ApJ, 663, 103, 2007

15 Pages·2007·1.7 MB·English

TheAstrophysicalJournal,663:103Y117,2007July1
#2007.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A.
THE INTRINSICALLY X-RAY WEAK QUASAR PHL 1811. I.
X-RAY OBSERVATIONS AND SPECTRAL ENERGY DISTRIBUTION1
Karen M. Leighly
HomerL.DodgeDepartmentofPhysicsandAstronomy,UniversityofOklahoma,440WestBrooksStreet,
Norman,OK73019;[email protected]
Jules P. Halpern
DepartmentofAstronomy,ColumbiaUniversity,550West120thStreet,NewYork,NY10027-6601
Edward B. Jenkins
PrincetonUniversityObservatory,Princeton,NJ08544-1001
Dirk Grupe
DepartmentofAstronomyandAstrophysics,PennsylvaniaStateUniversity,525DaveyLab,UniversityPark,PA16802
and
Jiehae Choi2 and Kimberly B. Prescott
HomerL.DodgeDepartmentof PhysicsandAstronomy,UniversityofOklahoma,440WestBrooksStreet,Norman,OK73019
Received2006March24;accepted2006October16
ABSTRACT
Thisisthefirstoftwopapersreportingobservationsandanalysisoftheunusuallybright(m ¼14:4),luminous
b
(M ¼(cid:1)25:5),nearby(z¼0:192)narrow-linequasarPHL1811,focusingontheX-raypropertiesandthespectral
B
energydistribution.TwoChandraobservationsrevealaweakX-raysourcewithasteepspectrum.Variabilitybya
factorof4betweenthetwoobservationsseparatedby12dayssuggeststhattheX-raysarenotscatteredemission.The
XMM-Newtonspectraaremodeledinthe0.3Y5keVbandbyasteeppowerlawwith(cid:1)¼2:3(cid:2)0:1,andtheupper
limitonintrinsicabsorptionis8:7;1020cm(cid:1)2.Thespectralslopesareconsistentwithpower-lawindicescommonly
observedinNLS1s,anditappearsthatweobservethecentralengineX-raysdirectly.IncludingtworecentSwiftToO
snapshots,afactorof(cid:3)5variabilitywasobservedamongthefiveX-rayobservationsreportedhere.Incontrast,theUV
photometry obtained by the XMM-Newton OM and Swift UVOT, and the HSTspectrum reveal no significant UV
variability.The(cid:1) inferredfromtheChandraandcontemporaneousHSTspectrumis(cid:1)2:3(cid:2)0:1,significantlysteeper
ox
thanobservedfromotherquasarsofthesameopticalluminosity.Thesteep,canonicalX-rayspectra,lackofabsorption,
and significant X-ray variability lead usto conclude that PHL 1811 isintrinsically X-ray weak. We also discuss an
accretiondiskmodelandthehostgalaxyof PHL1811.
Subject headingsg: quasars: emission lines — quasars: individual (PHL 1811) — X-rays: galaxies
1. INTRODUCTION ofannulilocallyemittingasblackbodies,thecontinuumspectrum
fromtheaccretiondiskshoulddependontheblackholemassand
The standard model for active galactic nuclei (AGNs) and
accretionrate(e.g.,Franketal.1992).Moresophisticatedaccre-
quasi-stellarobjects(QSOs)proposesthatthebroadbandoptical
tion disk models that include a range of physical processes ex-
and UV continuum originates in an accretion disk. The X-ray
pectedtobeimportantalsopredictarangeofshapes(e.g.,Kato
emissionisaseparatecomponent,producedinacoronalocatedin
et al. 1998). Another complication is that the type of accretion
thevicinityofthedisk,thatcreatestheX-raysbyinverseCompton
diskpresentispredictedtodependontheaccretionraterelative
scatteringthediskphotons.Thisbroadbandcontinuumisthen
totheEddingtonvalue(e.g.,Chenetal.1995),andthetypeof
thoughttoilluminatethegasthatformsthebroad-lineregion,
accretiondiskcanbedifferentatdifferentradii(e.g.,Svensson&
causingittoemitlinesviaphotoionization.
Zdziarski1994).TheX-rayemittingcoronaaddsanotherdimen-
Despitethecommonalityoftheoriginoftheopticalthrough
sionofcomplication,asitsoriginandgeometryarenotverywell
X-raycontinuumemissionandemissionlines,therearetheoret-
understood.Thus,inprinciple,thecoronalemissionmayormay
icalreasonsthatthespectrashouldvaryamongindividualobjects.
notbeimportantdependingonhowmuchoftheaccretionenergy
Theoriginofdifferencesmaybeextrinsic;forexample,thebright-
isfunneledtoit,andindeed,weseeevidenceforarangeofcoro-
nessand,tosomeextent,theshapeofthecontinuumspectrumof
nalactivityinX-raynovae(e.g.,Kubota&Done2004).
theaccretiondiskshouldvarywithviewingangle(e.g.,Laor&
Inadditiontothetheoreticalexpectationofarangeofpredicted
Netzer1989).Theoriginmayalsobeintrinsic.Evenforverysim-
spectralenergydistributionsamongAGNs,thereisobservational
plediskmodels,inwhichthespectrumisconstructedfromasum
evidence thatsucha range exists.In astudy ofthemultiwave-
lengthpropertiesofanX-rayselectedheterogeneoussampleof
1 BasedonobservationsobtainedwithXMM-Newton,anESAsciencemis- quasars, Elvis et al. (1994) observed a wide range of spectral
sionwithinstrumentsandcontributionsdirectlyfundedbyESAMemberStates
energy distributions (SEDs). Wilkes et al. (1994) and more re-
andNASA.
2 Currentaddress:DepartmentofAstronomy,NewMexicoStateUniversity, centlyBechtoldetal.(2003),Stratevaetal.(2005),andSteffen
P.O.Box30001,MSC4500,LasCruces,NM88003-8001. etal.(2006)foundthat(cid:1)ox,thepoint-to-pointslopebetweenthe
103
104 LEIGHLY ETAL. Vol. 663
TABLE1
ObservingLog
Exposure Bandpassor ExtractionRadius
ObservatoryandInstrument Date (s) EffectiveWavelength (arcsec)
ChandraACIS-S3............................................... 2001Dec5 9377 0.3Y10keV 3.94
ChandraACIS-S3............................................... 2001Dec17 9839 0.3Y10keV 3.94
XMM-NewtonPN................................................. 2004Nov1 27472 0.3Y12keV 27
XMM-NewtonMOS1........................................... ... 32126 0.5Y10keV 23
XMM-NewtonMOS2........................................... ... 32164 0.5Y10keV 23
XMM-NewtonOM(UVM2)................................ ... 25800 23108 12
SwiftXRT(PCmode)......................................... 2005Oct22 2462 0.3Y10keV 23.4
SwiftUVOT(UVW1)......................................... ... 787 26008 12
SwiftUVOT(UVM2)......................................... ... 844 22008 12
SwiftUVOT(UVW2)......................................... ... 844 19308 12
SwiftXRT(PCmode)......................................... 2006May12 1600 0.3Y10keV 23.4
SwiftUVOT(V).................................................. ... 130 54608 6
SwiftUVOT(B)................................................... ... 130 43508 6
SwiftUVOT(U).................................................. ... 130 34508 6
SwiftUVOT(UVW1)......................................... ... 259 26008 12
SwiftUVOT(UVM2)......................................... ... 372 22008 12
SwiftUVOT(UVW2)......................................... ... 526 19308 12
MDMMcGraw-Hill1.3+I+Templeton.......... 2004Oct14Y16 5,3,1a 80508 5.8Y10.0
MDMMcGraw-Hill1.3+V+Templeton......... ... 5,9,5a 53808 5.7Y10.1
MDMMcGraw-Hill1.3+B+Templeton......... ... 5,5,2a 43508 6.2Y10.0
MDMMcGraw-Hill1.3+U+Templeton......... ... 5,5,2a 36408 6.8Y10.2
a Numberofframesoneachofthethreedays.
