Are X-ray Emitting Coronae around Super Massive Black Holes Outflowing?

0.53 MB·

by  Teng Liu

DRAFTVERSIONJANUARY16,2014
PreprinttypesetusingLATEXstyleemulateapjv.04/17/13
AREX-RAYEMITTINGCORONAEAROUNDSUPERMASSIVEBLACKHOLESOUTFLOWING?
TENGLIU(刘腾),JUN-XIANWANG(王俊贤),HUANYANG(杨欢),FEI-FANZHU(朱飞凡),YOU-YUANZHOU(周又元)
CASKeyLaboratoryforResearchinGalaxiesandCosmology,DepartmentofAstronomy,UniversityofScienceandTechnologyofChina,Hefei,Anhui
230026,China;[email protected],[email protected]
DraftversionJanuary16,2014
ABSTRACT
4 Hard X-ray emission in radio-quiet active galactic nuclei (AGNs) is believed to be produced via inverse
1 Comptonscatteringby hotand compactcoronaenear the supermassive blackhole. Howeverthe originand
0 physicalpropertiesof the coronae, includinggeometry,kinematicsand dynamics, yet remainpoorlyknown.
2 Inthiswork,taking[OIV]25.89µmemissionlineasanisotropicindicatorofAGN’sintrinsicluminosity,we
comparethe intrinsic coronaX-ray emission between Seyfert1 and Compton-thinSeyfert2 galaxies, which
n
areviewedatdifferentinclinationsaccordingtotheunificationscheme.Wecompileasampleof130Compton-
a
J thinSeyfertgalaxieswithboth[OIV]25.89µmlineluminositiesmeasuredwithSpitzer-IRSandX-rayspectra
observed by XMM-Newton, Chandra, Suzaku or Swift. Known radio-loud sources are excluded. We fit the
5
X-ray spectra to obtain the absorption-corrected2 – 10 keV continuumluminosities. We find that Seyfert1
1
galaxiesareintrinsicallybrighterinintrinsic2–10keVemissionbyafactorof2.8+−00..54(2.2+−00..93inSwift-BAT
] 14– 195keVemission), comparingwith Compton-thinSeyfert2galaxies. TheSeyfert1 andCompton-thin
E Seyfert 2 galaxies follow a statistically identical correlation between the absorption-corrected 2 – 10 keV
H luminosityandthe14–195keVluminosity,indicatingthatourabsorptioncorrectiontothe 2–10keV flux
is sufficient. The difference in X-ray emission between the two populations is thus unlikely due to X-ray
.
h absorption,andinsteadimpliesanintrinsicanisotropyinthecoronaX-rayemission. Thisstrikinganisotropy
p ofX-rayemissioncanbeexplainedbyabipolaroutflowingcoronawithabulkvelocityof∼0.3−0.5c. This
- wouldprovideanaturallinkbetweentheso-calledcoronaeandweakjetsinthesesystems. Otherconsequences
o
ofoutflowingcoronaearealsodiscussed.
r
t Subjectheadings: galaxies: active–galaxies: nuclei–galaxies: Seyfert–galaxies: jets–X-rays: galaxies–
s
a relativisticprocesses
[

1
1. INTRODUCTION 2000;Molinaetal. 2009). Meanwhile,similartosolarcoro-
v
nae,theX-raycoronaeinblackholesystemsmightbeheated
4 StronghardX-rayemissionhasbeendetectedinblackhole
6 accreting systems spanning from stellar mass (∼10M⊙) to throughmagnetic processes and could produce magnetic in-
5 super massive scales (∼105−1010M ). Such systems are flow/outflow above the disk (Beloborodov1999). Bulk mo-
⊙ tionoftheflow,iffastenough,wouldproducebeamedX-ray
3 the dominantpopulationin the X-raysky. Inradio-quietsu-
emission. The X-ray emission is also likely producedin the
. per massive black hole (SMBH) accretion systems, the pri-
1 jetbase, whichisphysicallysimilarto an outflowingcorona
mary hard X-ray emission is power-law shaped in spectrum
0 (Markoffetal. 2005). Lightbendingeffectoftheblackhole
4 with a high energycutoffat &100 keV (Molinaetal. 2013; could also produce anisotropy in X-ray emission, by reduc-
1 Riccietal.2011). ItisnowwidelyacceptedthatX-rayemis-
ingtheX-rayfluxescapingupward(Miniutti&Fabian2004;
: sioninradio-quietAGNscomesfrominverseComptonscat-
v Chenetal. 2013). These arguments suggest that the corona
tering of the seed photons from the accretion disks through
i X-rayemissioninAGNsislikelyanisotropic,althoughweak
X hotand compactplasma, named corona(Haardt&Maraschi
if there is any as no observationalevidence has yet been re-
1991, 1993). Such coronae are also required in radio-loud
r ported. However, measuring the level of the anisotropy or
a AGNs, in which strong relativistic jets could produce extra
putting strong constraint to it is essential to understand the
emission from radio to X-ray and even gamma-ray, and in
originandphysicalpropertiesofthecorona.
stellar mass black hole accretion systems (e.g. Plotkinetal.
In the unification scheme of AGNs, type 1 sources
2012). However,westillknowverypoorabouttheoriginand
are viewed face-on while type 2 sources viewed edge-on
physicalnatureofthecorona.
(Antonucci 1993), providing an opportunity to measure the
Being free from dust extinction, hard X-ray emission has
anisotropy of the corona X-ray emission. In this work we
been widely used to represent the intrinsic power of AGNs,
presentanobservationalinvestigationtocomparethecorona
e.g.,tomeasuretheratiooftype1totype2AGNsandtode-
X-rayemissionintype1andtype2 AGNsatgivenintrinsic
rive the AGN bolometric luminosity (Meléndezetal. 2008;
blackholeaccretionpower.Todoso,weneedanindependent
Rigbyetal. 2009; Burlonetal. 2011; Maliziaetal. 2009;
AGNluminosityindicatorandsamplesofAGNswithdiffer-
Runnoeetal. 2012). However, we still know little observa-
entinclinationangles.
tionallytowhatlevelthecoronaX-rayemissionisisotropic.
Inspired by recent studies (Meléndezetal. 2008;
While a slab-like corona could produce weaker X-ray emis-
Rigbyetal. 2009; Diamond-Stanicetal. 2009; Weaveretal.
sionathigherinclinationduetoprojectioneffect(e.g.Zhang
2010; LaMassaetal. 2010; Liu&Wang 2010), we opt to
2005; Chenetal. 2013), the suggested patchy structure and
use[OIV] 25.89µmline emission, a forbiddenline produced
smallopacityofthecoronaindicatetheanisotropyproduced
in the so called narrow line region (NLR), as an intrinsic
this way is rather weak (Haardtetal. 1994; Zdziarskietal.
luminosityproxyofAGNs. Comparingwiththewidelyused
[OIII] 5007Å line, [OIV] line is significantly less attenuated ofopticalspectroscopiccatalogsofAGNswithexisting
by dust extinction (AV ∼ 39 corresponds to A25.89µm ∼ MBHestimatesand[OIII]5007Ådetectionsareinvolved
0.06-0.18;Goulding&Alexander2009),andhasrelatively in matchingwith allthe IRS targets. The finalsample
higherionization potential(54.9eV) thus is less affected by consistsof81AGNswithresolved[SIV],[NeIII],[OIV]
contamination from star formation in the host galaxy. The or[NeV]lines.
validityofusing[OIV]asintrinsicAGNluminosityindicator
has been confirmed observationally (Diamond-Stanicetal.
