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Optical Variability Properties of High Luminosity AGN Classes CS PDF

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J.Astrophys.Astr. (2004) 25,1–55 OpticalVariabilityPropertiesofHighLuminosityAGNClasses C.S.Stalin1;2;3,Gopal-Krishna2,RamSagar1 &PaulJ.Wiita4 1AryabhattaResearchInstituteofObservationalSciences(ARIES),ManoraPeak Nainital263129,India. 2NationalCentreforRadioAstrophysics,TIFR,PuneUniversityCampus,Pune411007,India. 3LaboratoiredePhysiqueCorpusculaireetCosmolgie,CollegedeFrance,11pl.Marcelin Berthelot,F-75231,ParisCedex5,France. 4DepartmentofPhysics&Astronomy,MSC8R0314,GeorgiaStateUniversity,Atlanta, Georgia30303-3088,USA. Received2003June24;accepted2004June22 Abstract. We present the results of a comparative study of the intra- night optical variability (INOV) characteristics of radio-loud and radio- quietquasars,whichinvolvesasystematicintra-nightopticalmonitoringof sevensetsofhighluminosityAGNscoveringtheredshiftrangez’0:2to z ’2:2.Thesample,matchedintheopticalluminosity–redshift.MB−z/ plane, consists of seven radio-quiet quasars (RQQs), eight radio lobe- dominatedquasars(LDQs),fiveradiocore-dominatedquasars(CDQs)and sixBLLacobjects(BLs).SystematicCCDobservations,aidedbyacare- ful data analysis procedure, have allowed us to detect INOV with ampli- tudesaslowasabout1%.Presentobservationscoveratotalof113nights (720hours)withonlyasinglequasarmonitoredascontinuouslyaspossi- bleonagivennight.Consideringthecasesofonlyunambiguousdetections ofINOVwehaveestimateddutycycles(DCs)of17%,12%,20%and61% forRQQs,LDQs,CDQs,andBLs,respectively.Themuchloweramplitude andDCofINOVshownbyRQQscomparedtoBLsmaybeunderstoodin terms of their having optical synchrotron jets which are modestly misdi- rectedfromus.Fromourfairlyextensivedataset,nogeneraltrendofacor- relationbetweentheINOVamplitudeandtheapparentopticalbrightnessof thequasarisnoticed.ThissuggeststhatthephysicalmechanismsofINOV andlongtermopticalvariability(LTOV)donothaveaone-to-onerelation- shipanddifferentfactorsareinvolved.Also,theabsenceofaclearnegative correlationbetweentheINOVandLTOVcharacteristicsofblazarsofour samplepointstowardaninconspicuouscontributionofaccretiondiskfluc- tuationstotheobservedINOV.TheINOVdutycycleoftheAGNsobserved in this program suggests that INOV is associated predominantly with the highly polarized optical emission components. We also report new VLA imagingoftwoRQQs.1029C329&1252C020/inoursamplewhichhas yieldeda5GHzdetectioninoneofthem.1252C020IS ’1mJy/. 5GHz Keywords. Galaxies:active—galaxies:jets—galaxies:photometry— quasars:general. 1 2 C.S.Stalinetal. 1. Introduction Thequestionofwhyonlyasmallfractionofquasarsareradio-loudhasbeendebatedfor almostfortyyears.Variousargumentshavebeenputforwardtoexplainthisapparent radio-loud/radio-quietdichotomy.Althoughtheexistenceofthedichotomyhaseven been questioned (e.g., Goldschmidt et al. 1999; White et al. 2000), a recent careful analysisofthetrickyselectioneffectsindicatesthatitmaybereal(Ivezicetal.2002). It has been argued recently that the radio emission correlates with the mass of the nuclear black hole (e.g., Dunlop et al. 2003 and references therein); however, this assertionhasbeenquestioned(Ho2002;Woo&Urry2002).McLure&Dunlop(2001) stressthepossibleimportanceofaccretionrateandchangesinaccretionmodetothis