25008and2keV,isinverselycorrelatedwiththeUVluminosity. XMM-Newton,andSwiftobservations,aswellastheresultsofa
Inmanyobjects,thelargerangeofspectralenergydistributions 3dayopticalphotometryrunatMDMObservatory.Wealsocom-
can be accounted for by extrinsic effects suchas reddening and pare the XMM-Newton spectrum of PHL 1811 with those from
absorption(e.g.,Brandtetal.2000).However,theseextrinsicef- othernarrow-lineSeyfert1galaxies(NLS1s).Inx3wecomment
fectscertainlycannotaccountfortherangeofSEDsinallobjects. onthelongtimescaleX-rayandUVvariability.Wepresentan
PHL 1811 is a nearby (z¼0:192), luminous (M ¼(cid:1)25:5) updatedspectralenergydistributioninx4.Inx5,wediscussthe
B
narrow-linequasar.PHL1811wasfirstcatalogedasablueob- natureoftheintrinsicX-rayweaknessandpresentanaccretion
ject in the Palomar-Haro-Luyten plate survey (Haro & Luyten diskmodelforPHL1811.Wealsocommentontheapparentspiral
1962). It was then rediscovered in the optical follow-up of the host galaxy discovered in the image presented by Jenkins et al.
VLAFaintImagesoftheRadioSkyatTwentycm(FIRST)sur- (2005).Wesummarizeourfindingsinx6.PaperIIdescribesthe
vey(Whiteetal.1997;Beckeretal.1995).Itisextremelybright HSTandground-basedopticalandUVobservationsandpresents
(B¼14:4,R¼14:1);itisthesecondbrightestquasaratz>0:1 Cloudymodelsthatexploretheunusualemission-lineproperties.
after3C273.Becauseitissobright,itisaverygoodbackground SomeoftheresultswerepresentedpreviouslyinLeighlyetal.
source for studies of the intergalactic and interstellar medium; (2004),Choietal.(2005),andPrescott(2006).Weassumeaflat
furthermore, a FUSE observation found its spectrum to have a universe with H ¼70 km s(cid:1)1 Mpc(cid:1)1 and (cid:2) ¼0:73 unless
0 vac
rareLymanlimitsystemthathasbeenstudiedbyJenkinsetal. otherwisespecified.
(2003,2005).Itwasodd,however,thatsuchabrightquasarwas
notdetectedintheROSATAllSkySurvey(RASS).Incompari- 2. OBSERVATIONS AND ANALYSIS
son with other quasars of its luminosity, the expected RASS
count rate is about 0.5 s(cid:1)1; we placed an upper limit of 1:3; 2.1. Chandra Observations and Analysis
10(cid:1)2countss(cid:1)1(Leighlyetal.2001).ApointedBeppoSAXob- TheChandraobservationsweremadeinimagingspectroscopy
servationdetectedtheobject,butitwasstillanomalouslyweak. modewiththeimageofPHL1811placedontheACIS-S3de-
Too fewphotonswereobtained in the BeppoSAXobservation tector.TheobservinglogisgiveninTable1.Weverifythatthe
tounambiguouslydeterminethecauseoftheX-rayweakness; positionoftheX-raysourceisconsistentwiththatofthequasar
Leighlyetal.(2001)speculatedthateitheritisintrinsicallyX-ray (Fig.1).
weak,oritisanearbybroadabsorptionlinequasarandtheX-ray Thelevel2eventfileswererecreatedusingthestandardpro-
emissionisabsorbed,oritishighlyvariable,andwecaughtitboth cedure.Thesmallcorrectionforthetime-dependentgainwasap-
timesinalowstate. pliedusingthecorr_tgainprogram,andthecorrectionforthe
In this paper and the companion paper (Leighly et al. 2007, time-dependentancillarymatrixwasmadeusingtheIDLprogram
hereafterPaperII)wereporttheresultsofseveralUVandX-ray acisabs.pro.3Thetotalcountratesobservedwithinacircular
observationsofPHL1811designedtoexploretheoriginandcon- region3.9400inradiuswere9:7;10(cid:1)3and4:0;10(cid:1)2countss(cid:1)1
sequencesoftheX-rayweaknessofthisobject.First,coordinated fromthefirstandsecondobservations,respectively.Theserates
ChandraandHSTobservationsweremadein2001.In2004,an arelowenoughthatpileupisnegligible.Between0.3and9.8keV,
XMM-Newtonobservationwasmade,andmostrecently,PHL
1811wasthetargetoftwoSwiftTargetofOpportunityobserva-
tions.Inx2wedescribetheresultsandanalysisoftheChandra, 3 Seehttp://www.astro.psu.edu/users/chartas/xcontdir/xcont.html.
No. 1, 2007 INTRINSICALLY X-RAY WEAK QUASAR PHL 1811. I. 105
Fig.1.—Chandraimage(left)andMDMopticalR-bandimage(right)showingthattheX-raysourceissecurelyidentifiedasPHL1811.
atotalof 81and374photonswereobtained.Basedontheback- warmabsorber,andironlineasnecessary.Thus,theChandra
groundcollectedfromsource-freeareasofthechips,weexpect1 spectrafromPHL1811,fittedover0.3Y5.0keV(0.36Y5.96rest
and3ofthesephotonstooriginateinthebackground.Thus,we frame),areconsistentwiththehardX-raypowerlawfoundin
canconcludethatweobserveasignificantchangeinfluxinthe ASCAspectraof NLS1s.
objectbyaboutafactorof4betweenthetwoobservationssep- Thebest-fittingphotonindexissteeperforthesecond,brighter
aratedby12days. observation,suggestingthatshape ofthe spectrumhas changed
Asufficientnumberofphotonswerecollectedinthesecond betweenthetwoobservations.However,theuncertaintiesindicate
observationtolookforshorttimescalevariability.Weusedthe thatthedifferenceisnotstatisticallysignificant.Fittingthespectra
BayesianBlocksprogramavailableintheISISsoftware4butdid simultaneouslyandusingtheF-testshowsthattheimprovement
not find any indication of variability during the observation at inthefitrepresentedbyachangeinthephotonindexissignificant
significancelevelsgreaterthanabout1(cid:2).Wealsotriedbinning atonlythe68%confidencelevel.Figure2showsthesemodelfits.
the light curve with 100 s bins and then grouping together by Inordertoseewhetherthereisanyevidenceforintrinsicab-
handpointsthatappearedloworhigh.The(cid:3)2foraconstantmodel sorption,wenextfitthespectrumfromthesecondobservation
was13.5for6degreesoffreedom(dof),indicatingthatvariability withamodelconsistingofapowerlaw,absorptionfixedatthe
wasmarginallydetectedwithconfidenceof96.4%,althoughthe Galacticvalue,andabsorptionintherestframeofthequasar.We
probabilitythatthisvariabilityissignificantmaybelowerbecause findnoimprovementinthefit,andthebest-fittingvalueofaddi-
theresultsarebiasedbypreselectingthebinsizes.Notethatthe tionalintrinsicabsorptioniszero.The(cid:3)(cid:3)2 ¼2:71upperlimit
(cid:3)(cid:3)2 ¼6:63uncertaintyonthefittedconstantmodelis14%,in- ontheintrinsicabsorptioncolumnis1:8;1020cm(cid:1)2.Theupper
dicatingthatweareonlysensitivetovariationslargerthanthis limitisratherlowdespitethepoorphotonstatisticsinthespec-
valueatthe99%confidencelevel.Suchhigh-amplitudevariations trumbecausethespectrumisconvex(Fig.2).
arerarebutnotunprecedentedinluminousNLS1s(e.g.,Leighly Theconvexresidualsofthepower-lawfittothesecondobser-
1999a). vationsuggestthepresenceofasoftexcess(Fig.2).Softexcess
The spectra were accumulated and grouped so there were componentsarecommoninthespectraofNLS1sandinthecase
(cid:3)20photonsineachbin.Wefirstfittedeachspectrumbetween ofpoorstatisticscanbefittedadequatelybyablackbodymodel
0.3and5keVseparatelywithapowerlawandfixedGalacticcol- (e.g., Leighly 1999b). We add a blackbody component to the
umn of 3:76;1020 cm(cid:1)2, obtained using the HEASARC n power-lawmodelforthesecondspectrumandfindthatthefit
H
tool.5WenotethattheGalacticLy(cid:1)lineinthemedium-resolution improves by (cid:3)(cid:3)2 ¼4:9, and the residuals are flat (Table 2).
