Chandra X-Ray Observations of Two Unusual BAL Quasars

0.69 MB·English

by  Jesse A. Rogerson

Chandra X-ray Observations of Two Unusual BAL Quasars
JesseA.Rogersona,PatrickB.Halla,StephanieA.Sneddenb,MichaelS.Brothertonc,ScottF.Andersond
aDepartmentofPhysicsandAstronomy,YorkUniversity,Toronto,OntarioM3J1P3,Canada
bApachePointObservatory,P.O.Box59,Sunspot,NM88349-0059
cDepartmentofPhysicsandAstronomy,UniversityofWyoming,Laramie,WY82071
1 dUniversityofWashington,DepartmentofAstronomy,Seattle,WA98195
1
0
2

n
a Abstract
J
We report sensitive Chandra X-ray non-detections of two unusual, luminous Iron Low-Ionization Broad
3
Absorption Line Quasars (FeLoBALs). The observations do detect a non-BAL, wide-binary companion
1
quasartooneoftheFeLoBALquasars. WecombineX-ray-derivedcolumndensitylowerlimits(assuming

] solar metallicity) with column densities measured from ultraviolet spectra and CLOUDY photoionization
O
simulations to explore whether constant-density slabs at broad-line region densities can match the physi-
C cal parameters of these two BAL outflows, and find that they cannot. In the “overlapping-trough”object
. SDSSJ0300+0048,wemeasurethecolumndensityoftheX-rayabsorbinggastobeN ≥1.8×1024cm−2.
h H
p FromthepresenceofFeiiUV78absorptionbutlackofFeiiUV195/UV196absorption,weinferthedensity
- inthatpartoftheabsorbingregiontoben ≃ 106 cm−3. Wedofindthataslabofgasatthatdensitymight
o e
beableto explainthisobject’sabsorption. Inthe Feiii-dominantobjectSDSS J2215−0045,the X-rayab-
r
t sorbingcolumndensityof N ≥ 3.4×1024 cm−2 isconsistentwiththeFeiii-derivedN ≥ 2×1022 cm−2
s H H
a providedtheionizationparameterislogU >1.0forboththen = 1011cm−3andn =1012 cm−3scenarios
e e
[ considered(suchdensitiesarerequiredtoproduceFeiiiabsorptionwithoutFeiiabsorption). However,the

2 velocitywidthoftheabsorptionrulesoutitsbeingconcentratedinasingleslabatthesedensities. Instead,
v this object’sspectrum can be explainedby a low density, high ionizationand high temperaturedisk wind
4 thatencountersandablateshigherdensity,lowerionizationFeiii-emittingclumps.
6
4 Keywords: quasars:general,absorptionlines,individual(SDSSJ030000.56+004828.0,SDSS
3 J221511.94−004549.9,SDSSJ025959.68+004813.6)
.
7
0
0
1. Introduction
1
:
v BroadAbsorptionLine(BAL) quasarspectraarenotascommonastypicalquasarspectrabutprovide
i a unique look at the central regions of Active Galactic Nuclei (AGN). A BAL quasar is characterized by
X
absorptiontroughsfrom gaswith blueshiftedoutflow velocitiesof typically 10%the speed of light(0.1c)
r
a (Weymannetal.1991). ThelowerlimitonthewidthofaBALtroughisinpartamatterofdefinition;the
traditionalrequiredminimumwidthis2000kms−1 (Weymannetal.1991),butaminimumwidthof1000
kms−1 hasalsobeenused(Trumpetal.2006). Suchvelocitywidthsarelargerthanessentiallyallgalactic
wind outflows, and thus ensure a sample of outflows driven predominantlyby AGN; of course, narrower
AGN-drivenoutflowscananddoexist.
Emailaddresses:[email protected](JesseA.Rogerson),[email protected](PatrickB.Hall)
PreprintsubmittedtoElsevier January17,2011
BAL quasars themselves are distributed into three subtypes: high-ionization(HiBAL), low-ionization
(LoBAL),andironLoBAL(FeLoBAL).HiBALquasarsshowabsorptionfromonlyrelativelyhigh-ionization
speciessuchasCiv,NvandSiiv. LoBALquasarsalsoshowabsorptionfromlow-ionizationspeciessuch
as Mgii, Aliii and Alii. FeLoBAL quasarsare LoBAL quasarswith absorptionfromone ormoreexcited
states of Feii or Feiii. Note that a range of ionization stages is seen even in HiBAL quasars, and that in
LoBALandFeLoBALquasarstheionizationsimplyextendstolowerionizationstages(Halletal.2002).
Approximately 10% of quasars in optically selected samples with spectra covering rest-frame 1400-
1550Åexhibitatroughortroughswhichcanbeconsideredbroadabsorption.Thetruefractionisexpectedto
behigherduetoselectioneffectswhichbiassurveysagainstthedetectionofBALquasars.HewettandFoltz
(2003) reported a corrected traditional BAL quasar fraction of 22 ± 4% using their sample of 42 bright
(B < 19) BAL quasars. Reichardetal. (2003) estimated a corrected traditional BAL quasar fraction of
J
15.9±1.4% using a sample of 224 BAL quasars with i . 20. Trumpetal. (2006) reportan uncorrected
traditional BAL quasar fraction of 10.4±0.2% using a sample of 1756 BAL quasars with i . 20, or an
uncorrectedfractionof26.0±0.3%usingalessconservative“absorptionindex”criteriontodefineaBAL
quasar (uncertainties from Poisson statistics only). By comparing the Trumpetal. (2006) catalog to the
near-infrared2MASSdatabase,Daietal.(2008)reportacorrectedBALfractionof43±2%forluminous
quasars (M . −30.1) using the absorption index criterion. However, Kniggeetal. (2008) find that the
Ks
absorptionindexBALquasarcriterionincludesalargenumberoffalsepositives. Takingthatintoaccount,
they find uncorrectedand correctedBAL fractionsof 13.6%and 17±3%. Scaling the results of Daietal.
