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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

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