UVspectrumofPHL1811(Jenkinsetal.2005)isconsistentwith However,theimprovementinthefitisnotstatisticallysignificant;
this value of N . This model fits both spectra well, yielding theF-testshowsthattheimprovementinfitissignificantatonly
H
photonindicesof 2:01þ0:37and2:58þ0:19forthefirstandsecond the 71% confidence level.Thus,wecannotconcludethata soft
(cid:1)0:36 (cid:1)0:18
Chandraobservations,respectively.Thesephotonindicesare excessispresent,becauseofthepoorstatistics.
consistentwiththoseobservedfromNLS1sbyASCA(Leighly Thepower-lawindexforthepowerlawplusblackbodyfitto
1999b).NotethattheASCAphotonindicesweretakenfrommod- the second observation is flatter than for the power law alone
els spanning the (cid:3)0.5Y10 keV band that include a soft excess, (2:22(cid:2)0:34 vs. 2:58þ0:19) and is now completely consistent
(cid:1)0:18
withthatofthefirstobservation(2:01þ0:37).Thissuggeststhat
(cid:1)0:36
4 Seehttp://space.mit.edu/CXC/analysis/SITAR/index.html. thespectralvariabilityoriginatesasanemergenceoftheblack-
5 Availableathttp://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3nh/w3nh.pl. bodycomponentwhentheobjectisbrighter.Toinvestigatethis
106 LEIGHLY ETAL. Vol. 663
possibility,wefitbothspectrasimultaneouslywithapowerlaw
plusblackbodymodel,fixingthenormalizationoftheblackbody
forthefirstobservationtozero.Themodelfitsthedataadequately
((cid:3)2 ¼0:67for16dof).Thejointlyfittedphotonindexis2:12(cid:2)
(cid:4)
0:25, and for this parameterization, the normalizations of the
powerlawdifferbyafactorof3.5.
Spectralratiosprovideacomplementaryandmodel-independent
approachtothequestionofspectralvariability.Werebinthespec-
trumfromthesecondobservationtothebinningofthefirstspec-
trumandtaketheirratio(Fig.2,inset).Thisshowsthatthespectral
variabilityispredominatelyinthesoftestband,supportingourhy-
pothesisthatavariablesoftexcessisresponsibleforthespectral
variability. In this case, the variability of the power-law compo-
nentisafactorof3.5.The(cid:3)2foraconstant-ratiomodelis3.8for
3degreesoffreedom,whichsignificantatthe71%confidence
level.
To summarize the results of the Chandra observations, we
findconclusiveevidenceforfactorof(cid:3)4variabilitybetweenthe
twoobservationsseparatedby12days.Detailedanalysisisham-
peredbypoorstatistics;however,wefindmarginalevidence(2(cid:2))
forvariabilityontimescalesofthousandsofsecondsinthesecond
observation,whentheobjectwasbrighter.Spectralfittingreveals
asteepspectrumwithnoevidenceforintrinsicabsorption,and
Fig. 2.—Top:Unfoldedbest-fittingpower-lawmodelsforthespectrafrom
marginalevidenceforspectralvariabilitybetweentheobserva-
thetwoChandraobservations.Bottom:Residualsfromapower-lawmodelfitto
thespectrumfromthesecondobservation,withaconvexshapesuggestingthe tions, with the spectrum becoming steeper when the object is
presenceofasoftexcess.Inset:Ratioof thesecondspectrumtothefirstspec- brighter.Thespectralfittingandtheratioofthespectrasuggest
trumshowingthatthespectralvariabilityispredominatelyconfinedtothesoftest thatthespectralvariabilityiscausedbytheemergenceofasoft
energies,suggestingtheemergenceofasoftexcesscomponentwhentheobjectis
excesscomponentwhentheobjectisbright.Themeasuredspec-
bright.
tralindicesrangebetween2.0and2.6,dependingonthemodel.
TheyareconsistentwiththephotonindicesmeasuredinASCA
observationsof NLS1s(Leighly1999b).Thisfactsuggeststhat
weseetheintrinsicX-rayemissionfromthecentralengineand
TABLE2
ChandraSpectralFittingResults
Parameter 2001Dec5 2001Dec17
Power-LawModel,SpectraFittedSeparately
Photonindex.................................................. 2:01þ0:37 2:58þ0:19
(cid:1)0:36 (cid:1)0:18
Normalizationa............................................... (1.17(cid:2)0.02) ; 10(cid:1)5 (5.10(cid:2)0.45) ; 10(cid:1)5
Flux(0.3Y5keV;ergcm(cid:1)2s(cid:1)1)................... 5.2 ; 10(cid:1)14 2.3 ; 10(cid:1)13
Luminosity(0.3Y5keV;ergs(cid:1)1).................. 5.6 ; 1042 2.4 ; 1043
(cid:3)2/dof............................................................. 1.96/2 13.2/15
PowerLaw+Blackbodyb
Photonindex.................................................. ... 2.22(cid:2)0.34
Power-lawnormalizationa.............................. ... 4:4þ0:83;10(cid:1)5
(cid:1)1:0
Blackbodytemperature(keV)....................... ... 0:096þ0:040
(cid:1)0:081
Blackbodynormalizationc.............................. ... 1:5þ1 ;10(cid:1)6
(cid:1)1:1
(cid:3)2/dof............................................................. ... 8.36/13
PowerLaw+Blackbody,bJointFit
Photonindex.................................................. 2.12(cid:2)0.25d 2.12(cid:2)0.25d
Power-lawnormalizationa.............................. (1.17(cid:2)0.22) ; 10(cid:1)5 4:14þ0:79;10(cid:1)5
(cid:1)0:84
Blackbodytemperature(keV)....................... 0:10þ0:035d 0:10þ0:035d
(cid:1)0:042 (cid:1)0:042
Blackbodynormalizationc.............................. 0e 1:5þ2:7 ;10(cid:1)6
(cid:1)0:82
(cid:3)2/dof............................................................. 10.8/16d 10.8/16d
a InunitsofphotonskeV(cid:1)1cm(cid:1)2s(cid:1)1at1keVintheobservedframe.
b Theblackbodytemperatureisgivenintherestframe.
c InunitsofL /D2,whereL isthesourceluminosityinunitsof1039ergs(cid:1)1,andD isthedistanceto
39 10 39 10
theobjectinunitsof10kpc.
d Parameterfittedjointly.
e Fixedparameter.
No. 1, 2007 INTRINSICALLY X-RAY WEAK QUASAR PHL 1811. I. 107
TABLE3
XMM-NewtonSpectralFittingResults
Parameter Measurement
Photonindex...................................................................... 2:28þ0:12
(cid:1)0:11
PNnormalizationa,b............................................................ 1.16(cid:2)0.98
MOS1normalizationoffseta.............................................. 1:25þ0:21
(cid:1)0:19
MOS2normalizationoffseta.............................................. 1:32þ0:22
(cid:1)0:20
Flux(0.3Y5keV;ergcm(cid:1)2s(cid:1)1)........................................ 5.1 ;10(cid:1)14
Luminosity(0.3Y5keV;ergs(cid:1)1)....................................... 5.4 ; 1042
(cid:3)2/dof................................................................................. 74.7/79
a ThespectralmodelwasCexp½NH(Gal)(cid:2)(E)(cid:4)E(cid:1)(cid:1),whereallparameterswere
tiedtogetherexceptfortheconstantC,whichwasfixedto1forthePNandallowed
tovaryfortheMOS1andMOS2.Unexpectedopticalloadingplausiblycausesthe
MOSspectratohaveahighernormalization;therefore,wequotefluxandlumi-
nosityvaluesfromthePNdataonly.
b InunitsofphotonskeV(cid:1)1cm(cid:1)2s(cid:1)1at1keVintheobservedframe.
Fig. 3.—XMM-Newtonbackground-subtractedlightcurvecomposedofthe
0.5Y5keVphotonsfromtheMOS1,MOS2,andPNdetectors.Thelowerlight
curveshowstheestimatedbackgroundinthesourceextractionregion.Thereis 10(cid:1)3countss(cid:1)1.Thelightcurveisstatisticallyconsistentwithno
nostrongevidenceforvariabilityoverthe31ksobservation. variability((cid:3)2 ¼33:4for30dofforaconstantmodel).Notethat
the(cid:3)(cid:3)2 ¼6:63uncertaintyonthefittedconstantmodelis9%,
that the X-rays are powered by inverse Compton scattering of indicatingthatweareonlysensitivetovariationslargerthanthis
softphotonsasinotherAGNs. valueatthe99%confidencelevel.Suchhigh-amplitudevariations
arerarebutnotunprecedentedinluminousNLS1s(e.g.,Leighly
2.2. XMM-Newton Observation and Analysis 1999a).
Thespectrawereextractedusingtheregionsdescribedabove
PHL1811wasobservedbyXMM-Newtonon2004November1
and grouped so that there were 15photonsper energybin.We
usingtheEPICPN(Stru¨deretal.2001)andMOS(Turneretal.
simultaneouslyfitthePNinthe0.3Y5keVrangeandMOSinthe
2001) instruments and the Optical Monitor (OM; Mason et al.