(2008)toaccountforthecontaminationfoundbyKniggeetal.(2008)yieldsaBALfractionof23%,which
is also the upperlimit BAL fraction quotedby Kniggeetal. (2008). Lastly, Gangulyetal. (2007) find an
uncorrectedBALfractionof11%,correctedto23%inGangulyandBrotherton(2008).Insummary,thereis
generalagreementthat≃23%ofquasarsexhibitBALtroughsafterselectioneffectsaretakenintoaccount.
The fraction of quasars exhibiting BAL troughs could be due to an orientation effect, such that all or
most quasars have BAL outflows covering some of the lines of sight along which they would be seen as
quasars(e.g.,Turnshek1986). Alternatively,itcouldbeduetoanevolutionaryeffect,suchthatallormost
quasars have BAL outflows for some fraction of their lifetime (e.g., SurdejandHutsemekers 1987). Of
course,somecombinationoftheaboveisalsopossible(e.g.,Morris1988). Inanycase,theseoutflowsmay
havemasslossratescomparabletotheaccretionratesrequiredtopowerquasars(Steenbruggeetal.2005).
Thus,understandingBALquasarsandBALoutflowsisimportantforunderstandingquasarsasawhole.
ObservationsshowthatBAL quasarsareX-rayweakcomparedwith regularquasars(e.g. Greenetal.
1995,Gallagheretal.2006,andreferencestherein). ThediskwindmodelofMurrayetal.(1995)explains
thisandotherpropertiesofBAL quasarswell. In thatmodel,the X-rayweaknessis attributedto intrinsic
absorption from ’hitch-hiking’ gas between the BAL outflow and the X-ray emitting region. The nearly
completely ionized metal atoms in the hitch-hiking gas are kept ionized by capturing electrons and then
immediately absorbingX-rays from the quasar, just as an Hii regionis kept ionizedby protonscapturing
electronsandthenimmediatelyabsorbingphotonsof energies13.6eV or greater. Because highlyionized
metalatomscanonlyefficientlyabsorbX-rays,thehitch-hikinggasprotectstheBAL outflowfromoveri-
onization by X-rays while transmitting the ultraviolet (UV) radiation that accelerates the BAL outflow to
high velocities.1 We assume a BAL quasar has an intrinsically normalX-ray spectrum but is shielded by
someabsorbinggas,sothatconstraintscanbeplacedonthehydrogencolumndensityrequiredtoabsorbthe
X-rayflux.
It is also possible that some or all BAL quasars are intrinsically X-ray weak. It has been shown by
1Thehitch-hikinggasmayalsoactasanX-rayreflectorandasoftX-rayemitter,butwedonotconsiderthoseeffectsherein.
2
Giustinietal.(2008)thatsomeBALquasarsmaybelessX-rayabsorbedthanpreviouslythought,andthe
X-rayweaknessisduetoadifferingspectrumfromtypicalquasars. Theauthorsadmittheremaybesome
factorsthatcouldaffectthisresult. TheseincludetheX-rayenergyrangeusedforspectralanalysis,partial
coveringofthesourcebytheX-rayabsorber,oranionizedX-rayabsorber.Streblyanskaetal.(2010)goon
toshowthatwhenanionizedabsorberisconsidered,thecolumndensityofintrinsicabsorptionsignificantly
increases. AlthoughthepossibilityofintrinsicX-rayweaknesscannotbefullyrejectedbycurrentdatasets,
traditionalBALquasars(withBI>0,whereBIistheBALnicityindex)appeartobealwaysX-rayabsorbed
(Streblyanskaetal.2010). BoththeabovestudiesusesomeBALquasarsfromTrumpetal.(2006),which
maynotbeactualBALquasars(Kniggeetal.2008).
Recent sky surveys have also uncovered a slew of BAL quasars which exhibit unusual properties as
comparedtotypicalBALquasars(e.g.,Halletal.2002;Brunneretal.2003;Ducetal.2002). Investigation
ofthesepropertiesisasworthwhileasthestudyofBALquasarsthemselves,asthemostunusualobjectswill
definetheparameterspacespannedbyBALoutflowsingeneral.Ifwearetohaveacompleteunderstanding
ofquasars,ourmodelsmustexplainallnormalandunusualquasarbehaviour.
In this paper we discuss continuedresearch into two such unusualBAL quasars. SDSS J030000.56+
004828.0(hereafterSDSSJ0300+0048,Fig.1a)isan“overlapping-trough”BALquasaratz=0.89191with
nearlycompleteUVabsorptionbelowrest-frame2800Å(Halletal.2002;Halletal.2003).ThisFeLoBAL
quasar has a non-BAL binary companion, SDSS J025959.68+004813.6 (hereafter SDSS J0259+0048),
whichislocated19′.′5awayfromSDSSJ0300+0048ataredshiftz=0.894(∆v=330±160kms−1).