E: Acollection(fromliterature,archive,andtheauthor’sown
2009; Meléndezetal. 2008; Rigbyetal. 2009). Particu-
observing programs) of Spitzer-IRS high-resolution
larly, Diamond-Stanic et al. (2009) showed that the [OIV]
spectra of 426 galaxies, including quasars, Seyferts,
luminosity distributions are indistinguishable for obscured
LINERs and HII galaxies (Pereira-Santaellaetal.
and unobscured AGNs in a well-defined galaxy-magnitude-
2010).
limited sample, while [OIII] luminosities are systematically
lower in obscured sources. However we note that unlike
These samples somehow overlap with each other. High-
[OIII], measurements of [OIV] line fluxes are available in
resolution data are adopted with higher priority when avail-
muchfewerAGNs(see§2).
able.High-resolution[OIV]fluxmeasurementsfromdifferent
We select to compare corona X-ray emission of type 1
subsamples for overlapped sources are generally consistent
AGNs with that of Compton-thin type 2 AGNs. Only se-
witheachother. Inafewofthem(7sources,5%ofthefinal
curely identified Compton-thin sources (with X-ray obscu-
sample),thedifferencesareaslargeasbyafactorof>1.5.We
ration column density N < 1024 cm−2) are included, be-
H simplyadopttheaverage[OIV]fluxesinlogarithmspacefor
cause for Compton-thick ones reliable correction to X-ray
theseoverlappedsources. The[OIV]fluxesareconvertedinto
obscuration is extremely hard or even impossible. Known
luminositiesadoptingthesamecosmologicalparameters1.
radio-loud sources are also excluded to avoid contamina-
We then match the composite [OIV] sample to XMM-
tions from known strong jet emission. 2 – 10 keV intrin-
Newton data archive to search for reliable X-ray spectra.
sic(i.e.,absorption-correctedcontinuum)luminositiesareob-
For a few sources without XMM observations, we supple-
tainedthroughspectralfitting.
ment the X-ray spectra collection with Chandra, Suzaku or
In §2, we describe the samples, and the processing of X-
Swift data (see Table 1). We perform literature search and
raydata. In§3, wecomparetheintrinsicX-rayemissionbe-
independent X-ray spectral fittings to get the classifications
tweentype1andtype2subsamplesatgiven[OIV]luminosity, of their X-ray obscuration nature. Possible Compton-thick
showingrelativelyweakerintrinsicX-rayemissionintype2
sourcesareexcluded,andonlysecurelyidentifiedCompton-
sources. In §4, we present discussions on the robustness of
thin ones are included. The criteria we adopted to iden-
theanisotropyofcoronaX-rayemission,andfinallyinterpret
tify secure Compton-thin sources are: 1) X-ray spectra fit-
itintermsofcoronaoutflowing.§5presentsthesummary.
ting could provide reliable measurements to X-ray obscura-
tionsat N <1024 cm−2, if with highqualityspectra; 2)for
2. SAMPLEANDDATAREDUCTION H
those sources without good-enough X-ray spectra, sources
2.1. [OIV]SampleCompilation with T ratio <1 (T ratio = f2−10keV/f[OIII], where f2−10keV
To gather a large sample of AGNs with both istheobservedX-rayflux,and f thedustextinctioncor-
[OIII]
[OIV] flux measurements and X-ray observations, we rectedlineflux)ornarrowFeKαlineEW>600eVareex-
first combine five major samples in literature with cludedascandidatesofCompton-thicksources(Bassanietal.
[OIV] 25.89µm line fluxes measured by Spitzer In- 1999). This is a rather conservative approach since some
frared Spectrometer (IRS, Diamond-Stanicetal. 2009; known Compton-thin sources also show T ratio <1 or line
Weaveretal. 2010; Tommasinetal. 2010; Dasyraetal. EW>600eV (Bassanietal. 1999). We exclude19 sources
2011; Pereira-Santaellaetal. 2010). The five samples are (15ofwhicharetype2)withtoofewX-raycounts(<100in
brieflydescribedasfollows: 2–10keV)inXMMdata,forwhichreliablemeasurementsto
theX-rayobscurationsarehard.Toavoidcontaminationfrom
A: Asubsampleof91SeyfertgalaxieswithSpitzer-IRShigh- strongrelativisticjettotheX-rayemission,knownradio-loud
resolutionspectroscopicobservations(Tommasinetal. sourcesareexcluded. InTable1wepresentthesamplewith
2010) from the 12µm flux limited sample of Seyfert referencesto their X-rayidentifications. Thefull sample in-
galaxies(116sources,Rushetal.1993). cludingpossibleCompton-thicksourceswillbepresentedin
afuturepaper.
B: Spitzer-IRS low-resolution spectroscopic observations
We also excluded5 X-rayun-obscuredSeyfert2galaxies,
of a spectroscopically selected, galaxy-magnitude-
i.e. those with X-ray obscuration column densities N <
H
limited sample of Seyfert galaxies from the revised 1022 cm−2 but without any evidence of broad line regions
Shapley-Ames (RSA) catalog (Diamond-Stanicetal.
(BLRs) detected, which could be physically different from
2009). The RSA Seyfert sample, containing 18
normal Seyfert 2 galaxies (Brightman&Nandra 2008). In-
type 1 and 71 type 2 Seyfert galaxies, is a well
cludingthemintoanalyseswouldhoweverslightlystrengthen
studied galaxy-magnitude-limited complete sample
ourconclusioninthiswork.
(Maiolino&Rieke1995;Hoetal.1997).
Finally,thecompositesampleincludes84type1AGNs(in-
cludingSeyfert1, Seyfert1.2andSeyfert1.5galaxies,here-
C: A subsample of 79 AGNs with high-resolution Spitzer-
after Sy1s) and 46 Compton-thin type 2 AGNs (including
IRS spectroscopy from the Swift-BAT hard X-ray se-
Seyfert1.8,Seyfert1.9andSeyfert2,hereafterSy2s).
lectedlocalAGNsample(Weaveretal.2010).
D: Spitzer-IRShigh-resolutionspectroscopyobservationsof 1H0=70kms−1Mpc−1,Ωm=0.27,andΩλ=0.73,adoptedthrough-
a composite AGN sample (Dasyraetal. 2011). A list outthispaper.
2.2. X-rayDataReductionandSpectralFitting
To obtain the intrinsic 2 – 10 keV luminositiesof sources
inoursample,wefirstcollectdatafromliterature(seeTable
1 and Appendix A). For a major fraction of the sample we
needtoperformourownspectralfittingtomeasuretheintrin-
sicX-rayfluxesaseithertheX-raydataarenotpublishedyet,
or no intrinsic 2 – 10 keV fluxesare givenin literature. For
sources with multiple XMM observations, only the observa-
tions duringwhich the sources were Compton-thinare kept,
and exposure-time weighted mean intrinsic luminosities are
calculatedinlogarithmspace.