0.5Y5keVband.Thespectrayield611,191,and204sourcepho-
2001).TheEPICobservationswerecarriedoutusingthethinfil-
tonsforthePN,MOS1,andMOS2,respectively.Thespectraare
terinPrimeFullWindowmode.Thedatawerereducedusingstan-
fittedverywellwithamodelconsistingofaconstant,apowerlaw,
dardselectioncriteria.Theobjectwasobservedfor32.1ksusing
andaGalacticabsorptioncolumndescribedinx2.1.Theresults
theMOSdetectorsandfor27.5ksusingthePN.Thedetailsofthe
arelistedinTable3,andthemodelfitisshowninFigure4.Note
observationaregiveninTable1.
thatunlikethesecondChandraobservation,theresidualsareflat,
BackgroundflaresareaconcerninXMM-Newtondataanaly-
andthereisnoevidenceforasoftexcesscomponent.
sis,andweobservedthebackgroundtovaryduringtheobserva-
Thespectraareadequatelyfittedusingasteeppowerlaw((cid:1)¼
tion.Formostoftheobservation,thebackgroundwasrelatively
2:3(cid:2)0:1), typical of that from NLS1s observed by ASCA
lowandstable.However,evenatthelowestrate,itappearstobe
slightlyelevatedcomparedwiththequiescentrate6byafactorof (Leighly 1999b). There is no evidence for additional absorp-
tion.Weaddaneutralabsorptioncomponentattheredshiftof
approximately2.3forthePNand1.4intheMOS1+MOS2in
boththesoft(0.5Y2.0keV)andhard(2.0Y10keV)bands.Inad- PHL1811tothemodelbutfindnosignificantreductionin(cid:3)2.
The90%confidenceupperlimit((cid:3)(cid:3)2 ¼2:7)onadditionalab-
dition,forthefirst3000sinthePNdetector,andthefirst5000s
sorptionisN ¼8:7;1020 cm(cid:1)2.
for the MOS detector, there occurred a small background flare H
thatwashigherthanthequiescentratebyafactorof2Y3.Weex-
tractedandanalyzedspectrawithandwithoutthisflaringperiod
andconcludethattheflareissosmallthatitdoesnotadversely
affecttheresults.Therefore,weanalyzetheentireexposure.
WeextractlightcurvesfromthePNandMOSdetectors.The
target is relatively weak, so we use a source extraction region
with a radius corresponding to an encircled energyfunction of
80%;theradiuswas2700 forthePNand2300 fortheMOS1and
MOS2.Backgroundlightcurvesandspectrawereextractedfrom
nearby,source-freeregionsofthedetector.Thebackgroundspec-
trumisflatterthanthesourcespectrum,andwefindthattheback-
groundcontributiontotheemissionintheextractionapertureis
equaltothatofthetargetat(cid:3)5keV.Thebackgrounddominatesat
higherenergies;therefore,weextractlightcurvesinthe0.5Y5keV
band. We use a bin size of 1000 s, which yields an average of
(cid:3)30sourcecountsperbin,andthebackgroundcontributesabout
12%ofthetotalcounts.Theresultingbackground-subtractedlight
curveisshowninFigure3.Themeancountrateisð2:8(cid:2)0:17Þ;
Fig. 4.—XMM-NewtonMOS1,MOS2,andPNspectra(opensquares,open
6 SeeXMM-NewtonUser’sHandbook,x3.3.8(http://heasarc.gsfc.nasa.gov/ triangles,andfilledcircles,respectively),wellfittedusingapowerlawandGalactic
docs/xmm/uhb/node38.html). absorption.TheupperlimitonintrinsicX-rayabsorptionis8:7;1020cm(cid:1)2.
108 LEIGHLY ETAL. Vol. 663
Theconstantinthemodelisfixedtoavalueof1forthePN the(cid:3)2is70.5for70degreesoffreedom.Thebest-fittingphoton
spectrumandallowedtobefreefortheMOS1andMOS2spec- index is 2:36þ0:12, again typical of an NLS1 (Leighly 1999b).
(cid:1)0:11
tra.ItcanbeseeninTable3thatthebest-fittingnormalizationsof Wenoticethatthenormalizationforthespectrumfromthefirst
MOS1andMOS2spectraare25%and32%higherthanthatof ChandraobservationisidenticaltothatoftheXMM-NewtonPN
thePN,respectively.Thisdifferencecannotbeexplainedbyre- spectrum; if we tie these two parameters together, there is no
sidual calibration uncertainty between the XMM-Newton EPIC changein(cid:3)2((cid:3)2 ¼70:5for71degreesoffreedom).Thesetwo
instrumentsasthatisnowquitelow(Stuhlingeretal.2006).We spectraseemtodescribetheobjectinthesamestate(seealsox3),
analyzetheX-rayspectrafromanotherobjectinthefield(located soweleavetheirparameterstiedtogetherthroughouttheremain-
at R:A:¼21h54m41s, decl:¼(cid:1)9(cid:5)2604900) that is about 50% derofthissection.
brighterthanPHL1811.ThenormalizationsoftheEPICspec- Next, we allow the photon index of the second, higher flux
traforthisobjectwerecompletelyconsistentwithoneanother Chandraobservationtobefittedindependentofthatofthefirst
(MOS1constant:1:00(cid:2)0:12;MOS2constant:1:00þ0:13).An- ChandraspectrumandtheXMM-Newtonspectrum.Weobtaina
(cid:1)0:12
otherobjectinthefieldofviewwasindependentlyanalyzedby somewhatbetterfitwith(cid:3)2 ¼64:1for70degreesoffreedom.
anotheroftheauthors(D.Grupe)withthesameresult. However,thedecreasein(cid:3)2issignificantatonlythe63%level
WhilethesmallnumbersofphotonsinthePHL1811spectra accordingtotheF-test.Theresultingphotonindicesare2:22(cid:2)
meanthatthedifferenceinnormalizationsamongthemodelsfor 0:14 for the low state (first Chandra observation and XMM-
thedifferentEPICspectraisnotstatisticallysignificant,therestill Newton PN spectrum) and 2:57þ0:19 for the second, brighter
(cid:1)0:18
seems to be a problem with the spectra that needs to be under- Chandraobservation.Theseindicesarestillwithintherangeof
stood.Webelievethattheproblemoriginatesinopticalloading. power-lawindicesobservedfromNLS1s(Leighly1999b).Inad-
ThenominallimitforopticalloadingusingthethinfilterisV (cid:6) dition,itisoftenfoundthattheNLS1spectrasoftenaswhenthey
12forboththePNandtheMOS(Smith2004;Altieri2003).PHL are brighter (e.g., Fig. 4 of Leighly 1999a); thus, intrinsic soft-
1811isafainteropticalsource(B¼14:4,R¼14:1);however,it eningofthespectrumisaplausibleexplanationforthemarginal
hasaverybluespectrumandisaveryweakX-raysource,andthus spectralvariabilityweobserve.