Thesecondobjectis SDSSJ221511.94−004549.9(hereafterSDSS J2215−0045,Fig. 1b), a reddened
FeLoBALwithdetachedMgii, Aliii, Alii, andFeiiiUV34,48absorption(Halletal.2002). Thisquasaris
atz=1.4755ascalculatedbytheassociatedMgiiabsorption.SDSSJ2215−0045hasabsorptionfromFeiii
buttherearenoabsorptionfeaturesfromFeii,makingitaveryrareandunusualfind.HallandHutseme´kers
(2003) discuss how the work of deKooletal. (2002) shows that the presence of Feiii but lack of Feii
absorptionrestrictstheoutflowtohighdensitiesandanarrowrangeofcolumndensities.
X-raydatahasbeenobtainedusingtheChandraX-rayObservatoryandanalyzedtohelpunderstandthese
twounusualBAL quasars. We discussourdataandmethodsin§2, presenttheresultsofourobservations
in§3and§4,discusstheirimplicationsin§5andsummarizeourresultsin§6. Weadoptthecosmologyof
Spergeletal.(2007):H =73.2,Ω =0.259,andΩ =0.741.
0 M Λ
2. X-rayDataandAnalysis
ObservationsofourtwotargetswerecarriedoutinChandraCycle4usingtheAdvancedCCDImaging
Spectrometer(ACIS).
We use the observed-frameenergyrange0.5-6.65keV:0.5keV is anobviouslowerenergycutoffdue
totheincreasedbackground(andhigherthroughputdegradation)atlowenergies,and6.65keVwaschosen
astheupperlimitbecauseithasthesameeffectiveareaasthe0.5keVlowerlimitandbecausetheeffective
observingareadropsquicklyatenergies>6.65keV.Thesourcecountrateintheenergyrange0.5-6.65keV
isexpectedtobeafactorof100higherthanintherange6.65-13keV,whiletheexpectedbackgroundcount
rateisthesameintheseenergyranges.2
GiventhesmallnumberofdetectionsofX-rayphotonsoverafinitetimeperiod,wearelimitedbysmall-
number statistics rather than by the background level. To determine upper limits at specified confidence
intervalsinthefaceofsmall-numberstatistics,weusetheBayesianmethodofKraftetal.(1991)ratherthan
2Seehttp://cxc.harvard.edu/proposer/POG/html/index.html
3
thefrequentistmethodoutlinedinGehrels(1986).TheBayesianmethodisfavoredduetoitsabilitytowork
withnon-zerobackgroundfluxwheninthezerosourcecountregime.
2.1. AGNX-ray-UVLuminosityCorrelation
TheX-rayandUVluminositiesofregular(non-BAL)quasarsareobservedtohaveastrongcorrelation
across multiple decades of UV luminosity; this offers a useful tool for measuring the X-ray weakness of
a BAL quasar, which tend to have far weaker X-ray luminosity than a typical quasar. The correlation is
usually expressed in terms of the logarithmof the ratio between the 2 keV (l ) and 2500Å(l ) rest-
2keV 2500
framespecificluminosities,denotedasα :
ox
α =0.372log(l /l ) (1)
ox 2keV 2500
ThiscorrelationhasbeenquantifiedcarefullyinarecentstudybySteffenetal.(2006)byobservingthel
2keV
andl of333AGNs(frommultiplesurveys).Thebestfittothedatais:
2500
log(l )=(0.721±0.011)log(l )+(4.531±0.688) (2)
2keV 2500
Thus,providedaUVluminosity,theexpectedX-rayluminositycanbecalculated. Fortypicalquasars,α
ox
canbewrittenasafunctionofl ,asinSteffenetal.(2006):
2500
α (UV)=(−0.137±0.008)log(l )+(2.638±0.240) (3)
ox 2500
TocharacterizetheX-rayweaknesswecomparetheobservedα value,calculatedusingtheobserved
ox
l andl inequation(1),totheexpectedα valueα (UV).Thedifferencebetweenthetwoisdenoted
2500 2keV ox ox
∆α :
ox
∆α =α −α (UV) (4)
ox ox ox
By exploiting the strong correlation between l and l in ordinary quasars, we can quantify the
2500 2keV
X-rayweaknessofourtargets.
To determine l for ourtargets requiresestimating f for their underlyingcontinua. We used the
2500 2500
spectra available for each object in SDSS Data Release Six (Adelman-McCarthyetal. 2008). We used
either the smoothed or extrapolated flux at 2500 Å rest-frame as f ; details are given in the individual
2500
objects’ discussions below. We have assumed a value of 10% as the uncertainty on the UV flux for the
followingdiscussion.TheluminositydistancetoeachquasarwascalculatedfollowingPen(1999)basedon
thequasar’sredshiftandouradoptedcosmology.
3. Results: SDSSJ0300+0048andSDSSJ0259+0048
SDSSJ0300+0048andSDSSJ0259+0048wereobservedsimultaneouslyfor6743.5secondsonDecem-
ber19,2002(UT).NoX-rayphotonsweredetectedwithina2′.′5radiusofthepositionofSDSSJ0300+0048.
SDSSJ0259+0048wasdetected,with39photonswithin2′.′5ofitsposition. Themeasuredbackgroundof
the imageis suchthatwe expectonly0.244±0.014observed-frame0.5-6.65keVbackgroundphotonsin
thedetectionapertures.
4
3.1. Results: SDSSJ0259+0048
We first analyze the SDSS J0259+0048results, as it is a non-BAL quasar and is thus expected to be
unremarkable. Asa testofourmethods,we seektoreproducetheobservedX-rayphotoncountof39, by
usingonlytheobservedUVfluxandtherelationsin§2.1.
Forthisquasaratz = 0.894wefind f = (0.55±0.05)×10−27 ergs−1 cm−2Hz−1 andl = (2.00±
2500 2500
0.20)×1030ergs−1Hz−1.