TheXMM-Newton data areprocessedwith the XMM-SAS
packageinthestandardway.Thesourceregionstoextractthe
spectra are optimizedwith the “eregionanalyse”task. Back-
groundextractionregionsareafewmanuallyselectedcircles
withproperradiiaroundthesourceregion,beingcentralsym-
metric as much as possible, and kept away from the CCD
edges, the out-of-timeeventsstrips and other sources. Each
exposure is checked for pile-up with the task “epatplot”. A
corecircleregionwithasmallradiusisexcludedinthespec-
tra extracting in case of pile-up. The circle radius is manu-
ally selected by increasing step by step until the pile-up ef-
fectnolongershowsupinthe“epatplot”result. Wegenerate
FIG.1.— The relations between the 2 – 10 keV intrinsic X-ray emis-
thesourceandbackgroundspectratogetherwiththeappropri- sion(absorption-correctedpower-lawemission)andthe[OIV]25.89µmline
ate redistributionmatrix and ancillaryresponsefile from the emissionforSy1s(type1Seyfertgalaxies,bluedots)andCompton-thinSy2s
source and backgroundregionsfor each exposure, using the (type2Seyfertgalaxies,redsquares). Sy2saredividedintotwotypeswith
task “especget”. The Chandra and Swift-XRT data of a few NHlower(solidsquares)orhigher(opensquares)than1023cm−2.Acouple
ofupperlimits to[OIV]luminosities areplotted inarrows. Linesplotthe
sourcesarealsoprocessedwiththeirstandardpipelines. best-fitcorrelations (through simplelinear regression), andshaded regions
Tomeasuretheabsorption-correctedpower-lawfluxinthe arethe1σconfidencebandsofthefits.Thetypical(median)errorsinL[OIV]
2–10keVband,wefitthe0.5–10keVspectrain“Xspec” andL2−10keV are5%and4%respectively,andarenotplottedforsimplicity.
withapower-lawabsorbedbybothGalacticandintrinsicab- Inthelowerpanelweplottheresidualsofthedatapointstothebest-fitlineof
Sy1s(alongthey-axis)forbothSy1sandSy2s. Sy2sareobviouslyweaker
sorbers,togetherwiththreeadditionalemissioncomponents–
inintrinsic2–10keVemission.
asoftexcess,acoldreflectioncomponentandanarrowFeKα
lineat6.4keV.Weareonlyinterestedindecomposingthead- Sy2s:y=(0.86±0.08)×(x−40.99)+42.52±0.09
ditionalcomponentsfromthepower-law,butnotinthecom-
Remarkably weaker intrinsic X-ray emission at given [OIV]
plexoriginsofsoftexcessandreflection. ForSy1s,wefitthe
luminosityintype2AGNsisseeninthisfigure. Weperform
softexcesswiththetraditionallyusedblackbodymodel,while
K-Stesttoexaminewhethertheresidualsofdatapointsfrom
forSy2swithanextrasoftpower-lawmodel,becausethesoft
Sy1sandSy2stothebest-fitslopeofSy1s(lowerpanelofFig.
excessdisplaysdifferentoriginsforunabsorbedandabsorbed
1) are extracted from the same population. At a confidence
AGNs (Corraletal. 2011). In some cases, additional black-
levelof99.94%theyarenot.Thedeviationofresidualsofthe
bodyorpower-laworionizedgasemissionisneededtomodel
two subsamples shows that Compton-thin type 2 AGNs are
thesoftexcess.Becausethespectralqualitiesofmostsources
are notgood enoughto put reasonableconstrainson the pa- fainterinintrinsic2–10keVemissionbyafactorof2.8+−00..54,
rametersof the reflectioncomponent(“pexrav”in “Xspec”), comparing with Sy1s. We note that orthogonal distance re-
wefixthecutoffenergyofthepower-lawat200keV,andthe gression(ODR,Isobeetal.1990)yieldsdifferentcorrelation
reflection fraction R at 0.5. Adopting different values of R slopes,butdoesnotalterotherresultsinthiswork. Thebest-
does not alter the results in this work (see §4.4). In a few fittedODRlinesare:
cases,abroadFeKαlineisalsoneededtoimprovethefitting Sy1s:y=(1.00±0.06)×(x−41.17)+43.15±0.06
statistics.
Sy2s:y=(0.99±0.08)×(x−40.99)+42.56±0.09
3. COMPARISONBETWEENTYPE1ANDTYPE2AGNS Inourstatisticalanalyses,weomit5sources(4Sy1sand1
In Fig. 1, we plot the intrinsic 2 – 10 keV X-ray lumi- Sy2, plotted in Fig. 1) with only [OIV] upper limits. These
nosities versus [OIV] luminositiesof our sample. Clear cor- [OIV] upperlimits couldbe taken into accountwith survival
relations between intrinsic 2 – 10 keV X-ray and [OIV] line analysesbyeithertakingtheintrinsic2–10keVluminosityas
emission,bothofwhichsomehowreflecttheintrinsicpower theindependentvariable,ortreatingtheupperlimitsto[OIV]
oftheSMBHaccretion,aredetectedforbothtype1andtype luminositiesas lower limits to 2 – 10 keV emission instead.
2 AGNs, confirmingpreviousstudies (Meléndezetal. 2008; Both approachesyield consistentresults. Here we adoptthe
Rigbyetal. 2009; Diamond-Stanicetal. 2009; Weaveretal. morestraightforwardapproachbyexcludingthe[OIV]upper
2010). Simple linear regression (Isobeetal. 1990, taking limitsandtaking[OIV]astheindependentvariable.
[OIV] as the independentvariable)is performedon the Sy1s Hard X-ray photons above 10 keV are insensitive to
andSy2ssubsamples.Thebest-fittedlinesare: Compton-thin obscuration. We match our sample to Swift-
BAT 70 month catalog (Baumgartneretal. 2013) to extract
Sy1s:y=(0.88±0.05)×(x−41.17)+43.13±0.06 their14–195keVX-rayfluxes. ForSwift-BATnon-detected
FIG.2.—Swift-BAT14–195keVversusintrinsic2–10keVluminosities.
Sy1sandCompton-thinSy2sfollowastatisticallyidenticaltightcorrelation.
Arrows plotthe upperlimits toBATnon-detected sources. Lines plot the
best-fitcorrelations,andshadedregionsthe1σconfidencebandsofthefits.
sources,weadoptanupperlimitof1.34×10−11ergcm−2s−1
to their 14 – 195 keV fluxes, which is the 5σ sensitiv-
ity limit of Swift-BAT all sky survey for 90% of the sky
(Baumgartneretal. 2013). We plot 14 – 195 keV versus
intrinsic 2 – 10 keV luminosities in Fig. 2, and perform FIG.3.—SimilartoFig. 1,butreplacingtheintrinsic2–10keVX-ray
luminositywiththeSwift-BAT14–195keVluminosity.Thetypicalerrorin
Buckley-James linear regression on type 1 and type 2 sub-
samples,takingaccountoftheupperlimits. Bothtype1and L14−195keVisplottedatthelowerrightcornerintheupperpanel.Arrowsplot
theupperlimitstoBATnon-detectedsources.
type2sourcesfollowastatisticallyidenticaltightcorrelation.