dichotomy. Onthetheoreticalside,twomainapproacheshavebeenputforwardtoexplainthis dichotomy. In one scenario, the jets in RQQs are absent or inherently weak. Some possiblemechanismsidentifythisdifferentiatingfactorwiththespinoftheblackhole (Wilson&Colbert1995;Blandford2000),ormagneticconfigurations(Meier2002). Wills (1996) suggested that all quasars launch jets but those aimed into a galactic diskaredestroyedbyinteractionswiththedensegasandthusappearradio-quiet.An extreme variant of this possibility is the hypothesis that the relativistic jets in RQQs arelargelysnuffedoutbeforeescapingthenuclearregionitselfduetoheavyinverse Comptonlosses(Kundt2002).Asaconsequence,jetsonradioemittingphysicalscales arequenched,eventhoughtheymightemitsubstantialamountsofnon-thermaloptical synchrotronemissiononmicro-arcsecondscales.Unfortunately,such(cid:22)arcsecscales are beyond the reach of any existing imaging telescopes and the only way to probe theirconditionsisthroughfluxvariabilityobservations. Althoughintra-nightopticalvariability(INOV)or“microvariability”wasconvinc- inglyestablishedforblazarsoveradecadeago(e.g.,Miller,Carini&Goodrich1989; Carini, Noble & Miller 1998), the question of whether RQQs also show INOV has remainedcontroversial(Gopal-Krishnaetal.1995,2000;Jang&Miller1995,1997; Rabbette et al. 1998; de Diego et al. 1998; Romero et al. 1999; Carini et al. 1999). Likewise,thecauseofINOVisstillamuchdebatedissue.However,forblazars(CDQs and BL Lacs) which are believed to be dominated by non-thermal Doppler boosted emission from jets (e.g., Blandford & Rees 1978), the occurrence of rapid intensity variations in both the radio and optical bands is believed to be due to shocks propa- gatingdowntheirrelativisticjets(e.g.,Marscher&Gear1985).Intra-nightvariability in blazars may well arise from instabilities or turbulent fluctuations in the flow of such jets (e.g., Hughes, Aller & Aller 1992; Marscher, Gear & Travis 1992). Alter- nate models, which invoke accretion disk instabilities or perturbations (e.g., Man- galam & Wiita 1993; for a review see Wiita 1996) may also explain INOV, partic- ularly in RQQs, where any contribution from the jets, if they are at all present, is weak. WhileconclusiveevidenceforthepresenceofjetsinRQQsisfarfromclear,deep VLAobservationshintatthepresenceofweakjetseveninRQQs(Kellermannetal. 1989;Miller,Rawlings&Saunders1993;Kellermannetal.1994;Papadopoulosetal. 1995; Kukula et al. 1998; Blundell & Beasley 1998; Blundell & Rawlings 2001). The existence of incipient nuclear jets in RQQs have also been inferred from radio spectralindexmeasurementsofopticallyselectedquasarsamples(Falcke,Patnaik& Sherwood1996).IfindeedopticalsynchrotronjetsexisteveninRQQs,thenafairly OpticalVariabilityinLuminousAGNs 3 robustsignatureofsuchun-imageable(becauseatthemicro-arcsecondscale)jetscan comefromdetectionofINOVatthelevelexhibitedbytheirradioloudcounterparts, namelytheLDQs.Topersuetheseissues,aprojecttosearchforINOVinthefourmajor classesofpowerfulAGNswasinitiatedin1998asacollaborativeeffortbetweenthe Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital and the National Centre for Radio Astrophysics (NCRA), Pune. The present paper presents thedetailedresultsofthisproject,partofwhichhasbeenpublishedelsewhere(Gopal- Krishnaetal.2003,hereafterGSSW03;Stalinetal.2004,SGSW04;Sagaretal.2004, SSGW04). 