isanunusualobjectcomparedwiththestellarcalibrationsources Itispossiblethatthevariabilityresultsfromvariablecoldab-
usedtodeterminetheloadinglimits.Anotherpointthatsupports sorption.Wetestthisscenariobyconstrainingthephotonindices
ourcontentionthatopticalloadingisimportantisthefacttheop- andnormalizationstobethesameforallthreespectra,andin-
ticalloadingintheMOS2isobservedtobeabout7%largerthan cludinganeutralabsorptioncolumninthequasarrestframein
that in the MOS1, possibly due to variations in the filter trans- themodel,allowingtheabsorptioncolumntovary.Thismodel
mission(Altieri2003);wealsoobserveahighernormalizationfor gives a very poor fit; the reduced (cid:3)2 ¼2:57 for 70 degrees of
the MOS2 spectrum. However, the degree of contamination by freedom.Thus,wecanrejectonstatisticalgroundstheideathat
opticalphotonscannotbeverylarge,becausetheMOSspectrado thespectralandfluxvariabilityoriginatessolelyinvariableneu-
notshowanyobservabledistortion;apower-lawfittothemalone tralabsorption.Allowingthenormalizationstoalsobefreeyields
yieldsanidenticalphotonindexwiththatobtainedfromthePN. agoodfit((cid:3)2 ¼65:3for69degreesoffreedom),althoughitisnot
Nevertheless, given the fact that the MOS spectra are possibly asignificantimprovementovertheno-absorptionmodel((cid:3)(cid:3)2 ¼
contaminatedbyopticalloading,wemeasurethefluxfromthePN 5:2, significant at the 58% confidence level according to the
spectrum,notingthatwecannotbesurethatthisspectrumisun- F-test). The additional absorption is consistent with zero for
contaminatedbyopticalloadingaswell,andmayrepresentanup- thebrighterChandraobservationwithanupperlimitof1:4;
perlimitonthefluxforthisobservation. 1020 cm(cid:1)2 and is equal to 5:0þ3:9 ;1020 cm(cid:1)2 for the fainter
(cid:1)3:6
PHL 1811was observed using the Optical Monitor withthe Chandra observation and the XMM-Newton observation. Al-
UVM2filter.Tenexposures,eachwithdurationof 2580s,were thoughthismodel(variablecoldabsorptionplusvariablepower-
made.TheSAStaskomichainwasruntoreprocessthedata.The lawnormalization)fitsthespectrawell,thescenarioseemsunlikely
countrateinformationwasextractedfromtheomichainoutput. becauseitrequiresthatabsorbervariabilitybecoordinatedwith
Inaddition,thecountrateswereextractedfromtheimagesus- intrinsicfluxvariability.
ingtheIRAFtaskphot.Thecountrateswerethenconvertedto
flux using the conversion factor for the UVM2 filter (2:17; 2.4. The X-Ray Spectra of NLS1s: How Does
10(cid:1)14ergs(cid:1)1cm(cid:1)28(cid:1)1count(cid:1)1).Theresultsfromthetwoex- PHL 1811 Compare?
tractionprocedureswereconsistenttowithin2%,andtherefore ASCA observations of NLS1s demonstrate that their spectra
hereafterwediscusstheresultsfromtheomichainoutput. cangenerallybedescribedbyahardpowerlawwithanaverage
OneofthegoalsoftheOMobservationwastolookforUV indexof2:19(cid:2)0:10andasoftexcessthatcanbemodeledusing
variability.Aconstantmodelfittothelightcurveyielded(cid:3)2 ¼ a blackbody component. The strength of the soft excess varies
16:86for9degreesoffreedom.Thus,aconstantmodelisrejected fromobjecttoobject,withtheobjectshavingtheoverallsteepest
atthe94.9%confidencelevel.However,wedonotconsiderthis spectraalsoshowingthehighestamplitudevariability(Leighly
evidence for marginally significant UV variability, because the 1999b).
fluctuationsaredifferentfortheomichainandIRAF-reduceddata. XMM-Newton,withitslargeeffectivearea,hasrevolutionized
The mean flux of the 10 observations was ð2:773(cid:2)0:007Þ; ourunderstandingoftheX-rayspectraofAGNs.Softexcesses
10(cid:1)14 ergs(cid:1)1cm(cid:1)28(cid:1)1. arenowseentoberelativelycommoninquasarsingeneral(e.g.,
Porquet et al. 2004). NLS1 spectra are still found to have soft
2.3.Joint Chandra and XMM-Newton Modeling
excessthatsometimescanbemodeledbyadualComptonization
In principle, better constraints on spectral variability can be model,inwhichsoftphotonsarescatteredbyComptonizingme-
obtainedbyjointlyfittingtheChandraandXMM-Newtonspec- diaoftwo differenttemperatures. Examplesofobjectswiththis
tra.WefirstfitwithapowerlawplusGalacticabsorptionmodel, typeofspectrumareTonS180(Vaughanetal.2002)andMrk896
allowingthenormalizationstobefreeamongthetwoChandra (Pageetal.2003).Inothercases,theX-rayspectrumisverycom-
spectraandtheXMM-NewtonPNspectrum,butwiththephoton plex,withaveryprominentsoftexcessandcomplexabsorption
indicesconstrainedtobeequal.Thespectraarefittedadequately; features at high energies. These spectra can be modeled using
No. 1, 2007 INTRINSICALLY X-RAY WEAK QUASAR PHL 1811. I. 109
1811 resembles an average NLS1 without a soft excess. The
power-law X-ray spectrum is believed to be produced by
Comptonupscatteringofsoftphotonsinahotplasma.Thisisthe
sameprocessbelievedtooperateinAGNsingeneral;thereason
thatthephotonindexissteeperinNLS1sthaninbroad-linequa-
sarsisthatthehotplasmahasbeenComptoncooled(e.g.,Pounds
etal.1995).
IsthelackofcomplexityinthePHL1811spectrumduetothe
lowfluxandpoorstatistics,ordoesitreallyhaveasimplespec-
trum?Wecanobtainsomeanswerstothisquestionbyextracting
sufficiently short segments of XMM-Newton data from other
NLS1ssothatweobtainspectrawithapproximately611photons
between0.3and5keV.WeperformthisexerciseontwoNLS1s:
TonS180and1H0707(cid:1)495.
Ton S180 has a spectrum with a mild soft excess. Vaughan
etal.(2002)modelitusingadualComptonizationmodel,andit
wasclassifiedbyGallo(2006)asanNLS1witha‘‘simple’’spec-
trum.TheTonS180dataareseeninthemiddlepanelofFigure5.
Thisobjectissobrightthatweneedanexposureofjust51.52sto
obtain572photonsbetween0.34Y5.61keV(theobserved-frame
rangecorrespondingtotherest-framerangeof0.36Y5.96keVfor
thisz¼0:062object).Wefitthespectrumwithapowerlawplus
Galacticabsorptionandobtainagoodfit,withaphotonindexof
Fig.5.—ComparisonoftheXMM-NewtonPNspectrumfromPHL1811with 3:04þ0:17and(cid:3)2 ¼0:99for33degreesoffreedom.Thefactthat
spectrafromotherNLS1sfromsegmentsofdatashortenoughforthestatisticsto (cid:1)0:16 (cid:4)
thereduced(cid:3)2islessthanonemeansthatthereisnostatistical
besimilar.Ineachpanel,theratioofthedatatoapowerlawplusGalacticab-
sorptionmodelisplotted.Theresultingpower-lawindexand(cid:3)2isgivenineach evidenceforspectralcomplexity;inaddition,weseenosugges-
(cid:4)
panel.ThetoppanelshowsthatPHL1811isstatisticallywelldescribedbya tiveresidualsinthedata-to-modelratio.However,incontrastto
power-lawmodel.Thephotonindexistypicalofthepower-lawindicesobserved PHL1811,thephotonindexissignificantlysteeperthantheav-
fromASCAobservationsof NLS1s(Leighly1999b).Inthemiddlepanel,Ton
eragefromNLS1sobservedbyASCA,clearlybecauseinthis
S180,anNLS1classifiedashavinga‘‘simple’’spectrumbyGallo(2006),is
statisticallywelldescribedbyapowerlaw;however,theindexissteep,sug- spectrumwearefittingthesoftexcesscomponentpredominantly.
gestingthatalongerexposurewouldrevealahardtail(asitdoes;Vaughanetal. If thisweretheonlyX-rayspectrumthatwehadofthisobject,
2002).Inthebottompanel,1H0707(cid:1)495,anNLS1classifiedashavinga‘‘com- wewouldsuspectthatspectralcomplexitymaybepresentanda
plex’’spectrumbyGallo(2006),showsaverysteepspectrumandsignificant
hardtailmightbeseeninalongerobservation,asitis(Vaughan
residuals,confirmingthenecessityofacomplexmodel(e.g.,Galloetal.2004b;
etal.2002).
Tanakaetal.2004).