Usingequation(2) we thereforeexpectlog(f ) = −31.2+0.8 ergs−1cm−2 Hz−1. Giventhe predicted
2keV −0.7
X-rayflux,weexpect2.8timestheobservedcountrateof1.59inthelog,butthedifferenceis<1σ. Thus,
ourmethodaccuratelyreproducestheobservedX-rayphotoncountofanunremarkablequasar.
Usingequation(1)wecalculatedanobservedα =−1.63±0.04forSDSSJ0259+0048.Theexpected
ox
α iscalculatedbyequation(3). Pluggingintheobservedl weexpectα (UV) = −1.51±0.34. Thus
ox 2500 ox
∆α = −0.12±0.34. Therefore, within the uncertainties, the observedand expectedvaluesfor α (and
ox ox
X-raycounts)arethesame.
3.2. Results: SDSSJ0300+0048
Using the same method as above, we analyze the observed and predicted X-ray quantities for the
FeLoBAL quasar SDSS J0300+0048 at z = 0.89194 to determine the column density of the intervening
matter.
The“detection”ofzerophotonsintheimageresultsinanupperlimitof3.00photonsat95%confidence
(Kraftetal.1991),equivalenttoacountrateof≤ 4.445×10−4photonss−1 intheobservedbandpass. This
corresponds to an observed X-ray flux upper limit of f ≤ 1.793× 10−33 erg s−1 cm−2 Hz−1 at 95%
2keV
confidence.
Tofind f wefitapowerlawin f tonarrownormalizationwindowscenteredat3060Åand4735Å
2500 ν
rest-frame.Theresultingfluxat2500Åis f =(3.77±0.38)×10−27ergs−1cm−2Hz−1.
2500
Equation(2)thenpredictstheX-rayfluxofSDSSJ0300+0048tobelog(f )= −30.58+0.77ergss−1
2keV −0.79
cm−2Hz−1,whichincludestheuncertaintyinthepowerlawslopeintheUVband. Therefore,weexpectto
see 2.6+0.8 X-raycountsinthelogintheimage. Thisishigherthantheobserved95%confidencelimitof
−0.7
0.5inthelog.
We characterize this X-ray weakness as follows. The expected α (from equation 3) is α (UV) =
ox ox
−1.64±0.35 and the observed α (from equation 1) is α ≤ −2.35 at 95% confidence. Therefore the
ox ox
deviationof the observedα fromthe predictedvalue is ∆α ≤ −0.72, which is a reductionin f by
ox ox 2keV
a factorof 164fromtheexpectedflux. Thisreductionin X-rayfluxis attributedto shieldinggasnear the
quasar,whichmusthaveahydrogencolumndensityofN ≥1.8×1024cm−2atthequasarredshift,assuming
H
solarmetallicity.Thatlowerlimitwascalculatedassumingneutralgas,butappliestoneutralorionizedgas.
Ifthegasispartlyorfullyionized,itwillabsorbfewerphotonsperunitcolumndensity,andthereforethe
requiredminimumcolumndensitywillbelargerthantheabovelowerlimit.
Gibsonetal. (2009) report α ≤ −2.33 and ∆α ≤ −0.70 for this object in their summary of BAL
ox ox
quasarsfromSDSSdatarelease5. Thevaluesareslightlydifferentfromvaluesquotedinthispaperbecause
slightlydifferentformulaewereused.Nevertheless,thevaluesareconsistentwithintheuncertainties.
WecancompareourresultwiththecolumndensitiesrequiredtoexplaintheUVabsorption,inparticular
theCaiicolumndensityN =(7.13±1.15)×1014cm−2(Halletal.2003). Becauseitsionizationpotential
CaII
islessthanthatofHi,CaiibecomesthedominantcalciumiononlyatlargecolumnsbehindanHiionization
3http://cxc.harvard.edu/toolkit/pimms.jsp: theonlineversionofPortable,InteractiveMulti-MissionSimulator,providingcount-rate
estimationsandpredictionsforChandra.
5
front(§5.2ofHalletal.2003). FerlandandPersson(1989)foundthatforcloudsofdensity109.5cm−3,Caii
is dominantonly at a column densities N ≥ 6.3×1024 cm−2, more than a factor of ten higher than the
H
columndensityoftheHiionizationfront.
WehaveusedthephotoionizationsimulatorCLOUDY4toinvestigatetheabsorptioninvariousionsfora
representativeBroadLineRegion(BLR)(densityofn =1011cm−3,logU =−1.5,whereUistheionization
e
parameter)(Baldwinetal.2003).NotethatourCLOUDYsimulationswererunwithsolarmetallicityandno
dustobscuration.EvenatN =1025cm−2,Caiiisnotthedominantcalciumion.However,theionicfraction
H
ofCaiiimmediatelybehindtheHiionizationfrontis∼1%forn =1011cm−3,significantlyhigherthanthe
e
∼0.3%seenforn =109.5cm−3. GiventhattheabundanceofCaiiis−5.64inthelog,eachN =1022cm−2
e H
behindtheHiionizationfrontyields10(22−2−5.64)worthofCaii: N =2.29×1014cm−2. Caihasasimilar
CaII
behaviortoCaii thoughitsionicabundanceimmediatelybehindtheHiionizationfrontisonly0.0001%.
Thus,tomatchtheCaiicolumnobservedinSDSSJ0300+0048withstandardBLRparametersrequires
only N = 3.11×1022 cm−2 behindthe hydrogenionizationfront. Thisagreeswith the constrainton the
H
totalN behindthefrontfromtheobservedupperlimitontheCaicolumndensity: N ≤7×1023cm−2.