2. Selectionofthesample AGNs in general, have very different observational characteristics including a huge range in luminosity, redshift and power across the electromagnetic spectrum. Also, the co-moving number density of quasars detected at a given absolute magnitude is foundtoundergoarapidevolutionwithredshift(Schmidt&Green1983;Boyleetal. 2000; Wisotzki 2000). Therefore, sample selection is crucial for studying INOV of quasars. To avoid selection biases introduced by differences in luminosity and red- shift,theobjectswereselectedsuchthatallobjectsinagivensethavesimilaroptical magnitudes, in addition to having very similar redshifts; they are thus well matched in the optical luminosity–redshift plane. Our sample, selected from the catalog of Ve´ron-Cetty&Ve´ron(1998),consistsofsevensetsofAGNscoveringatotalredshift range from z D 0:17 to z D 2:2. Each set was designed to consist of a radio-quiet quasar(RQQ),aradiolobe-dominatedquasar(LDQ),aradiocore-dominatedquasar (CDQ) and/or a BL Lac object (BL). These seven sets cover seven narrow redshift intervals centered at z D 0:21, 0.26, 0.35, 0.43, 0.51, 0.95 and 1.92. However, it was not possible to find seven complete sets that satisfied our other observationally dictatedcriteria(sufficientlybrighttoallowdensetemporalsamplingandhavingsev- eral suitable nearby comparison stars), so our entire sample actually consists of 26, not 28, QSOs with the distribution of 7 RQQs, 8 LDQs, 5 CDQs and 6 BLs. The slight unevenness in number is because the two QSOs 1308C326 and 1512C370 were initially selected as CDQs, but better radio data classified 1512 C 370 as an LDQ (see SGSW04), while the blazar 1308 C 326 was found to be a BL Lac (see SSGW04). Due to the paucity of BL Lacs at low z, one object .1215C303/ serves as a member of both Sets 1 and 2, and none was available in the highest redshift bin. The general properties of the objects monitored in this program are given in Table1. ThemagnituderangefortheQSOsinoursampleis−30:0<MB <−24:3(assum- ingH D50kms−1Mpc−1;q D0).Thisimpliesthatwearedealingwithluminous 0 0 AGNs,whicharelegitimatelyclassifiedasQSOs,andinallcasestheunderlyinggalaxy contaminationtotheluminosityoftheAGNis< 10%.Moreover,sincetheAGNof thefourdifferenttypesaresimilarlydistributedinthez−MB plane,thisselectioncri- terionminimisesselectionbiasessuchasK-corrections,evolutionaryeffectsandany otherdifferenceslinkedtoredshiftandluminosity.Conservatively,wehaveincluded in our sample only those RQQs for which the K-corrected ratio of 5GHz to 2500 Å flux densities is less than 1 i.e., R(cid:3) < 1 (see Table 1) even though the usual crite- rionusedtocallaQSOanRQQisR < 10.R D f.5GHz/=f.B/;Kellermannetal. 1989). 4 C.S.Stalinetal. ‡ 77636938372104548438719966 (cid:3)R og− < − − − − − l< < < < e. 1). gramm †%Pol(opt)—0.910.31*8.0—0.780.62—1.411.172.50—0.376.90**4.10——0.531.70—1.420.204.90——0.16 my200 o 1 1 1 11 a pr L nt & ese 2674843791408945677006402615246087304410629740181919 ers thepr z> semek din Hut e )37781632265279478750266810 ( monitor MB(mag−24.−24.−24.−24.−25.−25.−25.−26.−25.−25.−25.−26.−25.−25.−25.−26.−26.−26.−26.−29.−27.−28.−27.−29.−29.