1H0707(cid:1)495hasacomplexspectrumthatwasmodeledby
Galloetal.(2004b)usingpartialcovering,andwasclassifiedby
partialcovering(e.g.,Tanakaetal.2004)orreflection(Miniutti Gallo(2006)asanNLS1witha‘‘complex’’spectrum.Notethat
& Fabian 2004). Examples of this type of object include 1H thisistheclassthatGallo(2006)observestobesomewhatX-ray
0707(cid:1)495(Galloetal.2004b;Tanakaetal.2004),IRAS13224(cid:1) weak. The 1H 0707(cid:1)495 data are seen in the bottom panel of
3809 (Boller et al. 2003), and PHL 1092 (Gallo et al. 2004a). Figure5.Inthiscase,asegment289sinlengthyieldedaspec-
Gallo(2006)splitNLS1sintotwoclasses:thosewithandwithout trumwith602photonsbetween0.34and5.73keV(theobserved-
significantcomplexfeaturesintheirhigh-energyspectra.Hefinds frame range corresponding to the rest-frame range of 0.36Y
thatobjectswithcomplexhigh-energyspectratendtobeX-ray 5.96keVforthisz¼0:041object).Wefitthespectrumwitha
weak and proposes that the X-ray weakness is consistent with powerlawplusGalacticabsorption.Inthiscase,theresulting
eitherattenuationinthepartialcoveringscenarioorthefocusing photonindexisevensteeperthanforTonS180((cid:1)¼4:17þ0:18),
(cid:1)0:17
X-raysawayfromourlineofsightinthereflectionscenario. the reduced (cid:3)2 is significantly greater than 1 ((cid:3)2 ¼1:61 for
(cid:4)
HowdoestheXMM-Newtonspectrumof PHL1811compare 34degreesoffreedom),andsignificantresidualsareseeninthe
with those from other NLS1s? It is important to note that the ratio. Clearly, despite the poor statistics in the spectrum, the
quality of our spectrum is much poorer than those from many needforacomplexmodelisevident.Itisalsoclearthatalthough
XMM-Newtonobservationsof NLS1s,suchasthosementioned thisobjectisX-rayweak,theX-rayspectrumisverydissimilarto
above,duetoitslowX-rayflux.ThePNspectrumhas857pho- thatof PHL1811.
tonsbetween0.3and5keVintheobservedframe,corresponding ThisexerciseshowsthattheX-rayspectrumof PHL1811is
to0.36Y5.96keVintherestframe.Ofthese,246arebackground, clearlydifferentfromthoseoftwootherNLS1s,TonS180and
leaving611netsourcephotons.Thisspectrumisadequatelymod- 1H0707(cid:1)495.Theseobjectsarerepresentativeofthecomplex-
eledwithapowerlawandabsorptionoriginatinginourGalaxy. ity of spectra from NLS1s observed by XMM-Newton. The
Thereduced(cid:3)2is0.94for51degreesoffreedom,andthephoton XMM-Newton spectrum from PHL 1811, a simple power law
indexis2:25(cid:2)0:15.Theratioofthedatatothepowerlawplus withaphotonindexconsistentwiththemeanhardX-rayindex
GalacticabsorptionmodelisshowninFigure5.Thisfigureshows fromNLS1sobservedbyASCA,suggeststhattheX-rayemis-
thattherearenoresidualsthatsuggestamorecomplexmodel,and sion mechanism is simple Compton upscattering of soft pho-
indeed,becausethereduced(cid:3)2islessthanone,amorecomplex tonsinahotplasma,asinotherAGNs,andthatthespectrumis
modelwouldoverparameterizethedataandwouldnotbejustified unaltered by any extrinsic effects such as partial covering or
statistically.Itisimportanttonotethatthisphotonindexisconsis- reflection.
tentwiththeaveragehardX-rayphotonindexobservedinASCA ItmayalsobeimportantthatPHL1811ismoreUV-luminous
spectra from NLS1s (2:19(cid:2)0:10; Leighly 1999b). Thus, PHL thanotherNLS1s.Themonochromaticluminosityat25008is
110 LEIGHLY ETAL. Vol. 663
TABLE4
SwiftUVOTResults
FluxDensity(10(cid:1)14ergs(cid:1)1cm(cid:1)28(cid:1)1)a
Observation UVW2 UVM2 UVW1 U B V
2005Oct22............... 3.75(cid:2)0.03 2.75(cid:2)0.02 2.45(cid:2)0.02 ... ... ...
2006May12.............. 3.66(cid:2)0.03 2.71(cid:2)0.03 2.47(cid:2)0.03 1.89(cid:2)0.04 0.99(cid:2)0.02 0.61(cid:2)0.02
a Observedfluxes;uncorrectedforGalacticreddening.
30.9(PaperII);thus,itisseentobeabout5timesmoreluminous October15,asthesamplinganddataqualitywasmuchworseon
thanthemostluminousobjectshowninFigure2ofGallo(2006). October16.
AlsonotethataverysimilarfigureisshowninLeighly(2001)and The images were reduced using standard IRAF procedures.
Matsumotoetal.(2004). AperturephotometrywasperformedonPHL1811andfourfield
starsusinganaperturetwicethesizeoftheimagePSFFWHM.
2.5. Swift Observations and Analysis
Theaperturewaschosentoensurethatessentiallyallofthepho-
PHL1811wasobservedbytheSwiftGamma-RayBurstEx- tons from the object were measured, especially in cases where
plorerMission(Gehrelsetal.2004)on2005October22starting theimagewasslightlytrailedduetocloudsandlossoftracking.
at09:52UTforatotalof2.5ks.Theobservationswereperformed The ratio between PHL 1811 and thefieldstarswas computed
simultaneouslywiththeX-RayTelescope(XRT;Burrowsetal. anderrorswerepropagated.PHL1811isabrightobject,sothe
2005)inthe0.3Y10.0keVenergyrangeandtheUV-OpticalTele- meansignal-to-noiseratiosintheseratiosarehigh,rangingfrom
scope(UVOT;Romingetal.2005)inthe1700Y65008wave- 50to210.
lengthrange.Itwasobservedagainon2006May12for1.6ks. We tested for variability in the ratio light curves using the
ThedetailsoftheobservationsaregiveninTable1. ‘‘excessvariance,’’atechniquecommonlyusedinX-rayastron-
The XRT data reduction was performed by the task omy.Theexcessvarianceisameasureoftheobservedvariance
xrtpipeline,version0.9.9,whichisincludedintheHEAsoft in the light curve subtracting the variance due to measurement
package,version6.0.4.Sourcephotonswereselectedinacircle errors.Innearlyallcases,thismeasurewasnegative.Weconclude
witharadiusof23.400 andthebackgroundphotonsinasource- thatnoevidenceforopticalvariabilitywasfound.
free region close by with a radius of 9500. Those photons were
extractedandreadintoseparateeventfileswithXSELECT,ver- 3. LONG TIMESCALE X-RAYAND UV VARIABILITY
sion 2.3. Twenty-two photons were detected during the first
PHL1811hasnowbeenobservedintheX-raybandpassseven
observation,and10weredetectedduringthesecondobserva-
timesbetween1990and2006:duringtheROSATAllSkySurvey,
tionforcountratesof(8:8(cid:2)1:9);10(cid:1)3s(cid:1)1and(6:2(cid:2)2:0);
byBeppoSAXin2000(Leighlyetal.2001),andinthetwoChan-
10(cid:1)3 s(cid:1)1,respectively.
draobservations,theXMM-Newtonobservation,andthetwoSwift
In the 2005 October 22 observation, the UVOT photometry
observationsreportedhere.InFigure6weshowthelong-termlight
wasperformedwiththeUVfiltersUVW1,UVM2,andUVW2.
curvecomposedofthesecuremeasurements,i.e.,thelastfiveob-
Duringthe 2006May 12 observation, all six (optical andUV)
servations.WedonotplottheRASSupperlimitortheBeppoSAX
filterswereused.ThedetailsaregiveninTable1.Aftertheas-
observationthat,withthelargedetectorPSF,wascertainlycon-
pect correction the exposures in each filter were co-added into
taminatedbyX-rayemissionfromneighboringobjects.Forthe
one image with uvotimsum, and the magnitudes and fluxes in
ChandraandXMM-Newtonobservations,theX-rayfluxdensities
eachfilterweredeterminedwiththetaskuvotsource.There-
sults (observed fluxes, uncorrected for Galactic reddening) are
listedinTable4.