H H
However, the UV-derived N ≃ 3.11× 1022 cm−2 is much less than the X-ray-derived N ≥ 1.8×
H H
1024 cm−2. This can be explained by assuming a higher ionization parameter at the cloud face. Using
CLOUDY, we investigated the behavior of Caii with higher values for the ionization parameter; namely
logU = 0.5,1.0,1.5,and2.0. Wealsovariedthedensity: separaterunsforadensityofn = 1011cm−3 and
e
n =1012cm−3werecarriedoutforeachoftheabovevaluesoflogU. TheresultsareshowninFigure2.
e
TheshadedregionsaredefinedbytheUV-derivedCaiicolumndensity(horizontalregion)andtheX-ray
derived lower limit on the hydrogen column density (vertical regions). The lower limit on the hydrogen
columnisshownat1σ(lightestgrey),2σ,and3σ(darkestgrey). ThecurvesrepresentthedifferentlogU
ofinterestforthisobject. ForaspecificlogU tomatchtheobservations,itscurvemustfallintheallowed
regionforboththecolumndensitiesplotted. InFigure2, theallowedregionforeachlogU isrepresented
by a cyan highlight. Thus, this objectrequireslogU > 0.0 for both n = 1011 cm−3 and n = 1012 cm−3
e e
(Figures2aand2b,respectively). ThislowerlimitisconsistentwiththeconstraintsposedbyCai(plotted
inFigure3),andalsotheconstraintsfromabsorptioninMgi,Mgii,andFeii.
However,asweshallseein§5.1,observationsofFeii*ruleoutthelogU >0,n ≃1011cm−3scenario;
e
gaswithsuchparametersmayformpartoftheabsorberinthisobject,butitcannotformtheentireabsorber.
4. Results: SDSSJ2215−0045
SDSSJ2215−0045wasobservedfor6615.9secondsonJune21,2003(UT).Themeasuredbackground
oftheimageissuchthatweexpectonly0.102±0.004observed-frame0.5-6.65keVbackgroundphotonsin
thedetectionapertures.
Tofind f wefitapowerlawin f tonarrownormalizationwindowscenteredat2668Åand3035Å
2500 ν
rest-frame.Theresultis f =(7.26±0.73)×10−27ergs−1 cm−2Hz−1. Thetrue f couldbesomewhat
2500 2500
highersincewedonotaccountforpossibleMgiiabsorptionintheshorter-wavelengthnormalizationregion,
nor do we account for the inferred continuum reddening of E(B−V)≃0.06 (HallandHutseme´kers 2003).
Accountingforeitherofthoseeffectswouldincrease f ,whichwouldincreasethepredicted f ,which
2500 2keV
wouldinturnincreasetheabsorptionneededtobringthepredicted f belowtheupperlimitwemeasure.
2keV
Thus,thisvalueof f isconservativewithrespecttotheamountofX-rayabsorptionweinfer.
2500
4http://www.ferland.org/cloudy/
6
Nophotonsaredetectedintheobserved0.5−6.65keVbandpass,sowehaveaconservativeupperlimit
of3.00photonsat95%confidence.5 Thislimitcorrespondstoacountrateof≤ 4.544×10−4 photonss−1.
TheobservedX-rayfluxlimitistherefore f ≤1.776×10−33ergs−1cm−2Hz−1. FromtheobservedX-ray
2keV
fluxupperlimitwecalculatetheobservedα upperlimit,usingequation(1),andfindα ≤−2.45at95%
ox ox
confidence.
TheexpectedX-rayfluxislog(f )= −30.53+0.78ergs−1 cm−2 Hz−1,whichincludestheuncertainty
2keV −0.77
inthepowerlawslopeintheUVband. Giventhatflux,2.70+0.77countsinthelogshouldbeobservedover
−0.80
6615.9s,whichishigherthantheobserved95%confidencelimitof0.5inthelog.Wealsoexpectα (UV)=
ox
−1.74±0.35. Thedeviationof theobservedα fromthepredictedvalueis ∆α ≤ −0.71. Comparedto
ox ox
SDSSJ0300+0048,weprobehigher-energyphotonsinSDSSJ2215−0045duetoitshigherredshift,andso
toproducethesameobserved∆α ≤ −0.71requiresahigherabsorbingcolumn: N ≥ 3.4×1024 cm−2 at
ox H
thequasarredshift,assumingsolarmetallicity.Again,thisisahardlowerlimitassuminganeutralabsorber.
Gibsonetal. (2009) also evaluated values for this object. They find α < −2.71 and ∆α ≤ −0.96.
ox ox
Again, due to slightly different formulae, the values derived in Gibsonetal. (2009) are slightly different
thanthosecalculatedhere.Inthiscase,the∆α valueis<1.5σdifferent.
ox
Thepresenceofdoublyionizediron(Feiii)absorptionwithoutanysignificantsinglyionizediron(Feii)
absorptionis a rareoccurrence. deKooletal. (2002) showedthatthis ironabsorptionbehaviourcould be
produced with a density of n = 1011 cm−3, an ionization parameter logU = −1.5, and a very narrow
e
rangeoftotalhydrogencolumndensityaroundlogN ∼ 22.4. We investigatedthisresultwithCLOUDY
H
simulationsusingthesameparametersasforSDSSJ0300+0048.TheresultsforFeiiiarelocatedinFigure
4andtheresultsforFeiiinFigure5.