10 – 200 keV emission comparing with Sy1s by a factor
Thebest-fittedlinesinFig. 2are:
of 1.9±0.5. This decrement in 10 – 200 keV emission in
Sy1s:y=(0.93±0.03)×(x−43.23)+43.63±0.03 Compton-thinSy2s,althoughstatisticallyinsignificant(91%),
isingoodagreementwithours.Howeveritishardtoattribute
Sy2s:y=(0.92±0.05)×(x−42.63)+43.14±0.04
it to obscuration since it would require an average obscura-
Thisconfirmsthatbothquantitiesmeasuretheintrinsiccorona tion of logNH = 24.3±0.1 cm−2, which is too high com-
emission. It also proves that our absorption corrections to paring with the median column density for their Compton-
the2–10keVfluxesaresufficient, otherwisewe wouldsee thinsample(logN =23.0cm−2)measuredbasedon2–10
H
weaker2–10keVemission(relativeto14–195keVemis- keVspectralfitting. We notethatthesmaller samplesize of
sion)inSy2s. Rigbyetal. (2009) and the fact that a significant fraction of
Comparing 14 – 195 keV emission with [OIV] emission theirsourcesonlyhaveupperlimitsto10–200keVemission
(Fig. 3)wellconfirmsthepatternshowninFig. 1. Residuals mighthavepreventedthemtodetectastatisticallysignificant
for Sy1s and Sy2s are calculated in the same way as above differencebetween Sy1s and Compton-thinSy2s. A similar
(lowerpanelinFig. 3) andthencomparedwith logranksur- trendwasalsoseeninLiu&Wang(2010). Byutilizing[OIV]
vivalanalysis. Wefindasignificantdifferenceataconfidence emissionasintrinsicluminosityindicator,theyfoundthatthe
level of 99.1%, and a flux decrementin type 2 sources by a monochromatic6.4keVcontinuuminSy1sis4.2±1.6times
factorof2.2+0.9. Thebest-fitted(Buckley-Jamesregression) strongerthanCompton-thinSy2s,whichonaveragerequires
linesinFig.−30a.3re: aN =8.6±1.9×1023cm−2 inCompton-thinSy2s,similar
H
tothevaluereportedbyRigbyetal.(2009).
Sy1s:y=(0.83±0.06)×(x−41.17)+43.50±0.06
Sy2s:y=(0.71±0.10)×(x−40.99)+43.06±0.09 4.2. OntheIsotropyof[OIV]25.89µmLineEmission
Although [OIV] 25.89µm emission line is believed to
Sy2s tend to have a slightly flatter correlation slope (though
be a good isotropic indicator of AGN intrinsic lumi-
statisticallyinsignificant)comparingwithSy1s. Itispossible
nosity (Diamond-Stanicetal. 2009; Meléndezetal. 2008;
that the difference between Sy1s and Sy2s is more signifi-
Rigbyetal. 2009), below we discuss possible factors which
cantathighluminosities. Howevermuchlargersamplesare
mightaffectitsisotropy.
neededtodemonstratethispostulation.
Zhangetal.(2008)foundthatSy1shavesmaller[NII]/Hα
ratioson the BPT diagramcomparingwith Sy2s. They pro-
4. DISCUSSION
posedthatthe innerNLRofAGNislikely heavilyobscured
4.1. HintsinPreviousStudies
in type 2 sources by the extendingregionof the torus. This
It is widely assumed that the corona emission in AGNs is effect, if confirmed, could yield weaker [OIV] 25.89µm and
isotropic.However,lowtomoderatelevelofanisotropycan’t [OIII]emissioninSy2sduetoheavyobscurations.
be ruled out based on previous studies. Actually, in several Another factor is that, AGN with smaller covering factor
studieshintsforanisotropicX-rayemissionarevisible. of the obscuring torus (and thus larger open-cone angle of
Rigbyetal.(2009)comparedthe[OIV]25.89µmemission the NLR), is more likely to be viewed as Type 1, than that
with the 10 – 200 keV X-ray emission for the RSA Seyfert with larger torus covering factor (Turneretal. 2009). Nat-
sample. They found that Compton-thin Sy2s are weaker in urally, Sy1s tend to have stronger NLR emission than Sy2s
Sy2s within the two samples, considering a few upper lim-
Sy1 its to [OIV] luminosities. We find no statistical differencein
Sy2 [OIV] luminosity distributions between Sy1s and Sy2s, con-
firming the isotropy of [OIV] line emission as reported by
Diamond-Stanicetal. (2009). However,if correctingthe as-
101 sumed 3.2 times increment in the [OIV] fluxes of Sy2s, the
er distributionsbecomesignificantlydifferentbetweenSy1sand
b
um Sy2s(ataconfidencelevelof99.6%forthe12µmsampleand
N
96.6% for the RSA sample respectively). We conclude that
Sy2s unlikely have relatively stronger [OIV] emission com-
paringwithSy1s.
4.3. OntheSampleIncompletenessandBias
100
0.00 0.05 0.10 0.15 0.20 The sample we complied is based on five [OIV] subsam-
z
ples, thus is not homogeneous. We test the robustness of
FIG.4.—RedshiftdistributionsofSy1sandCompton-thinSy2s. our results with the two complete samples used above – the
12µm Seyfert sample and the RSA Seyfert sample, both of
(Lawrenceetal.2013). whicharehighlycompletein[OIV]measurements. TheirX-
Possibleslitlossin[OIV]measurementsmightalsoleadob- ray completenesses are also high, with available X-ray data
servationalbiasif Sy1sandSy2shave differentredshiftdis- for91outofthe11312µmsourcesand75outofthe89RSA
tributions. InFig. 4weseethatSy2stendtohavelowerred- sources. Also, the X-ray incompleteness is more severe to
shifts in our composite sample, thus slit loss could be more type2AGNsasobscuredsourcesaremuchfainterin X-ray,
severeinSy2sthaninSy1s. ButnoteslitlossinSpitzerspec- therefore correcting such incompleteness (if possible in the
tracouldberatherweakanywayduetothelargeSpitzer-IRS future) would further strengthen our finding. For the 12µm
slitwidth(4.7–11.1′′forobservationsadoptedinthiswork). sampleonly,we findSy2sarefainterinintrinsic2–10keV
Furthermore,the[OIV]emissionregionwasfoundtobemore emission at given[OIV] luminosity comparingwith Sy1s by
compactthat[OIII](Dasyraetal.2011). afactorof2.5+0.7 withaconfidencelevelof95.4%(38Sy1s
−0.5
However, all three possible factors above lead to higher versus 18 Compton-thin Sy2s). Considering only the RSA
[OIV]emissioninSy1scomparingwithSy2s,thereforecould sample,weobtainafactorof2.2+0.6 withaconfidencelevel
noteasetherelativestrongerintrinsichardX-rayemissionin of89.7%(22Sy1sversus17 Com−0p.t5on-thinSy2s). Combin-
Sy1s that we have detected. Instead, our findings would be
ingthe12µmsampleandtheRSAsampleweobtainafactor
furtherstrengthened. of 2.4+0.5 with a confidencelevelof 97.9%(44 Sy1s versus
Onthecontrary,couldthe[OIV]emissionbeinfactintrin- −0.5
29Compton-thinSy2s)Consideringtheremainingsourcesin
sically stronger in Sy2s than in Sy1s? Based on the large
ourcompositesampleweobtainstatisticallyconsistentnum-
dispersion in the ratios of [OIII] to hard X-ray luminosities, bers,afactorof3.2+0.9 withaconfidencelevelof96.6%(36
Trouille&Barger (2010) postulated that some AGNs may −0.8
Sy1sversus16Compton-thinSy2s).Thereforeourresultsare
havelow[OIII]luminositiesbecauseofthecomplexityofthe
independentoftheselectionofsubsamples.
NLRstructure. Theyalsosuggestedthat[OIV]emissionmay
Another issue is the possible bias in the optical identi-
be similarly affected by the NLR structure complexity. Is it
fications of Sy1s and Sy2s due to strong nuclei variations
possiblethattheNLRofSy2sabsorbalargerfractionofion-
in some sources. In other words, a Sy1 could be mis-
izationemission–becauseofmoregasinSy2s,eitheraround
identifiedasSy2ifitsnucleiactivitysignificantlyreducedre-
thenucleioratpc–kpcscalesinthehostgalaxies–thuspro-
cently,yieldingmuchweakerbroademissionlinesandX-ray
vide stronger narrow emission lines? Diamond-Stanicetal.