2.6. MDM Optical Photometry
Optical photometry data were taken on PHL 1811 at MDM
Observatory using the 1.3 m McGraw-Hill telescope on the
nights of 2004 October 14, 15, and 16 as part of a project to
searchforopticalvariabilityinthenarrow-linequasarPHL1092
(Galloetal.2004a).WeusedthethinnedCCD‘‘Templeton’’and
thetelescopeinthef/7.6configuration,yieldingaangularsizeof
0.5000pixel(cid:1)1.TheweatherwasgoodonOctober14,withtypical
seeingof1.700,althoughtheskywasnotphotometric.Theseeing
was worse on October 15 (average of 2.300) and it was inter-
mittentlycloudy.OnOctober16,theweatherhaddeteriorated
further,andfewusableframeswereobtained.
WeobservedPHL1811usingtheI,V,B,andUfilters,obtain-
ingseveralexposuresineachfilter.Thetotalnumberofframes Fig. 6.—Long-termX-rayfluxvariabilityofPHL1811intermsof(cid:4)F at
(cid:4)
analyzedwere9,19,12,and11fortheI,V,B,andUfilters,re- 2keVintherestframe.Theuncertaintiesforthespectroscopicdata(Chandraand
XMM-Newton)arepropagated1(cid:2)errorsinthepower-lawnormalizationand
spectively.Withineachnight,theobservationswereallobtained
index.Forthedetectiondata(Swift)theuncertaintiesareproportionaltothecount
withinatimespanofabout30minutes,sowecouldtestthein-
rateerror.Notethelogarithmicfluxaxis.Alsoshownisthepredictedfluxfor
terday optical variability principally between October 14 and (cid:1)ox¼(cid:1)1:6basedontheUVfluxfromtheHSTobservations.
No. 1, 2007 INTRINSICALLY X-RAY WEAK QUASAR PHL 1811. I. 111
Fig.7.—XMM-NewtonOMandSwiftUVOTphotometrypointsoverlaidon
theobservedmergedopticalandHSTUVspectrumobserved2001December3
anddiscussedinPaperII.NostrongevidenceforUVvariabilityisseenamong Fig.8.—SpectralenergydistributionofPHL1811,plottedasafunctionofthe
theobservations,whichspan4.5yr. rest-framefrequency.ContoursfromeachofthetwoChandraobservationsare
shown,generatedbysuccessivelysettingeachparametertoits(cid:3)(cid:3)2¼2:71value
andcomputingthemodel,thendeterminingthemaximumandminimumofallof
were estimated from the best-fitting power-law models, and the themodels.Thedashedlineshowstheexpected2keVfluxforanaveragequasarof
uncertainties were obtained by propagation of the errors on the thisluminosity,basedontheregressionpresentedbyWilkesetal.(1994),whilethe
dottedlineshowstherangeobservedbyWilkesetal.(1994).Theaveragequasar
normalizationsandphotonindices.FortheSwiftobservations,the
SEDfromElvisetal.(1994)scaledtothe1(cid:5)minflectionisalsoshown.
fluxdensitieswereestimatedfromthecountratesusingPIMMS,7
assumingtheXMM-Newtonphotonindex((cid:1)¼2:3)andGalactic
absorption, and the uncertainties in the flux densities were as- The effective wavelengths of the Swift UVOT photometry
sumedproportionaltotheuncertaintyinthecountrate. pointsarelistedinTable1,andtheinferredfluxesareplottedin
WefindthatPHL1811hasvariedsignificantlybyafactorof Figure7.NotethatthefiltertransmissionfunctionsintheSwift
(cid:3)5 in this time period. The first Chandra observation and the UVOTaredifferentfromthoseintheXMM-NewtonOM,even
XMM-Newtonobservationfoundittobeinarelativelylowstate, thoughthefilternamesarethesame;hence,theeffectivewave-
with(cid:4)F(cid:4)at2keVrestframeequalto(cid:3)1:0;10(cid:1)14ergs(cid:1)1cm(cid:1)2; lengthsaresomewhatdifferent.LiketheXMM-NewtonOMpho-
thefluxesinthesetwoobservationsareconsistentwitheachother. tometry,thecorrespondencebetweentheUVOTphotometryand
ThesecondChandraobservationandthetwoSwiftobservations the HSTspectrum is very good, with the exception of the pho-
showasignificantlyhigherfluxbyafactorof3.5Y5.5,andthe tometryusingtheUVW2filter.Thatfilterisespeciallydifficultto
fluxesofthesethreeareroughlyconsistentwithoneanother,al- calibrateasthecompletefiltertransmissioncurveisnotknown9
thoughtheuncertaintiesareevaluateddifferently.Althoughthere andbecauseofthepaucityofsuitablecalibrationtargets.There-
areonlyfivepoints,thesedatasuggestthattheX-rayfluxoscil- fore,wedonotconsiderthedisagreementwiththeHSTspectrum
latesbetweentwostatesthatdifferbyafactorof4Y5.TheNLS1 indicativeofaspectralchange.Weconcludethatweobserveno
1H 0707(cid:1)495 seems to behave the same way, oscillating be- evidenceforanyUVvariabilitybetweenthefourUVobservations
tweentwofluxstatesthatdifferbyafactorof(cid:3)10(Leighlyetal. thatspanatimeperiodof4.5yr.
2002). Tosummarize,PHL1811hasnowbeenobservedseventimes
Aswillbediscussedinx4,quasarswithPHL1811’soptical intheX-raysoveraperiodspanning16yr.Duringthistime,the
luminosityarestatisticallyexpectedhavevaluesof(cid:1)ox8equalto X-ray flux has been observed to vary by a factor of (cid:3)5, but it
(cid:1)1.6.ThedashedlineinFigure6showsthepredictedX-rayflux remainswellbelowthatofatypicalquasarof itsUVluminosity.
for(cid:1)ox ¼(cid:1)1:6basedontheUVfluxoftheHSTspectrum.Al- Incontrast,thefourUVobservationsmadeoveraperiodof4.5yr
though PHL 1811 varies significantly, it never approaches the donotshowanyconvincingevidenceof UVvariability.
nominalX-rayfluxforanobjectofitsUVluminosity.
WehavefourepochsofUVobservations:theHSTSTISspec-
4.THE SPECTRAL ENERGY DISTRIBUTION
troscopicobservationmade2001December3(discussedinde-
tailinPaperII),theXMM-NewtonOMobservationmade2004 WepresentedthefirstspectralenergydistributionofPHL1811
November1,andthetwoSwiftUVOTobservationsmade2005 inLeighlyetal.(2001)basedonanopticalspectrum,aROSAT
October22and2006May12.WesearchforpossibleUVvaria- upperlimit,aBeppoSAXobservation,andmultiwavelengthpho-
bility by comparing the latter three photometry measurements tometry.InFigure8,wepresentanupdatedspectralenergydis-
withtheHSTspectrum. tribution.FortheopticalandUV,weusedthemergedspectrum
The effective wavelength of the XMM-Newton OM UVM2 describedinPaperII.TheChandraresultsarerepresentedbyre-
filter is 2310 8. We plot the observed fluxes on the observed- gionsontheSEDplotthatwereconstructedusingthethirdjoint
framemergedHSTandopticalspectrumfromPaperIIinFigure7. model fit (Table 2). The contours were constructed by succes-
Wefindthatthephotometryfluxiscompletelyconsistentwiththe sively setting each variable parameter to its (cid:3)(cid:3)2 ¼2:71 value
observedspectrum. and computing the model, then determining the maximum and
minimumofallthemodels.
7 Availableathttp://cxc.harvard.edu/toolkit/pimms.jsp.
8 Theparameter(cid:1) isdefinedasthepoint-to-pointslopebetween25008 9 Seehttp://swift.gsfc.nasa.gov/docs/heasarc/caldb/swift/docs/uvot/uvot_caldb_
ox
and2keV;i.e.,(cid:1)ox¼log(F2500/F2keV)/2:61. filtertransmission_02.pdf.
