallower absorp-
with time as well, suggesting that the absorber is inho-
tion feature, centered on component 2, has an un-
mogeneous and its internal structure plays an important
physical ratio of its blue and red members. Proba-
role in the variability of the mini-BAL profile. Thus, a
blythebluememberiscontaminatedbyotherlines.
more plausible scenario is the one depicted in Figure 2b
Only component 1 is useful, and it yields C = 1.
f
of Hamann & Sabra (2004) and in Figure 5 of de Kool,
In this system, we againdo not see any evidence of
Korista, & Arav (2002), where the absorbing medium is
intrinsic properties.
inhomogeneous, with clumps spanning a range of column
System I — This system was detected only in the densities scattered over the cylinder of sight.
epoch 8 spectrum, because it was not covered in There is a remaining difficulty with this scenario, how-
the other epochs. We used 7 components to fit the ever. Thefactthatallthetroughsinthemini-BALvaryin
CIV NAL (see Figure 4i). Component 4 may show concert would require a coincidence between the motions
partialcoverage,butits C valuediffers fromunity of the individual clumps. More specifically, the projected
f
by only slightly more than 1σ. The case for partial areas of all the clumps within the cylinder of sight should
coverage for component 6 is somewhat more com- vary in the same way (i.e., rise and fall with time in the
pelling, since the minfit fitting procedure yielded same manner for all clumps). We consider such a coordi-
a value C = 0.87± 0.02. Component 4 is not nated sequence of variations rather unlikely, therefore, we
f
blended with other components, and component 6 regard this interpretation as implausible. We note that if
is only weakly blended. Although the S/N for this future observationsshowthis patterntobe common,then
spectrum is not very high in the relevant region, this interpretation can be conclusively rejected.
this system may be another candidate for an in-
5.2. Internal Motions in the Absorbing Gas
trinsic system.
At first glance, compression of the absorber might be
5. discussion: the origin of time variability abletoproduceatrendsimilartothatofFigure7b,where
the coverage fraction decreases as the column density in-
In view of the observational constraints derived above,
creases in one of the kinematic components of the mini-
we evaluate here different scenarios for the origin of the
BAL.However,oncloserexamination,suchacompression
variability of the CIV mini-BAL. A fundamental assump-
would require highly supersonic motions within the ab-
tion underlying our discussion is that the kinematic com-
sorber. Assuming the background continuum source size
ponents making up system A represent parcels of absorb-
is 0.02 pc (this is the UV-emitting region of the accretion
ing gas at different positions along the line of sight. In
disk; see Misawa et al. 2005), a change in the coverage
effect, we assume that the absorberis embedded in an ac-
fractionfrom 0.36to 0.27 within 0.13 years(epochs 5 and
celerating outflow from the quasar,thus the blueshift of a
7) implies a speed of order 104 km s−1. In comparison,
kinematic component increases with its distance from the
quasar continuum source. the speed of sound in a ∼ 104 K gas is only 10 km s−1.
A very important observationalclue from the data pre- Thus we consider such a scenario untenable.
sentedaboveisthefactthatallthekinematiccomponents
5.3. Scattering of Continuum Photons Around the
ofthemini-BALsystemvaryinconcert(i.e.,thedepthsof
Absorber
all troughs increase or decrease together). These changes
Dramatically Variable Mini-BAL in HS 1603+3820 7
The observed partial coverage signature can, in prin- tive recombinationcoefficients of5.3×10−12 cm3 s−1 and
ciple, be explained if continuum photons are re-directed 2.8×10−12 cm3 s−1 (from Arnaud & Rothenflugh 1986;
by a scattering medium towards the observer, when they assuming a nominal gas temperature of 20,000 K, follow-
were initially traveling in a different direction. Thus, the ingHamann1997). Thusweobtainalimitontheelectron
absorber can be homogeneous and its projected size need density of n > 1×105 cm−3, using the methodology of
e
not be smaller than the cylinder of sight towards the con- Narayanan et al. (2004) and a bolometric luminosity of
tinuum source. The dilution of the troughs by photons L =2.5×1048 erg s−1 (Misawaetal. 2005). Toconvert
bol
scattered around the absorber can change with time as thislimittoalimitonthedistanceoftheabsorberfromthe
the absorber moves relative to the scattering medium or continuum source, r, we must assume a value of the ion-
as the conditions in the scattering medium change. In izationparameter. IftheCIVcolumndensityfluctuations
the context of this scenario, the apparent coverage frac- reflectconversionsofCIII↔CIVinalower-ionizationab-
tion can vary according to the behavior of the scattering sorber,thenthecalculationsofHamann(1997)indicatean
medium. Moreover,the variationsofthecoveragefraction ionization parameter of U ∼0.002 and lead to r <8 kpc.