−30. (cid:3)(cid:3)and ) cts 990 obje Bmag)6.455.455.606. 92).al.1 ac (11111111111111111111111111 19et ominatedandBLL Dec(2000)C433518−010913C351523C300701−002714C124856C311407C014413C330935C365050C200630C314111−005253−011954C174219C324021C365607C322402C430208C291938−071102C322044C163700C274402C305230C312838 wingStockeetal.((cid:3)dwith(Berriman d oe core- 000)59.456.117.752.033.526.209.419.741.343.048.807.031.835.007.406.009.420.939.612.330.628.738.956.635.924.8 dfollmark minated, RA(209482351131212170516100711311255013715140854110411061218073810320713095802220751035213100238101900151228 calculateeptthose o sc quiet,lobe-d TypeRQQLDQCDQBLRQQLDQCDQRQQLDQLDQBLRQQLDQCDQBLRQQLDQCDQBLRQQLDQBLBLRQQLDQCDQ fluxdensitieal.(1992)ex mationonradio- OthernameUS995−PKS234901CPG1309355CB2121530−1E05140030CPG1004130CB2112831CQ125202003C48.0CB2151237OJ287TON52−PKS1103006−PKS1216010CPKS073517CSO50CB20709373C2323C66ACQJ075129193C94CB2130832CAO0235164TON34CB2001230CB2122531 ÅGHzto2500efromWillset r 5ar Generalinfo ObjectC945438−349014C309355C215303−514005C004130C128315C252020C134329C512370C851202C101319−103006−216010C735178C029329C709370C955326C219428C748294−350073C308326C235164C017279C012305C225317 eratioofthepolarization Table1. Setno.1.02112.0113.10104.11105.10006.00107.101 (cid:3)‡Risth†Optical OpticalVariabilityinLuminousAGNs 5 3. Radioobservationsandreductions SinceradiofluxdatawerenotavailablefortwoofourQSOs(1252C020and1029C 329),inordertoascertaintheirradioclassification,wetookVLA1snapshotsat5GHz in the BnC configuration, in a dual intermediate frequency mode with a total on- sourceintegrationtimeof10minutes.Theresultingimages,madeusingtheCLEAN algorithmwithintheAIPSsoftware,areshowninFig.1(rmsnoise(cid:24)50(cid:22)Jy).While theQSO1252C020wasfoundbeassociatedwithanextendedweakradiosourceof 1(cid:6)0:1mJy,theotherQSO,1029C329,hasnoradiocounterpartdowntoa3-sigma limitof0.15mJy.ThereforebothareproperlyclassifiedasRQQs,withR D0:52and R <0:23,respectively. 1252+020 (Radio Quiet Quasar) (a) 01 45 00 44 45 0) 30 0 0 2 J N ( O 15 TI A N LI C E D 00 43 45 30 12 55 23 22 21 20 19 18 17 16 RIGHT ASCENSION (J2000) Cont peak flux = 8.4612E-04 JY/BEAM Levs = 1.500E-04 * (-2, 1, 2, 4, 8, 16, 32, 64, 128, 256, 512) Figure1. (Continued) 1TheVeryLargeArray(VLA)oftheNationalRadioAstronomyObservatoryisoperated byAssociatedUniversities,Inc.underacooperativeagreementwiththeNationalScience Foundation. 6 C.S.Stalinetal. 1029+329 (Radio Quiet Quasar) (b) 32 42 00 41 30 0) 0 00 0 2 J N ( O TI 40 30 A N LI C E D 00 39 30 00 10 32 14 12 10 08 06 04 02 00 31 58 RIGHT ASCENSION (J2000) Cont peak flux = 7.5534E-04 JY/BEAM Levs = 2.550E-04 * (-2, 1, 2, 4, 8, 16, 32, 64, 128, 256, 512) Figure1(a–b). VLA5GHztotalintensityimagesofthequasars1252C020(left)and1029C329 (right).Thesynthesisedbeamsareshownintheinsetboxes. 