112 LEIGHLY ETAL. Vol. 663
From the best-fit Chandra model and the merged HST and independent, so in this scenario we would expect to see the in-
opticalspectrum,wecompute(cid:1) tobe(cid:1)2.40forthefirstob- trinsicpowerlawwithattenuatedflux.InSeyfert2s,theelectron
ox
servation,whichwasnearestintimetotheHSTobservation,and scatteringoccursoveranextendedarea,andthuswhiletheintrin-
(cid:1)2.19forthesecondobservation.AsseeninFigures6and7,the sicX-rayemissionmayvary,thescatteredemissiondoesnot,be-
ChandraandHSTfluxesarecomparablewiththeXMM-Newton cause variability is washed out as it scatters over the extended
and Swift fluxes, so we confine our discussion to the Chandra region.Thefactthatweseesignificantvariabilitybetweenthetwo
andHSTdataherewithoutlossofgenerality. Chandraobservations,separatedby12days,arguesthatweare
Wilkes et al. (1994) computed a regression between optical notseeingscatteredlight. Thisconclusiondependsonthe com-
luminosityand(cid:1) foraheterogeneoussampleofquasars.Using pactnessofthe electron-scattering mirror;ifunusually compact,
ox
theircosmology(H ¼50kms(cid:1)1Mpc(cid:1)1,q ¼0),weobtainalu- observationofvariabilitywouldbepossible.Regardless,theob-
0 0
minositydistanceforPHL1811of1261Mpcandacorresponding servationofsignificantvariabilitybetweentwoobservationssep-
luminositydensityat25008of1:44;1031 ergs(cid:1)1Hz(cid:1)1.Then aratedby12daysfromthisluminousquasarsuggeststhatweare
using their regression, we predict (cid:1) to be (cid:1)1.6. We plot the seeingtheintrinsicemissionfromtheAGN.
ox
predictedX-rayfluxassumingthisvalueof(cid:1) onFigure8,aswell Narrow-line Seyfert 1 galaxies are known for their high-
ox
asaverticalbarthatindicatestherangeof X-rayluminositiesob- amplitudeX-rayvariability(e.g.,Leighly1999a).Asdiscussed
servedbyWilkesetal.(1994). inLeighlyetal.(2001)itwaspossible,atthattime,whenwe
Morerecently,Stratevaetal.(2005)andSteffenetal.(2006) hadonlytheRASSupperlimitandtheBeppoSAXdata,thatwe
updated the (cid:1) regression using a large sample of optically hadcoincidentallyonlyobservedPHL1811whileitwasinatran-
ox
selected active galaxies that span a large range in redshift and sient low state. In this paper, we report five more X-ray obser-
luminosityyethavehighlycomplete X-ray data.Fortheircos- vations, all of which find it to be a significantly weak X-ray
mology(H ¼70kms(cid:1)1Mpc(cid:1)1,(cid:2) ¼0:3,and(cid:2) ¼0:7)we source.Thus,theprobabilitythatwecoincidentallyobserveit
0 M (cid:4)
obtain a luminosity distance of 936 Mpc and a corresponding inalowstateisdecreasing.PHL1811appearstobeintrinsically
luminositydensityat25008of 7:92;1030ergs(cid:1)1Hz(cid:1)1.Their X-rayweak.
regression yields a predicted (cid:1) of (cid:1)1.60 as well. From their
ox
Figure4,wefindthattheenvelopeof(cid:1)oxobservedforquasarsof 5. DISCUSSION
this luminosity spans approximately (cid:1)1.75 to (cid:1)1.4. Our ob-
servedX-rayluminositydensitiesarefactorsof 130Y450below 5.1. PHL 1811 is Intrinsically X-Ray Weak
thehighvalueand13Y45belowthelowvalue.Thus,PHL1811 InLeighlyetal.(2001)wereportedthefirstX-raydetection
isobservedtobesignificantlyX-rayweakcomparedwithother of PHL1811byBeppoSAX.ThatobservationshowedthatPHL
quasars. 1811appearedtobeX-rayweak,butthe65netphotonspectrum
Brandtetal.(2000)compilethedistributionof(cid:1) 10fromthe wasnotsufficienttodeterminetheoriginoftheX-rayweakness.
ox
PG quasar sample studied by Boroson & Green (1992). They WepresentedthreealternativesfortheX-rayweakness:(1)PHL
findasuggestionofabimodaldistributionwith10ofthe87ob- 1811isaBALQSO,andtheX-rayemissionisabsorbed;(2)since
jectsclassifiedasX-rayweakwith(cid:1) (cid:7)(cid:1)2.Thentheyfinda PHL1811 isanNLS1,itishighlyX-ray variable, and wehap-
ox
connectionbetween(cid:1) andthepresenceofsignificantCivab- penedtocatchitinalowstate;or(3)PHL1811isintrinsically
ox
sorptionlines,suchthatmostofthesoftX-rayweakobjectshave X-rayweak.TheHSTobservationdiscussedinPaperIIandthe
absorption-line equivalent widths greater than 5 8. They infer ChandraobservationsreportedhereshowthatitisnotaBALQSO,
theseresultstoimplythatX-rayabsorptionistheprimaryorigin asthereisnoevidenceforUVabsorptionlines.Furthermore,there
ofsoftX-rayweaknessinAGNs. isnoevidenceforabsorptionintheX-rayspectrum,andthesig-
ClearlyPHL1811doesnotfollowthetrendobservedbyBrandt nificantvariabilitybetweenthetwoChandraobservationssuggests
etal.(2000).ItissoftX-rayweak,but,asdiscussedinPaperII, thattheX-rayemissionisnotscattered.Sothefirsthypothesisis
thereisnoevidenceforanysignificantintrinsicCivabsorption firmlyruledout.
lines.IntheBrandtetal.(2000)sample,objectswithsimilar(cid:1) We can never conclusively rule out the second hypothesis,
ox
asobservedfromPHL1811haveCivabsorptionlineequivalent thatPHL1811ishighlyX-rayvariableandwealwaysjusthap-
widthsof5Y208. pentocatchitinalowstate.However,ithasnowbeenobserved
Furthermore, the X-ray spectrum shows no evidence for in- seventimesbetween(cid:3)1990(duringtheROSATAllSkySurvey)
trinsicabsorption.IflowYcolumn-density absorption werepre- and 2006 May (in a Swift observation), and it has never been
sent,wewouldexpecttoobserveaflatspectrum;incontrast,for observed to be bright. In fact, since it has already varied by a
asinglepower-lawmodel,wemeasurephotonindicesof 2Y2.6, factorof(cid:3)5amongtheobservationsreportedhere,itmayhave
consistentwithunabsorbedquasars,andanupperlimitonintrin- alreadybeenasbrightasitcanget.Itseemsincreasinglyunlikely
sicabsorptionof8:7;1020 cm(cid:1)2. thatitwilleverbeasX-raybrightasotherquasars.
AnotherpossibilityisthatahighYcolumn-densityorCompton- Thus,weconcludePHL1811isintrinsicallyX-rayfaint.Most
thickabsorberispresentinourlineofsight,sothatweseenodi- quasars are bright X-ray sources, so what property of the PHL
rectcontinuumemission.Ourspectradonotprobehighenough 1811centralenginecausesittobeX-rayfaint?Aswepointedout
energiestoseewhetherthereisahighlyabsorbedcomponent,as inLeighlyetal.(2001;alsoGrupeetal.2001),thereisnoob-
hasbeenfoundinbroadabsorptionlinequasars(BALQSOs;e.g., vious reason why intrinsically X-ray weak quasars should not
Gallagheretal.2002;Greenetal.2001).Ifthecontinuumemis- exist.Briefly,aquasararguablycannotexistwithoutaccretionasa
sionwerecompletelyabsorbed,thentheX-rayspectrumthatwe sourceoffuel.InanobjectsuchasPHL1811,thataccretionpro-
seemighthavebeenscatteredintoourlineofsight(i.e.,similarto bablyoccursthroughanopticallythick,geometricallythinaccre-
a Seyfert 2 galaxy or BALQSO). Electron scattering is energy tiondiskthatemitstheobservedstrongopticalandUVcontinuum
(Fig.8).SuchadiskwouldneverbehotenoughtoemitX-rays,
whichareprobablyemittedbythecorona,aseparatecomponent.
10 Brandtetal.(2000)useanalternativedefinitionof(cid:1) usingthefluxdensity
at30008ratherthan25008.Theyestimatethat(cid:1)ox(250ox0)¼1:03(cid:1)ox(3000)(cid:1) We postulated that there may be situations in which the corona
0:03(cid:1) ,where(cid:1) istheslopeofthespectrumbetween2500and30008. maynotexistormaybeweak.
u u