InFigure4,theshadedregionisshapedbythelowerlimitsofboththeX-rayderivedhydrogencolumn
(vertical lower limits) and the UV-derived Feiii column density (horizontal lower limits; details given in
§5.2). Thecurvesmustfallinsidethisregiontocorrectlydescribetheobservations. Thecurvesmustalso
satisfytheupperlimitontheFeiicolumndensity(alsodiscussedin§5.2)showninFigure5. Theshaded
region,inthiscase,isdefinedbyanupperlimitFeiicolumn.
ThecyansegmentsinFigure4representtheintervaloverwhicheachcurvesmatchtheobservationsby
fallingwithintheshadedregionsofbothFigures4and5. WecanplacealowerlimitoflogU > 1.0onthe
ionizationparameterforboththen =1011cm−3andn =1012cm−3cases(Figures5aand5b,respectively).
e e
ThislowerlimitisconsistentwithconstraintsimposedbyMgii,Alii,andAliii.
ThephysicalbasisbehindtheseconstraintsisthatFeiiiabsorptionwithoutsignificantFeiiabsorptionis
seenwhentheabsorberhasjustbarelyenoughcolumndensitytoformahydrogenionizationfront,because
FeiiiisonlyabundantjustbeforesuchafrontandFeiiisabundantjustafterone. Furthermore,tocreatean
Feiii column matchingthe observationsin the narrow regionjust before the frontrequiresa high density,
becausesuchhighdensitiesforceFeivtorecombinetoFeiii(BautistaandPradhan1998).
5. Discussion
WehaveshownthatbothSDSSJ0300+0048andSDSSJ2215−0045areundetectedinsnapshotX-ray
observations. DespitetheshortexposuretimesoftheX-rayobservations,non-detectionsoftheseoptically
brightquasarsrequireverylargeabsorbingcolumnsandsmalllevelsofunabsorbedX-rayscattering(<1.3%
at 95%confidence).6 Note thatour columndensity lower limits assume neutralabsorption. As shown by
5Onephotonwithenergy12.7keVisdetectedwithin2′.′5ofthepositionofSDSSJ2215−0045,butphotonswithsuchhighenergies
aremuchmorelikelytobebackgroundphotons,asdiscussedin§2.
7
Streblyanskaetal.(2010),whenanionizedabsorbermodelisused,BALquasarX-rayabsorbingcolumns
canbe1-2ordersofmagnitudelargerthanintheneutralcase. Thus,thecolumndensitieswequotehereare
hardlowerlimits,andthetruecolumndensitiesarealmostcertainlylarger.Wenowdiscusstheimplications
oftheseresultsinmoredetail.
5.1. SDSSJ0300+0048
InSDSSJ0300+0048,atoutflowvelocitiesof2000<v<4000kms−1weseeMgii,Mgi,CaiiandFeii
absorption. At4000< v . 10850kms−1,weseeMgii,Feiiandexcited-stateFeii(Feii*)absorptionfrom
gas which must be located closer to the quasar than the Caii-absorbing region. (If it were located farther
fromthequasarthantheCaii-absorbingregion,itwouldbeshieldedbythatgasandwoulditselfshowCaii.)
Halletal.(2003)andHallandHutseme´kers(2004)suggestedthattheSDSSJ0300+0048outflowcould
beproducedbygasinanaccretiondiskwindifwearelookingacrossthewindratherthandownthewind.
Inthismodel,high-ionizationgasislaunchedfromclosertothequasarthanlow-ionizationgasis,withCaii
andMgibeinglaunchedfromoutsideahydrogenionizationfront.7
We canconstrainthedensityintheFeii*-absorbingregionbystudyingthecriticaldensitiesofexcited
levels from which we either see or do not see Feii* absorption. We see absorption from the Feii UV78
multiplet, whose lower level has a critical density of ≃ 105.5 cm−3. We do not see absorption from Feii
UV195/UV196 multiplets, with relevant critical densities of ≃ 107.5 cm−3. Thus, we can constrain the
densityintheFeii*-absorbingregiontobe≃106.0±0.5cm−3.
Wenowexaminetheionizationbehaviourofaslabofgasatthisdensity,toseeifasingle-slabmodelcan
explainboththeX-rayandUVabsorption. InFigure6wepresenttwoCLOUDYrunsforn = 106 cm−3,
e
with logU = 1.0 and logU = −0.5. For logU = 1.0, the observed Feii column (Figure 6a) can be
reachedwithreasonablehydrogencolumndensities,buttheCaiicolumn(Figure6b)canonlybeachieved
with N ∼ 1025 cm−2. When the hydrogencolumn approaches ∼ 1025 cm−2 between us and the quasar
H
UV emission region,the electronscattering opticaldepth rises aboveτ ∼ 2, increasingthe alreadyhigh
es
intrinsicopticalandUVluminosityofthisobject.(Ofcourse,theX-rayabsorptionmaycoveronlytheX-ray
continuum-emittingregion,andnotthelargerUVcontinuum-emittingregion,butatthemomentweareonly
consideringwhetherasingleuniformabsorbercanexplainthedata.) ForlogU = −0.5,theobservedCaii
column(Figure6b)isreachedat∼ 3×1024 cm−2, atwhichpointthepredictedFeiicolumniswellabove
the observationally inferred lower limit and the temperature within a factor of two of the observationally
inferredupperlimit.