(2009)showedthatthe[OIV]luminositydistributionsarein- emission but relatively stronger [OIV] emission, since [OIV]
takes a longer time to respond to nuclei variation. How-
distinguishableforobscuredandunobscuredAGNsinacom-
ever, sources with such state transitions would more likely
plete AGN sample – the RSA Seyfert sample. This pro-
be identified as Compton-thick in X-ray, due to the signifi-
vides a strong support to the isotropy of [OIV] emission in
cantweaknessofthecentralX-rayemission(Guainazzietal.
thesensethatSy1sandSy2shaveintrinsicallysimilar[OIV]
2005), which would have been excluded from our sample.
emission. Below we extend such test to examine whether
To further address this issue, we re-divide our whole sam-
weakanisotropyin[OIV]emissioncouldbetoleratedbycur- ple into two subsamples with N <1022 cm−2 (66 sources)
rent data. Considering the best-fitted slope of Sy1s in the H
intrinsic 2 – 10 keV luminosity – [OIV] luminosity distri- and NH >1022 cm−2 (59 sources) respectively, independent
of their optical identifications. Following the method as de-
bution (0.88, see §3), a 2.8 times decrement in the X-ray
emission of Sy2s can be explainedby a 3.2 times increment scribed in §3, we find that Seyfert galaxies with 1022 cm−2
in their [OIV] emission. We test this possibility by com- < NH < 1024 cm−2 have weaker intrinsic X-ray emission
paring the [OIV] luminosity distributions of Sy1s and Sy2s at given [OIV] luminosity than sources with lower NH (by a
withintwocompleteAGNsamples–the12µmSeyfertsam- factor of 1.70+0.30, with a confident level of 99.5%). This
−0.25
ple (Rushetal. 1993; Tommasinetal. 2008, 2010) and the demonstrates that the possible bias in optical identifications
RSASeyfertsample(Maiolino&Rieke1995;Hoetal.1997; is unableto erase the observeddifferencebetween Sy1sand
Diamond-Stanicetal.2009),asmentionedin§2.1.Bothsam- Sy2sinoursample.
ples are highly complete in [OIV] line measurements in our
4.4. OntheAbsorptionCorrection
composite[OIV] sample, with 113outof 116sourcesof the
12µmsampleandall89oftheRSAsample. Weperformlo- In§3,wehaveshownthatourabsorptioncorrectiontothe2
granktestsonthe[OIV]luminositydistributionsofSy1sand –10keVfluxisgenerallysufficient,asSy1sandSy2sfollow
a statistically identical correlation between intrinsic 2 – 10
keVluminosityand14–195keVluminosity. Wealsofound
that both intrinsic 2 – 10 keV emission and 14 – 195 keV
R=0
emissionare relativelyweakerin Sy2sthan in Sy1satgiven 101 Sy1
[OIV] luminosity. We note previous studies also reported Sy2
that Sy2s have relatively weaker hard (> 10 keV) X-ray
emission than Sy1s (Rigbyetal. 2009; Weaveretal. 2010; 100
0.6 0.8 1.0 1.2 1.4
LaMassaetal. 2010), and interpreted this result in terms of
heavy X-ray absorptions. Particularly, Rigbyetal. (2009) R=1
foundthatknownCompton-thinSy2sareweakerin14–195 101 Sy1
keVemissioncomparingwithSy1sbyafactorof1.9±0.5, Sy2
andattributedittoapossibleobscurationwhichismuchheav- ber100
ierthanvaluesobtainedthroughspectralfittingandcouldat- um 0.6 0.8 1.0 1.2 1.4
N
tenuate even 14 – 195 keV X-ray emission. However, such
R=2
interpretation is unlikely for Compton-thin Sy2s, otherwise 101 Sy1
peculiarthickabsorberswhichareessentiallyimpenetrableto Sy2
X-rays would be required in most (if not all) Compton-thin
100
Sy2s, andsuchabsorberscan notbeaccountedforwith cur- 0.6 0.8 1.0 1.2 1.4
rentcommonX-rayabsorptionfittingprocedures.
One may suspect that the absorption correction might be 101 R=3
Sy1
inadequateforheavilyobscuredSy2s(i.e. thosewith1023<
Sy2
N <1024 cm−2),astheirobscurationsmighthavebeensig-
H
nificantly underestimated. However, in Fig. 1 we show that 100
0.6 0.8 1.0 1.2 1.4
Sy2s with 1023<NH <1024 cm−2 (24 out of 45) are statis- Flux/FluxR=0.5
tically indistinguishablefrom Sy2s with N <1023 cm−2 in
H FIG.5.—Distributionsoftheratiosoftheintrinsic2–10keVfluxesmea-
termsoftheirintrinsic2–10keVemission(relativeto[OIV]), suredwithR=0,1,2,3tothosemeasuredwithR=0.5.
rulingoutsuchpossibility.
Below we present further discussions on issues related to oursample.
absorptioncorrection. Tombesietal. (2010) reported detections of highly ion-
We use the traditional photoelectric absorption model ized X-ray obscurations in local AGNs with N ∼ 1022−
H
“wabs” in “Xspec” in spectra fitting. Using a more mod- 1024 cm−2,whichcan’tbecorrectedthroughourspectralfit-
ern obscuration model “tbabs” instead of “wabs” does not ting. However, most of the ionized absorbers are not thick
affect the absorption-corrected fluxes. The mean ratios of enough to significantly attenuate the 2 – 10 keV continuum
theabsorption-correctedfluxesusingtwodifferentmodelsare throughComptonscattering. Furthermore,thesimilardetec-
99.97%and99.76%forSy1sandSy2srespectively. tionratesofsuchabsorbersintype1andtype2AGNssuggest
Duringspectrafitting,wefixthereflectionfractionRofthe thattheyhave nofavoredinclination. Theirlargeskycover-
pexrav model at 0.5. We show in Fig. 5 that adopting dif- age,ifconfirmedwithhighS/Ndetection,alsoindicatesthat
ferentR valuesonlyproducesminorchangesto the intrinsic theywouldnotproduceinclinationdependentbiasestoX-ray
2–10keVfluxes,comparingwiththedifferencewefindbe- fluxes.
tween Sy1s and Sy2s. Also, it was suggested that reflection
inSy2sislikelystrongerthaninSy1s(Riccietal.2011),thus 4.5. OutflowingCorona
adoptinga uniformR for both Sy1s and Sy2s would lead to
It was proposed that there could be projection effect in
asystematic(althoughratherweak)overestimationofthein-
corona’s X-ray emission, as if the corona is flat and opti-
trinsicX-rayemissioninSy2s. Correctiontothiseffectcould
cally thick, it will produce weaker radiation at larger incli-
furtherstrengthenourresult.
nation angle (Zhang 2005). However, studies have shown
Inacoupleoftype1AGNs,hardX-rayexcesseswerede-
that the corona can only cover a minor fraction of the disk,
tectedabove20keV,suggestingtheyarepartiallyobscuredby
and should be patchy instead of flat/disk-like (Haardtetal.