4. Opticalobservationsanddatareductions 4.1 Theinstrument The optical observations of the selected quasars were carried out using the 104cm Sampurnanand telescope of the ARIES, Nainital. This is an RC system with a f/13 beam(Sagar1999).Thedetectorusedfortheobservationswasacryogenicallycooled 2048(cid:2)2048pixel2 CCD,exceptpriortoOctober1999,whenasmallerCCDofsize 1024(cid:2)1024pixel2 was used. In each CCD, a pixel corresponds to a square of 0.38 arcsecsize,coveringatotalskyareaofabout120(cid:2)120 inthecaseoflargerCCDand about60(cid:2)60inthecaseofsmallerCCD.ToincreasetheS/Nratio,observationswere carriedoutin2(cid:2)2binnedmode.Practicallyalltheobservationswerecarriedoutusing Rfilter,exceptontwonights,wherequasi-simultaneousRandIfilterobservationswere done.ThechoiceoftheRfilterinthisobservationalprogramisbecauseofitsbeing atthemaximumresponseoftheCCDsystems;thusthetimeresolutionachievablefor eachobjectismaximised. OpticalVariabilityinLuminousAGNs 7 4.2 Observingstrategy The main goal of this program was to obtain lengthy temporally dense data trains for each object on each night it was observed, in order to be able to detect low level intra-night variations at high S/N. The optical monitoring of blazars has shown that theprobabilityofobservingINOVinagivennightisgreatlyenhancedbycontinuous monitoringforatleast3to4hours(Carini1990).Accordingly,weattemptedtomonitor eachobjectforaminimumof5hourspernight,withatimeresolutionoftheorderof 10minutes.However,onoccasionswhentheobjectwasinafaintstate,thesampling timecouldbeupto30minutes. Another important strategy was to center the field of view so as to cover as many suitablecomparisonstarsaspossiblewithintheCCDframe.Foreachofourquasars wecouldgetatleasttwo,butoftenmore,comparisonstarswithin1magnitudeofthe quasar’s brightness in the frame. Several putative sources were eliminated from our sampleiftheyhadnosuitablecomparisonstarsintheirvicinities.Theavailabilityof multiple comparison stars allowed us to identify and discount any comparison star whichitselfvariedduringagivennight,thusallowingreliabledifferentialphotometry of the QSO. The positions and apparent B and R magnitudes of all the comparison starsusedinourdifferentialphotometryaregiveninSGSW04andSSGW04andso willnotberepeatedhere.Theentireprogrammeconsistsofobservationson113nights fromOctober1998throughMay2002,addingupto720hoursofusefulobservations (i.e.,6.4hours/nightonaverage).Alogoftheobservationsalongwiththebasicresults aregiveninTable2. 4.3 Datareduction Preliminary processing of the images as well as the photometry was done using the IRAF2 software. The bias level of the CCD is determined from several bias frames (generally more than seven) taken intermittently during our observations over the night. A mean bias frame was formed using the task zerocombine in IRAF which was then subtracted from all the image frames of the night. Care was also taken in formingthemeanbiasframesuchthattheyarenotaffectedbycosmic-ray(CR)hits. TheroutinestepofdarkframesubtractionwasnotperformedastheCCDsusedinthe observationswerecryogenicallycooledto−120(cid:14)Catwhichtemperaturetheamount of thermal charge is negligible for the exposure times of the present observations. Flatfieldingwasdonebytakingseveraltwilightskyframeswhichwerethenmedian combined to generate the flat field template which was then used to derive the final frames; any errors in the flat field template were well within 0.1%. Finally CR hits seen in the flat fielded target frames were removed using the facilities available in MIDAS3. 2IRAFisdistributedbytheNationalOpticalAstronomyObservatories,whichisoperated by the Association of Universities for Research in Astronomy, Inc. under co-operative agreementwiththeNationalScienceFoundation. 