need not be tied in any way to variations of other param- If,ontheotherhand,theCIVcolumndensityfluctuations
etersofthe absorber,suchasthe CIVcolumndensity,the reflectconversionsofCV↔CIVinahigher-ionizationab-
b-parameter,or the shift velocity. This idea can be tested sorber,thenthecalculationsofHamann(1997)indicatean
observationallythroughspectropolarimetry. Inparticular, ionization parameter of U ∼ 0.06 and lead to r < 1 kpc.
we wouldexpect the fractionalpolarizationinthe absorp- Inthiscontextthesimultaneousvariabilityofallthemini-
tionmini-BALtroughstobehigherthaninthecontinuum BAL troughs implies that these troughs should represent
(see the analogous test in BAL quasars by Brotherton et gas parcels of similar densities for the following reason.
al. 1997,Ogleetal. 1999,andLamy&Hutsem´ekers2000, Since all absorbing gas parcels must be within the cylin-
forexample). Furthermore,wewouldalsoexpectthisfrac- derofsighttothecontinuumsource,thetimelagbetween
tional polarizationto increaseas the coveragefractionde- changes in the continuum intensity at the source and our
creases. observationofachangeinthemini-BALprofileisthesum
of the light traveltimes fromthe continuum source to the
5.4. Change of Ionization State of the Absorbing Gas absorber and from the absorber to the observer, plus the
recombination time of the gas. The sum of light travel
Changes in the level of the ionizing continuum can also
times is the sameregardlessofthe locationofagasparcel
bring about absorption-line variability since they change along the cylinder of sight, therefore, a difference in time
the relativeionic populations in the absorber(in this par-
lagcanonlyresultfromdifferentrecombinationtimes,i.e.,
ticular case, the relative populations of C III, C IV and different gas densities. We can then conclude that the
C V). In principle, a monotonic change in the continuum density does not vary significantly across the line profile,
intensity can produce a non-monotonic change in the ab-
whichimpliesinturnthattheshapeofthemini-BALpro-
sorption line strength. For example, an absorber initially
file is determined primarily by the combinationof column
at a low ionization state that sees a rising ionizing flux
density (i.e., physical thickness) and coverage fraction as
will first respond with an increase in the strength of its
a function of velocity.
C IV absorption lines (as C III → C IV) and then with a Fluctuations in the ionization state of the absorber are
decrease (as C IV → C V). The variability time scale of anattractiveexplanationofthe observedvariationsinthe
the absorptionlinesprovidesanupper limitontheioniza-
strengthofthemini-BALinHS1603+3820. However,the
tion or recombinationtime of the absorbinggas,hence its
mini-BAL appears to vary much faster than any expected
electrondensity(see the detaileddiscussioninHamannet
variationsintheUVcontinuumofsuchaluminousquasar
al. 1997b and Narayanan et al. 2004). The density limit
(see for example, Giveon et al. 1999, Hawkins 2001, and
can be obtained from n ≥ (α t )−1, where α is a re-
e r var r Kaspietal. 2007). Moreover,Fleming&Kennefick(2006)
combinationcoefficientandt isavariabilitytime scale.
var foundno variabilityofthis particularquasarin 10daysin
Thisisasimplifiedpictureinwhichthegasisinionization the observed frame. 12 Therefore we propose that the
equilibrium and CIV is the dominant ionic species of C. variations of the ionizing continuum seen by the absorber
Since the C IV ionic column densities have fluctuated arecausedbyascreenofvariableopticaldepthbetweenit
up and down more than once over the course of our mon-
andthe (relatively steady)continuum source. This screen
itoring observations, the ionizing continuum seen by the
could be the ionized, clumpy, inner part of the outflow,
absorber must be fluctuating too. Moreover, the contin-
i.e., the shielding or hitchhiking gas invoked in the out-
uum intensitymustbe fluctuating byatleasta factorof3
flow models of Murrayet al. (1995),andarisingnaturally
onrest-frametimescalesof6monthsorlesstoproducethe
in the more detailed calculations of Proga et al. (2000).