Compton-thickclouds(Turneretal.2009;Reevesetal.2009;
1994).Also,observationsshowedthatcoronaopacityissmall
Risalitietal. 2009a), and such partially covering absorber
(Zdziarskietal. 2000; Molinaetal. 2009). Therefore, the
couldbecommonintype1AGNs(Tatumetal.2013). X-ray
projectioneffectshouldberatherweakinX-raycoronaemis-
spectral fitting at <10 keV thus could have underestimated
sion.Lightbendingeffectcausedbythestronggravityaround
theintrinsic2–10keVemissioninAGNs. Thiseffect,how-
theSMBHcouldalsoproduceanisotropy(Fabian&Vaughan
ever doesnot affectthe major results in this work, since the
2003; Chenetal. 2013) in X-ray emission, however in the
partiallycoveringabsorber,ifubiquitous,producesconsistent
waycontrarytoourfinding.
biasestotype1andtype2sources,sincewehaveshownthey
Thereareotherfactorsthatmaycontroltherelativestrength
followanidenticalcorrelationbetween14–195keVandin-
of X-ray emission in AGNs, i.e., luminosity and Edding-
trinsic 2 – 10 keV emission (see Fig. 2). The biases could
tonratio(normalizedaccretionrate),inthesensethatAGNs
havebeencanceledwhile we comparebetweenthem. Actu-
with higher luminosity and/or Eddington ratio tend to have
ally,theaverageratioof14–195keVfluxtointrinsic2–10
weaker X-ray emission relative to the accretion power (e.g.
keV flux in our sample is consistentwith a power-lawspec-
Fanalietal.2013).Ouranalysesareperformednormalizedto
trumwithaphotonindexΓ=1.78+0.18(Γ=1.87+0.17assum-
−0.37 −0.38 [OIV]luminosity,thushavecorrectedthepossibleeffectoflu-
ingapower-lawspectrumwithareflectionfractionR=0.5),
minosity. Sy1sand Sy2s generallyshow consistentphysical
suggesting that partial covering absorption effect is weak in
parameters, including black hole mass, luminosity and Ed-
dingtonratio(Singhetal.2011). X-rayspectralslopesΓalso
measuresthe Eddingtonratio(Fanalietal. 2013). Our spec-
tral fittings yield consistent Γ between Sy1s and Sy2s (Fig.
7), indicatingthattwosubsampleshaveonaveragethesame
Eddington ratio. Therefore the observed relatively stronger
intrinsic hard X-ray emission in Sy1s than in Sy2s are not
causedbydifferentEddingtonratiosbetweenthetwosubsam-
ples.
Under the scheme of AGN unification model, the differ-
ence we find between Sy1 and Compton-thin Sy2 indicates
anisotropyofthe X-rayemission, whichcan be attributedto
thebeamingeffectofcoronaoutflowingalongtheaxisofthe
accretionsystem.
Below we calculate the bulk outflowing velocity of the
corona required to produce the observed anisotropy. Under
theschemeofAGNunificationmodel,weattributetheclassi- 4
ficationofSy1,Compton-thinSy2andCompton-thickSy2to
differentrangesofinclinationangles. Based onstudyof the 3.5
12µm galaxy sample, an obscured fraction of AGN of 62%
andaCompton-thickfractionofAGNof∼20%werereported 3
(Brightman&Nandra2011). Accordingto these values, we FSy2
define0◦−52◦and52◦−78◦astherangesofinclinationan- F/Sy12.5
glesofSy1sandCompton-thinSy2s(seeFig. 6).
Assuminganoptical-thincoronawithanoutflowingveloc- 2
ity of β =v/c, the beaming factor is defined as the relative
intensityobservedfromaninclinationanglei: 1.5
0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6
b=(γ−1(1−βcosi)−1)3+α Bulkvelocityβ
whereγ =(1−β2)−1/2,αistheenergyindexofthepower- FIG.6.—Intheupperpanel,weplottheprobabilitythataSeyfertgalaxy
couldappearasSy1orSy2asafunction oftheinclination anglei(orthe
law. Here a typical value of α=0.8 is taken. We calculate
solidangle1-cosi)inourtoymodels.Thefilledblueandredregionsmark
theratioofthebeamingfactorsaveragedwithintherangesof thedistinctinclinationanglerangesof0◦−52◦forSy1sand52◦−78◦for
Sy1sandCompton-thinSy2satdifferentbulkvelocities,and Compton-thinSy2s. Withtheseassumptions, therequiredbulkoutflowing
we find that a flux decrement of 2.8+0.5 times of Compton- velocitytoexplaintheaveragedifferenceintheintrinsicX-rayemissionbe-
−0.4 tweenSy1sandCompton-thinSy2s(FSy1/FSy2)isplottedwithasolidlinein
thin Sy2s comparing with Sy1s indicates an outflowing ve- thelowerpanel. Thehachuredregionsundertheblue(Sy1)andred(Sy2)
locity of0.47±0.05c (see Fig. 6). Thiscorrespondsto the curvesintheupperpanelplotacomparisonmodel. Insuchatoymodel,the
escape speed at ∼7−12 GM/c2, and the electron thermal pfuronbctaiboinlitoyfothfeasSoelyidfearntgglael,axwyhbicehinrgedvuiecwesedfraosma1Sya1t0(b◦lutoe0cu.5rvaet)5i2s◦a,cionstihnee
speedinplasmawithtemperatureT∼64keV(E=kT).The sensethatattheotherwiseassumedtransitionangleofi=52◦betweenSy1s
factor of 1.70+0.30 (if simply comparingsources with N > andSy2s,aSeyfertgalaxyhasequalpossibilitytoappearasaSy1oraSy2.
−0.25 H Underthissituation,therequiredbulkoutflowingvelocityishigher,asshown
1022cm−2withthosewithsmallerNH,regardlessoftheirop- bythedashedlineinthelowerpanel.
ticalidentifications,see§4.3)canbetranslatedtoanoutflow-
ingvelocityof0.29+−00..0068c,andthefluxdecrementof2.2+−00..93 accurate calculations rely on future exact knowledge on the
in14–195keVemission(seeFig. 3)correspondstoanout- inclinationsofSy1sandCompton-thinSy2s.
flowingvelocityof0.38+0.12c. In radio-loudAGNswhich constitute only ∼10−20%of
−0.06
ThedifferenceininclinationanglebetweenSy1sandSy2s theAGNpopulation(Kellermannetal.1989),theradioemis-
is likely not as distinct as we have assumed, as suggested sionoriginatesfrompowerfulrelativisticjetsuptokpc–Mpc
by the revised unification model (Elitzur 2012), the clumpy scales,thelaunchofwhichisanoutstandingpuzzleinastro-
torus model (Nenkovaetal. 2008), the possibility that some physics(Nemmenetal.2012).Inradio-quietones,jet-likera-
Sy2s are due to large scale obscurationsin the hostgalaxies diomorphologiesarealsooftendetected(Panessa&Giroletti
(Matt 2000; Malkanetal. 1998; Rutkowskietal. 2013), and 2013), though much weaker and more compact. Learning
the misalignmentbetweenthe axisof jet and the axisof ob- how weak and compact jets are launched can help to un-
scuration in some AGNs (Rabanetal. 2009). For example, derstand the nature of more powerful ones. Strong corre-
theclumpytorusmodelcouldallowforSy1sviewededge-on lations between X-ray and radio emission were detected in
and Sy2s viewed face-on, although with very low probabil- radio-quiet AGNs (Panessaetal. 2007; Laor&Behar 2008;
ities. These effects would reduce the difference in inclina- Wuetal. 2013) and in X-ray binaries (Merlonietal. 2003),
tion between two populations (Sy1s and Sy2s), and require suggestinga close corona-jetcoupling. How couldthishap-
even higher outflowing velocities. To illustrate such effect, penisstillunclear.Populartheoreticalinterpretationsinclude:
we adopta simple toy modelby assumingthat the probabil- bothX-rayandcoreradioemissionoriginatefromstatic(non-
ity of a Seyfert galaxy appearing as Sy1 gradually reduces outflowing) coronal activity (Laor&Behar 2008); magneti-
fromface-ontoedge-on(asacosinefunctionofthesolidan- callydominatedcoronaproducesjet/outflowthatisresponsi-
gle,seetheupperpanelofFig. 6andthecaptionfordetails), bleforradioemission(Merloni&Fabian2002);abortedjets
andcomparetheoutflowingvelocityrequiredinthiscasewith (Ghisellinietal. 2004) produce both radio and X-ray emis-
thatin the abovecase (see the lowerpanelof Fig. 6). More sion; or the role of X-ray corona itself could be subsumed
1.5 1.5 2.0 2.5 3.0 Although this pattern is statistically insignificant yet due to
thelimiteddataquality,thispointsadirectiontofutureobser-
1.0 vations.