3MIDAS stands for Munich Image and Data Analysis System and has been designed anddevelopedbytheEuropeanSouthernObservatory(ESO)inMunich,Germany. 8 C.S.Stalinetal. kkk kkk kk kkkkkkkk k k k kk rrr rrr rr rrrrrrrr r r r rr ooo ooo oo oooooooo o o o oo toCswww040404www04ww04wwwwwwww04w04w04w04ww Ref.DLsentsentsentSWSWSWsentsentsentGWsentsentGWsentsentsentsentsentsentsentsentSWsentSWsentSWsentSWsentsent rerereGGGrerereSrereSrererererererereGreGreGreGrere PPPSSSPPPSPPSPPPPPPPPSPSPSPSPP §(cid:14)obs 1.8 1.0 §(cid:17)obs 0.74 0.05 0.04 ‡ r) P h ( (cid:28)‡ (hr) 5.0 4.2 >6.8 >4.3 † % 3.5 1.8 2.3 0.9 Os. †Ceff 5.5 4.9 3.3 3.6 S Q e ofth NOV?atusNVNVNVVVVNVNVNVVNVNVVNVNVNVNVNVNVNVNVNVNVNVNVVNVVNVNV V Ist O N I sof T (hr) ult s e r ) sic N obs443124343940393241212827232523353036212642503127283629192019 a ( b d n a 901111911900211199001112290012 s e900000900900000099000000090000 ation Dat5. v 122111200223101121231210120021 r e s b o ogof bjectype)C438Q) −01Q) C355Q) C303 −005Q) C130Q) C315Q) C020Q) Al O(t45Q 49D 09D 15L) 14Q 04D 28D 52Q 9R 3L 3C 2B 5R 0L 1C 2R 2. 0( 2( 1( 1( 0( 1( 1( 1( e bl Ta Setno.1. 2. 3. OpticalVariabilityinLuminousAGNs 9 kkkkkkkk kk kk k kk rrrrrrrr rr rr r rr oooooooo oo oo o oo toCswwwwwwww04040404ww04ww040404w040404040404ww04 Ref.DLsentsentsentsentsentsentsentsentGWGWGWGWsentsentSWsentsentSWSWSWsentSWGWGWGWGWSWsentsentGW rererererererereSSSSrereGrereGGGreGSSSSGrereS PPPPPPPPSSSSPPSPPSSSPSSSSSSPPS §(cid:14)obs 1.60 1.17 1.62 1.421.261.73 1.00 §(cid:17)obs 0.53 0.11 0.55 0.05 6 5. P‡ (hr) 8,3.4, 3.8 1. (cid:28)‡ (hr) 0.4 0.4> 0.8 0.6>8.2 >8.1 † % 2.6 1.2 2.41.2 1.0 2696 8 †Ceff 2.8 2.6 3.12.1 2. NOV?atusNVNVNVNVNVVNVVVVVVNVNVVNVNVVPVNVNVPVVVVVNVNVNVV Ist T (hr) N obs)333246242811151919292248392221212340132819152224112249654343 ( 111222228901901199011222228901 e000000009900900099000000009900 at1. D111000001100000000000000001111 001222202321102211021211112322 ) d e u 9 0 2 9 6 0 8 Contin Object(type)C3432DQ) C1237DQ) C5120L) C0131QQ) −0300DQ) −1601DQ) C3517L) ( 1L 5L 8B 1R 1L 2C 7B 2. 0( 1( 0( 1( 1( 1( 0( e bl Ta Setno.3. 4. 10 C.S.Stalinetal. k k kkk kkk k kkkk kkk r r rrr rrr r rrrr rrr o o ooo ooo o oooo ooo toCs w04w0404www04www040404040404w04wwww0404www Ref.DL sentSWsentSWSWsentsentsentSWsentsentsentGWGWGWGWGWGWsentGWsentsentsentsentSWSWsentsentsent —reGreGGrerereGrerereSSSSSSreSrererereGGrerere PSPSSPPPSPPPSSSSSSPSPPPPSSPPP §(cid:14)obs— 1.21 1.832.501.941.871.39 2.12 7 bs— 3 0 41946 4 §o 1 0 09312 1 (cid:17) 0. 0. 2. P‡ (hr)— 7.0 (cid:28)‡ (hr)— >6.2 4.4 >6.5>5.9> >5.1 3 1 4 745322 0 † %— 1. 1. 1. 8. 3 8 1 206859 6 †Ceff— 4. 2. 3. 2.6.> >6. NOV?atus—NVVNVNVVNVNVNVVNVNVNVPVVVVVVNVVNVNVNVNVNVNVNVNVNV Ist )403851521955395711011631404657 T hr8. ( 1 s)599017991986748332319266820165 N b411231223443331278075252233322 o 11 1 ( 900122111119008900000899001111 e900000000009009900000999000000 at3. D000000000110001111111101111111 100200222221001122020110202111 ) d e u 9 0 6 8 4 3 Contin Object(type)C2932QQ) C0937DQ) C5532DQ) C1942L) C4829QQ) −5007DQ) ( 0R 7L 9C 2B 7R 3L 2. 1( 0( 0( 0( 0( 0( e bl Ta Setno.5. 6.

AGNs, which are legitimately classified as QSOs, and in all cases the underlying galaxy contamination to the luminosity of the AGN is < 10%. Moreover
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