Thus,inlightoftheX-raydata,wefindthatwecannearlymatchthepropertiesofboththeX-rayandUV
absorbersinSDSSJ0300+0048withoneslabofgasofconstantdensityn = 106 cm−3 withlogU = −0.5
e
at its ionized face, implying an absorber at a distance of ∼60 pc from the black hole with a thickness of
∼3×1018cmor∼1pc
IftheabsorbinggasinSDSSJ0300+0048canbeapproximatedbygaswithauniformdensityofn =
e
106 cm−3, it is difficult to reconcile the resulting ∼60 pc distance of the gas from the black hole with the
6TheseX-rayscatteringconstraintsarebroadlyconsistentwithopticalpolarizationmeasurementsforbothobjects(Hall,Smith,et
al.,unpublished):SDSSJ0300+0048hasR-bandP=1.58±0.10%andSDSSJ2215−0045hasawhite-lightP=0.40±0.06%.
7Theradialdependence ofthedensityintheoutflowatlaunchisunclear. AsmentionedinHalletal.(2003),inregion(a)ofa
ShakuraandSunyaev(1973)diskthedensityincreaseswithradius.Adiskwindlaunchedfromthatregionwouldhaveahigherdensity
intheCaiiregionthanintheFeii*region.Alowtemperature(T .1100K)wouldberequiredintheCaiiregiontoavoidexcitingFeii*
there. Ontheotherhand,thelow-ionizationabsorptioninthisobjectcoverstheMgii(andFeii)broademissionlineregion,located
atradiiwheretheShakuraandSunyaev(1973)diskmodelpredictsaradiallydecreasingdensity. Ifthediskwindislaunchedatthese
largerradiianditsdensityisdecreasingwithdistance,thedensityintheCaii-absorbingregionshouldbethelowestintheoutflow. A
densityofne.103cm−3wouldberequiredtoavoidpopulatingthelowerlevelsofFeii*transitionswhicharenotobserved.
8
original disk wind scenario of Halletal. (2003). Such a distance is more consistent with the alternative
scenario to a disk wind discussed in Halletal. (2003); namely, that the outflow sweeps up gas at large
distancesfromitsoriginanddeceleratesintheprocess. Inthisswept-up-gasscenario,theFeii*-absorbing
gaswouldbegaswhichhasbeencompressedton &103cm−3andacceleratedto4000<v<10850kms−1
e
by a high-ionization,low-density wind (seen at velocities up to at least v = 10850kms−1 in this object).
TheCaii-absorbinggaswouldbegaswhichhasbeenovertakenbythewindmorerecentlyandwhichhas
therefore been compressed and accelerated to a lesser degree than the Feii*-absorbing gas. However, it
remainstobeseen(throughdetailedoutflowmodelingbeyondthescopeofthisinvestigation)whethergas
can be accelerated to 2000 < v < 4000 kms−1 without being compressed to a density n > 103 cm−3
e
or heatedto a temperatureT > 1100K. Meanwhile, as suggestedby Halletal. (2003), this swept-up-gas
scenariocanbetestedbylookingforalong-termincreaseoftheCaiiBAL outflowvelocity. (Noincrease
wasseenoverarest-frametimespanofupto205daysinmultipleSDSSspectraplusourVLTspectrumof
thisobject.)
5.2. SDSSJ2215−0045
The conservative upper limit of logN < 14.7 (Figure 5) for this object was estimated using the
FeII
plausible unabsorbedcontinuumfrom HallandHutseme´kers 2003 (hereafterHH03), shown in Figure 1b.
ThedetailsoftheestimationofthelowerlimitsfortheFeiiicolumndensityshowninFigure4aregivenin
theAppendix.
We have found in §4 that if a single absorber is responsible for both the observed UV and X-ray ab-
sorptioninSDSSJ2215−0045,itmusthaveN > 3.4×1024cm−2,logn & 9.5cm−3 andlogU ≥ 1,with
H e
fullcoverageoftheX-raysourceandatleast50%coverageoftheUVcontinuumsource.Wenowconsider
whetheranabsorptionsystemwiththosepropertiesisinfactplausible.
IfwetakeN =3×1024cm−2andlogn =9.5cm−3,thentheabsorberisonly1015cmthick. Alarger
H e
N would make fora thickerabsorberbutwould also increase the electron scatteringopticaldepthabove
H
τ ≃2,increasingthealreadyhighintrinsicluminosityofthisquasar.TheX-raycontinuumregionis∼1015
es
cm in radius(Chartasetal. 2009), while the UV continuumregionis ∼1016 cm in radius(Kochaneketal.
2007). To cover both regions, the absorber would have to be at least 20 times larger in the transverse
directionthanitisintheline-of-sightdirection. Moreover,theline-of-sightvelocitywidthoftheabsorber
is12,000kms−1. Ifthatisaturbulent∆v,theabsorberwoulddoubleitsthicknessandhalveitsdensityon
atimescaleequaltothecrossingtimeof∼10days,butobservationsofSDSSJ2215−0045overrest-frame
timescalesmuchlongerthanthatrevealnochangesinitsabsorptiontroughs(HH03).Ifthat∆visacoherent
velocityspreadalongourlineofsight,theaccelerationrequiredtoproduceitin1015cmis∼100ms−2. For
comparison,inthediskwindmodelofMurrayetal.(1995)theradiativeaccelerationis.100cms−2.
Itishardtoseehowtobringtheaboveextremeparametersintotherealmoftheplausible.Forexample,
asupersolarFeabundancewouldreducetheN requiredtomatchtheobservedN ,andLyαpumpingof
H FeIII
FeiiiUV34(Johanssonetal.2000)wouldreducetheamountofFeneededtoexplainagivenN . Those
FeIII
effectscouldeliminatetheτ problem,butwouldshrinktheline-of-sightwidthoftheabsorberandmake
es
thedynamicalproblemsworse.