0.5 Another consequence of bipolar outflowing corona model
al is relativelyweaker reflection componentfromthe accretion
esidu 0.0 disk beneath the corona comparing with the static corona
R−0.5 model (Beloborodov 1999; Miniuttietal. 2010), because of
−1.0 Fixed the relativistic beaming effect. An outflowing velocity of
Sy1
0.4c would reduce the relative strength of disk reflection by
−11.65 Sy2 a factor of ∼ 2 – 8 comparing with a static corona ( view-
Number1112468024 SSyy12 iddneigstkeicnitnciolsinnoamotfieosnAtrGdoenNpgsenr(edefl.egen.ct,TtiBaonnealockboaomertpoaodlno.ev1n91t99f59r;o9mR).eHythnoeowladecvscer1re9,ti9toh7ne;
0
1.5 2.0 2.5 3.0 Nandraetal. 1997), such as the broad Fe Kα line emission,
Γ
does not necessarily argue against the model of outflowing
FIG.7.—Theresiduals inFig. 1asafunctionofX-rayspectralphoton corona. Inadditiontotheinclinationangleeffect,itispossi-
indexΓ(upperpanel),whichisaniceindicatorofEddingtonratioinAGNs. ble that the corona outflowing velocity in AGNs is not uni-
ThetrendofweakerX-rayemissioninSy2sisvisiblefromlowtohighΓ form. It could be very low in some AGNs, producing no
values. ForacoupleofsourceswithtoofewX-raycounts,Γisfixedat1.8
(greensquares).Themedian(mean)ΓofSy1sandSy2sare1.81(1.86)and beaming effect, thus could explain (likely together with the
1.82(1.83)respectively. K-Stestalsoshowsthereisnostatisticaldifference light bending effect) the detected strong broad Fe Kα lines.
betweenthetwodistributionsofΓ(lowerpanel). Higher outflowing velocity leading to stronger beaming ef-
fect, could also naturally explain the non-detectionof broad
bythejetbase(Markoffetal.2005). Ourfindingremarkably
Fe Kαlines inmanyAGNs(e.g.Nandraetal. 2007). Alter-
matches the scenario that the role of corona could be sub-
natively,therecouldbebothanoutflowingcorona,andanon-
sumedbythebaseofjetinAGNs(Markoffetal.2005),and
outflowing (or even inflowing like magnetic loops) corona
couldnaturallyexplainthemysteriouscorrelationbetweenra-
(Wilkins&Fabian2012). Inthisscheme,thedirectobserved
dioandX-rayemissioninradio-quietAGNs. Twooutstand-
continuum is dominated by the emission from the outflow-
ingpuzzlesin astrophysics,thelaunchesofcoronaeandrel-
ingcorona, while the disk is mainly illuminatedby the non-
ativistic jets, could thereforebe directly bound, and even be
outflowing(orinflowing)corona.Asthevariationsoftheout-
merged.Thesamepicturecouldbeappliedtootherblackhole
flowingcoronaandthenon-outflowing(orinflowing)corona
accretionsystems,suchasX-raybinaries.
do not necessarily correlate, this diagram may naturally ex-
plainthepuzzlingconstantdisk reflectionstrengthwhilethe
4.6. FinalRemarks
continuumemission is highlyvariablein a few sources(e.g.
We have foundthat X-ray coronaemission is intrinsically Fabianetal. 2002), andthe non-uniformvariationpattern of
weaker in type 2 AGNs than in type 1 AGNs. Studies tak- the disk reflection relative to X-ray continuum in different
inghardX-rayemissionasisotropicluminosityproxytocal- sources (e.g. Iwasawaetal. 1996; Wangetal. 1999, 2001;
ibrate emission in other bandscould thereforehave been bi- Shuetal.2010).
ased. ThisalsoimpliesthathardX-raysurveys(evenat>10 Theoreticalcalculations(Malzacetal.2001)showthatout-
keV) are biased against not only Compton-thick AGNs, but flowingcoronaeproducehardX-rayspectrawhicharemuch
also Compton-thin type 2 sources, and the obscured frac- flatterthanobservedvaluesinAGNs(Γ∼1.4versus∼1.9)
tion of AGNs yielded from hard X-ray surveys could have with a bulk-outflowing velocity of ∼ 0.4c. However, this
been underestimated. Interestingly, by taking the anisotropy relies on the assumption that the disk reprocessed radia-
ofhardX-rayemission(althoughinterpreteddifferently)into tion is the main cooler of the coronal plasma, while soft
account,onecantackletheinconsistenceintheobservedob- radiation produced viscously from the accretion disk and
scured AGN fraction and its dependence to luminosity be- bremsstrahlung/cyclo-synchrotron emission in the corona,
tweenX-raysamplesandoptical,radioandIRselectedones have been neglected (Malzacetal. 2001). Such assumption
(Zhang 2005; Mayo&Lawrence 2013), providing an inde- couldonlybevalidifthecoronaisinformofstrongconcen-
pendentsupporttotheanisotropyofcoronaX-rayemission. trated flares and locally the reprocessed flux from the disk
Outflowingcoronaemayproducesystematicallyhighercut- dominates the soft seed radiation (Haardtetal. 1994), and
offenergiesinthehardX-rayspectraintype1AGNsthanin maynotholdinreality.
type2sourcesduetoDopplershift.Assumingtheviewingan- WenotethelargedispersionsinthecorrelationbetweenX-
glestotype1AGNsspanfrom0◦−52◦and52◦−90◦totype ray and [OIV] luminosities. The standard deviations of the
2AGNs(Brightman&Nandra2011),theexpecteddifference residuals (as defined in §3, see Fig. 1) are 0.5 and 0.7 for
incut-offenergybetweentwopopulationsisbyafactorof1.4, Sy1sandSy2srespectively,both(slightly)largerthantheav-
adoptingabulkoutflowingvelocityof0.47c. Thisdifference eragedifferencebetweenSy1sandSy2s(log2.8∼0.45).The
ishowevertoosmalltobedetectedwithcurrentobservations largescattercouldbecausedbymanyfactors,includingX-ray
consideringthe large uncertaintiesand huge intrinsic scatter variation, the structureof NLR, the spectral energydistribu-
in the measured cut-off energies (Molinaetal. 2013). Nev- tionofAGNs, etc. Interestingly,adispersionintheoutflow-
ertheless, it could be an efficient approach to measuring the ingvelocityofthecoronainAGNscouldalsoproducealarge
coronaoutflowinthefuture.Riccietal.(2011)comparedthe scatterin thecorrelationbetweenX-rayand[OIV] luminosi-
composite INTEGRAL spectrum of Sy2s with that of Sy1s. ties. Further investigationsare requiredto interpretboth the
Interestingly, from their Fig. 7 we can see a likely drop at large scatter and the offset in the correlations between Sy1s
above 80 keV in the ratio between the composite spectra of andSy2ssimultaneously.
Sy2sandSy1s,suggestingSy2shavelowercut-offenergies.