observedchangesintheCIVioniccolumndensity(seethe Thetwo-dimensional,axisymmetricmodelsofProgaetal.
calculations of Hamann 1997). Taking this interpretation
(2000)also show that disk winds can be acceleratedup to
at face value (but see discussion below), we can use the 15,000 km s−1, and that they are unsteady and generate
shortest variability time scale that we have observed (16
dense knots periodically. The time variability seen in the
days in the quasar rest frame, epochs 5 → 7; see Fig. 5)
mini-BALsystemofHS1603+3820mayberelatedtothis
to constrain the density of the gas. We consider both an
mechanism. The conditions in the screen may be similar
increase and a decrease in the ionization state of the gas
tothoseof“warm”absorbers,observedinthe X-rayspec-
as a cause for the observed change in the C IV column tra of many Seyfert galaxies,where it often co-exists with
density (i.e., C III → C IV or C V → C IV), with respec-
12 Unfortunately, our spectra cannot provide information on variations of the continuum level because we cannot calibrate their flux scale.
Thisisaconsequence ofthefactthattheblazefunctionoftheHDSonSubaruchanges withtimeoverthecourseofanight.
8 Misawa et al.
a UV absorber (e.g., Crenshaw et al. 1999). We note, systemsin the quasarspectrum. The partialcover-
by way of example, that models for the warm absorbers age signature and time variability of the mini-BAL
in NGC 5548, IRAS 1339+2438, and NGC 3783 require show it to be intrinsic to the quasar. Two of the
multiple ionizationphasesforanadequatefittotheX-ray NALsystems(DandI)mayalsobeintrinsic,based
spectrum (Kaastra et al. 2000, Sako et al. 2001, Blustin upon our partial coverage analysis.
et al. 2002), one of which may fulfill the requirements for
the inner screen. 2. Weareabletofitmodelsonlytothebluestportion
Iftheopticaldepthofthe screenatwavelengthsaround of the CIV mini-BAL profile where self-blending is
the C IV edge is close to unity, then small fluctuations not severe. All fit parameters (i.e., column density,
in the column density can cause large changes in the Doppler parameter,coveragefraction,and shift ve-
shapeandintensityofthe transmittedionizingcontinuum locity) as well as the total equivalent width of the
(e.g., Fig. 6 of Murray et al. 1995, or Fig. 1 of Hamann system vary significantly with time, even on short
1995),leadingtotheobservedchangesinthemini-BALof time scales. However, the profile parameters do
HS1603+3820. Thismaybeanalogoustothebest-studied not appear to change in concert with each other,
warm absorber, in the Seyfert galaxy NGC 3783, where withoneexception: theequivalentwidthsofallthe
fluctuations in the ionization structure of the absorbing troughs in this system vary together and approxi-
gas have been observed over the course of 6 months (e.g., matelyfollowthevariationsofthecoveragefraction
Netzeretal. 2003). Thereisalsoevidencethattheopacity determined from the model fits.
of the warm absorber in NGC 3783 fluctuates over time
3. We have examined a number of ways of explain-
intervals as short as a month, in response to variations
in the X-ray continuum by a factor of 2 (Krongold et al. ing the above variations of the CIV mini-BAL and
we have found two viable possibilities. The first
2005). Within this context we can also understand why
possibilityisthattheobservedpartialcoveragesig-
thevariationsinthecoveragefractionofthemini-BALgas
nature is the result of continuum photons scatter-
are unrelated to variations of its column density. Because
ing around the absorber and into our cylinder of
both media can be clumpy and since differential rotation
sight. The observed changes in mini-BAL equiv-
of the wind causes the inner screen to move faster than
alent widths are thus produced by variations in
the mini-BAL gas across the cylinder of sight, the inten-
thescatteredcontinuumthatdilutestheabsorption
sity of the continuum transmitted throughthe screen and
troughs. In the second possibility, the illumination
receivedbythemini-BALgaswouldvarywithtime. More-
of the UV absorber fluctuates on short time scales
over,thecoveragefractionneednotbecorrelatedwiththe
by a factor of up to 3. We suggest that these fluc-
intensity of the transmitted continuum.