Thus, a single, uniformabsorberwith the propertiesrequiredto explainthe UV and X-ray absorption
in thissystem isextremelyunlikelytoexist. Instead,it islikely thatthe X-rayabsorberisCompton-thick
(&3×1024cm−2)butcompact(radius∼1015cm),sothatatmostasmallpartoftheUVcontinuumregion
isobscuredbyit. TheUVabsorbermustbelarger(radius∼1016cm)andhaveacolumndensityjustunder
thatrequiredtoformahydrogenionizationfront(N . U ×1023 cm−2)sothatitcontainsverylittleFeii.
H
We can constrain −1 . logU . 1 for the UV absorber. The upperlimit comes from setting the electron
scattering optical depth to the UV continuum source to be at most unity. The lower limit comes via our
9
measuredlogN > 16.62andlogN < 14.7; giventhose values, Table2 ofdeKooletal. (2002) and
FeIII FeII
ourFigures5and4showthatlogn &9.5cm−3andlogU &−1.8
e
We can reconcile the high density required for the Feiii column with the large velocity and velocity
spreadofFeiiiifweassumetheFeiiiabsorptionarisesindenseclumpsembeddedinalowerdensity,higher
ionization wind seen in Civ and Siiv, along the lines suggested by Voitetal. (1993). The clump widths
along the line of sight would be d < N /n , or d < 1013.5±1.0 cm. This maximum size is larger than the
H e
typicalsizeof1012 cmpredictedforputativeBLRclouds(Koristaetal.1997). Tocoverhalfther ∼ 1016
cmUVsource,alargenumberofsuchclumps(∼105±2iftheclumpsareroughlyspherical)arerequired.
Inthisobject,Feiiiabsorptionisseenat6000 < v < 18000kms−1 alongourlineofsight,whereas
FeIII
Civ and Siiv absorption is seen at 0 < v < 25000± 3000 kms−1 (Figure 1 of HH03). Given that the
troughsinthisobjecthavehigheroutflowvelocitiesandvelocitywidthsthantheaverageBALtrough,itis
reasonabletoassumethatmuchoftheaccelerationofthegasoccursalongourlineofsight;inotherwords,
theoutflowstreamlinesarelargelyparalleltoourlineofsight. Ifweimagineanaccelerating,relativelylow
densitywind(seeninCivandSiiv)collidingwithdenseFeiiiclumps,thewindwillformashockaround
theclumpsandcaninprincipleaccelerate,ablate,compressandbypasstheclumps. Theaccelerationmay
helpexplaintheFeiiitroughvelocity,alongwithradiativeandpossiblymagnetohydrodynamicacceleration;
theablationhelpsexplainstheFeiiitroughwidth;thecompressionhelpsexplaintheinferredhighdensities;
andthebypassingexplainstheCivandSiivtroughextendingtobothsmallerandlargeroutflowvelocities
thantheFeiiitrough(Voitetal.1993).
Insummary,ourpictureofthisquasaristhattheX-ray-emittingregioniscompletelycoveredbyanX-
rayabsorberandthatthemoredistantUV-emittingregionisnearlyorcompletelycoveredbyaUV-absorbing
windseeninCivandSiivabsorption.Thiswindisaccelerated(atleastinpartalongourlineofsight)from
0kms−1to25000±3000kms−1. Atsomedistanceorrangeofdistancesfromthequasar,thewindincludes
dense,Feiii-emittingclumps(withoverallcoveringfactor∼50%andlowvolumefillingfactor)whichhave
beenacceleratedbythewindtovelocitiesof6000<v <18000kms−1. Aparaboloidalshocksurrounds
FeIII
eachclump,wherethewindgasencounterstheslower-movingclumpgas.Theshock-heatedgasisnolonger
visible in Civ or Siiv, butbetween a clumpand its shocklies a regionwhere gasablatedfromthe clump
willabsorbin thoseions, atvelocitiesv ≃ v . Thewind downstreamfromthe clumps—consistingof
FeIII
shockedgasand,ifthevolumefillingfactoroftheclumpsissmall,unshockedgasaswell—isnotseento
recombinetoobservableionizationstageswhenshadowedbyaclump,soitsrecombinationtimescalemust
belongerthanthetimeitspendsinashadow.
Inthispicture, the Feiii-emitting clumpsarebeingablatedand so the Feiii troughdepthwilldecrease
withtimeunlessthewindencountersnewclumps.Suchclumpsshouldfirstappearatlowvelocity;fromtheir
absence,weconcludethatthewindisnotcurrentlyencounteringnewclumps. Spectroscopicmonitoringof
thequasartolookforweakeningFeiiitroughs(aswellasanychangesinFeiii troughvelocityorvelocity
width)wouldbeworthwhile.
Lastly, we note the possibility that this quasar may have developedfrom a typical FeLoBAL. In such
an object, Feii is seen at low velocities and Civ at both high and low velocities, consistent with a high-
ionization wind which has encountered clouds optically thick in the Lyman limit and has only begun to
accelerate them. As time progresses, the cloudswill be accelerated and ablated. Eventually,they may be
opticallythinintheLymanlimit(andthusseeninFeiiiandnotFeii)andwillbefoundathighervelocities.
8WeassumethattheUVabsorberisfullyshieldedbyadust-freeX-rayabsorber. Inthatcase,theX-rayabsorberwillabsorba
relativelysmallfractionofhydrogen-ionizingphotons,andthestructureoftheabsorber’sStromgrenspherewillbemuchlessaffected
thanthestructureofitspartiallyionizedzone.Wecanthereforeuseourconstant-densityCLOUDYsimulationstoinfertheconditions
oftheFeiiiclumps,eventhoughthehigher-ionizationregionsintheoutflowmusthavealowerdensity.
10