tuations are caused by a screen of variable optical
We have considered the possibility that one of the ob-
depth between the mini-BAL gas and the contin-
served NAL systems at smaller blueshifts relative to the
uum source. This screen might be identified with
quasarcouldrepresentthe inner screen. However,none of
the shielding gas invokedor predicted in some out-
the three candidate C IV systems, B, C, and D, can sat-
flowmodels. Moreover,itcouldbeanalogoustothe
isfy the requirements for the screen. The very fact that
“warm”absorbersobservedin the X-ray spectra of
they have CIV absorptionlines suggeststhat their ioniza-
Seyfertgalaxiesandsomequasars. Thispicturecan
tionstateistoolow,andmoreover,systemsBandDhave
also explain the variations in the coveragefraction,
even lower-ionizationlines. In addition, none of the three
which appear to be unrelated to the ionic column
systems are variable, which is necessary in order for the
density changes.
screen to perform the desired function. More promising
waysoffindingthescreensignatureareX-rayobservations
in search of the classic warm absorber feature, O VII and
O VIII edges. Thus, we will be able to determine which This workwassupportedby NASA grantNAG5-10817.
of the Lyα absorption lines seen at shorter wavelengths Wearegratefultothe staffoftheSubarutelescope,which
might be associated with the screen material. is operated by the National Astronomical Observatory of
Japan. We wouldalsolike tothank ChristopherChurchill
6. summary and conclusions for providing us with the minfit software package. ME
We have been monitoring the absorption lines in the acknowledges partial support from the Theoretical As-
spectrum of the quasar HS 1603+3820 since 2002, us- trophysics Visitors’ Fund at Northwestern University and
ing the Subaru and Hobby-Eberly Telescopes. We have thanks the members of the theoretical astrophysics group
obtained six Subaru+HDS and two HET+MRS spectra for their warm hospitality. The Hobby-Eberly Telescope
spanning an interval of approximately 1.2 years in the (HET) is a joint project of the University of Texas at
quasar rest frame and probing rest-frame time scales as Austin, the Pennsylvania State University, Stanford Uni-
short as 16 days. We have determined the physical pa- versity, Ludwig-Maximillians-Universit¨at Mu¨nchen, and
rameters of the 9 absorption-line systems by fitting Voigt Georg-August-Universit¨atG¨ottingen. The HET is named
modelstothelineprofiles. Ourmainresultsareasfollows: inhonorofitsprincipalbenefactors,WilliamP.Hobbyand
Robert E. Eberly. We also thank the anonymous referee
1. We detect one CIV mini-BAL and eight CIV NAL for helpful comments and suggestions.
Dramatically Variable Mini-BAL in HS 1603+3820 9
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10 Misawa et al.
Table 1
Log of Monitoring Observations
Timea WavelengthCoverage Exposure Resolving
Epoch Date (years) Instrument (˚A) (s) Power S/Nb
(1) (2) (3) (4) (5) (6) (7) (8)
1c 2002Mar23 0.00 Subaru+HDS 5080–6420 2700 45,000 46.5
2c 2003Jul7 0.36 Subaru+HDS 3520–4850,4930–6260 6000 45,000 48.7
3 2005Feb26 0.83 Subaru+HDS 3520–4855,4925–6275 7100 36,000 43.0
4 2005May10 0.89 HET+MRS 4500–6200 1800 9,200 17.4
5 2005Jun29 0.92 Subaru+HDS 3530–4855,4925–6280 3600 45,000 31.1
6 2005Aug3,8 0.95 HET+MRS 4500–6170 4000 9,200 28.5
7 2005Aug19 0.96 Subaru+HDS 3520–4850,4920–6235 3600 36,000 43.6
8 2006May31–Jun1 1.18 Subaru+HDS 4320–5630,5740–7050 9000 45,000 52.5
aTherelativetimesincethefirstobservation,inthequasarrestframe.
bS/Nperbinaround5370˚Aafterrebinning. Thefinalbinsizeis0.03˚AfortheSubaru+HDSspectraand0.15˚Aforthe
HET+MRSspectra.
cThesespectraweoriginallypresentedanddiscussedbyMisawaetal. (2003,2005).