5. SUMMARY 3: Under the scheme of AGN unification model, this result
suggests the coronae in AGNs are bipolar outflowing
To investigate whether the corona emission in radio-quiet
withavelocityof∼0.3−0.5c. Suchoutflowingcoro-
AGNsisisotropic,wecompilealargesampleofAGNswith
naecouldnaturallybelinkedtothebasesofweakjets
both[OIV]25.89µmemissionlinefluxmeasurementsandX-
insuchsystems.
ray spectra. Only secularly identified Compton-thinsources
are included. Known radio-loud sources are also excluded.
4: Wediscusstheimplicationsofoutflowingcoronae,includ-
Taking [OIV] luminosity as a proxy of AGN intrinsic lumi- ing the relative strength of disk reflection component
nosity,wecomparetheabsorption-corrected2–10keV(also
(i.e. the broad Fe Kα line), the selection bias in hard
Swift-BAT14–195keV)luminositiesofSy1sandCompton-
X-ray surveys, the validity of using hard X-ray emis-
thinSy2s. Ourresultsaresummarizedasfollows:
sion as AGN luminosity proxy, the obscured fraction
of AGNs in X-raysurveys, and a particularprediction
1: At given [OIV] luminosity, the absorption-corrected 2 – that Sy2sshould have smaller X-raypower-law cutoff
10 keV X-ray emission is stronger in Sy1s than in energiescomparingwithSy1s,whichcouldbetestified
Compton-thin Sy2s at a confidence level >99.9% by withfuturehardX-rayobservations.
a factorof2.8+0.5. ConsistentpatternisseeninSwift-
−0.4
BAT14–195keVemission.
This workissupportedby Chinese NSF throughgrant
2: Basedoncarefulanalyses,wearguethatthedifferencecan 10825312&11233002. JXWacknowledgessupportfrom
not be attributed to sample selection, insufficient ab- Chinese Top-notch Young Talents Program. We thank
sorption corrections, or anisotropy in [OIV] line emis- TingguiWang,FengYuan,AndrzejZdziarskiandFuguo
sion. This is the first solid detection of moderate Xiefordiscussions,andtheanonymousrefereeforhelpful
anisotropyinAGN’scoronaX-rayemission. comments.
APPENDIX
SHORTNOTESONINDIVIDUALSOURCES
A short note is given to each source whose intrinsic 2 – 10 keV flux is taken from literature. Generally their X-ray spectra
areofhighqualityandhavebeennicelyinterpretedinpreviousstudies. Forsourceswithmultipleobservations,weadopttheir
exposure-timeweightedmeanintrinsicluminositiesinlogarithmspace.
NGC3516: Turneretal.(2005)performeddetailedspectralanalysisonthreeXMM/ChandraobservationsofNGC3516,revealed
threedistinctabsorbingcomponents.
2MASXJ05580206-3820043(H0557-385): Longinottietal. (2009) foundwith XMM data that 2MASX J05580206-3820043
is absorbed by both ionized gas and partially covering neutral gas. It showed extreme flux variation by a factor of 10,
entirelyduetointerveningline-of-sightclouds.
ESO434-G040(MCG-5-23-16): Braitoetal.(2007)revealedwithXMM-Newton-RGSspectrumthatthesoftX-rayemission
of ESO 434-G040 is likely dominated by several emission lines superimposed on an unabsorbed scattered power-law
continuum.ThisSy1.9hasabroadFeKαline.
UGC03973(Mrk79): Galloetal.(2011)fittedthespectraofUGC03973(XMM/Suzaku)withapower-lawwithionizedreflec-
tionandabsorption,plusabroadFeKαline.
NGC3227: Markowitzetal.(2009)modeledthetime-averagedspectrumofNGC3227(observedbyXMM)asamoderatelyflat
power-lawwithtwoionizedabsorbers.
NGC3783: Krongoldetal.(2003)analyzedtheChandraHETGSspectrumofNGC3783.Theirmodelconsistsofatwo-phase
ionizedabsorber.
NGC4388: Beckmannetal. (2004) presented INTEGRAL and XMM observationsof NGC 4388, and foundheavy absorption
(2.7×1023cm−2).
NGC4395: Nardini&Risaliti (2011) analyzed the XMM and Suzaku spectra of the dwarf Seyfert NGC 4395, attributing its
spectralvariationtovaryingpartialcoveringabsorber.
NGC4507: Braitoetal.(2013)presentedtheSuzakuobservationofNGC4507,foundavariableabsorberandastrongreflected
component.
ESO323-G077: Jiménez-Bailónetal.(2008)analyzedtheXMMspectraofESO323-G077,foundaneutralabsorber(withN
H
=5.82×1022cm−2),andtwoionizedabsorberplusabroadFeKαline.
NGC1365: Risalitietal.(2009b)detectedanabsorptionwithN ∼3.5×1023cm−2 crossingthelineofsightbasedona60ks
H
XMMobservation.AbroadFeKαlineisalsodetected.
FAIRALL0049(IRAS18325-5926): Moczetal.(2011)detectedablue-shiftedionizedabsorberintheChandraHETGSspec-
trumofIRAS18325-5926.
NGC5506: Guainazzietal.(2010)foundabroadFeKαlineintheX-rayobscuredNLS1NGC5506usingXMMdata.
MRK0273: The ULIRG Mrk 273 is mergingwith an unabsorbedSy2 Mrk 273x, which lies 1.2 arcmin away. Balestraetal.
(2005)usedthreecollisionallyionizedplasmacomponentsinadditiontoanabsorbedpower-lawtofittheXMMspectrum
ofMrk273.Theintrinsicfluxis22.1×10−13.
MCG-03-34-064: Miniuttietal. (2007) analyzed the XMM spectrum of MCG-3-34-64 (IRAS 13197-1627), recovering
Compton-thinabsorptionandabroadFeKαemissionline.
NGC4151: Lubin´skietal.(2010)presentacomprehensivespectralanalysisofNGC4151,usingtheINTEGRAL,RXTE,XMM,
SwiftandSuzaku,consideringionizedabsorber,partialcoveringabsorber,andreflection.
MRK0463E: Bianchietal. (2008)analyzedtheChandra,XMMandHSTdataofthedoublenucleusULIRGMrk463,which
is consistof Mrk463EandMrk463Wwith a projectedseparationof3.83′′. Their2– 10keVintrinsic luminositiesare
1.5×10−43and3.8×10−42ergs−1respectively,asmeasuredwithChandradata.The[OIV]fluxofMrk463Eismeasured
withSpitzerIRSLHmodule(slitwidth11.1′′),thusmaybepollutedbyMrk463W.
NGC4253(Mrk766): The NLS1 Mrk766 is highlyvariablein the hardX-ray flux ontime-scales asshortas a few hundred
seconds. Turneretal.(2007)attributesthespectralvariabilityofMrk766tothevariationsinacomplexandmulti-layered
absorber.
NGC6860: Winter&Mushotzky(2010)analyzedtheXMMandSuzakuobservationsofNGC6860,andfoundatwo-component
warmionizedabsorberandabroadFeKαline.
NGC0788,NGC6300,NGC7172,ESO103-G035: The2–10keVintrinsicluminositiesofthesesourcesaretakenfromthe
broad-bandX-rayspectralanalysisbydeRosaetal.(2012)usingvariousavailableinstrumentsincludingXMM,Chandra,
Suzaku,INTEGRALandBeppoSAX.
FAIRALL0009: Lohfinketal. (2012) fitted the multi-epoch spectra of FAIRALL 9 as observed by XMM and Suzaku with a
modelincludingarelativisticallyionizedreflectioncomponent.
NGC1052: Brennemanetal. (2009) analyzedthe 101ksSuzakuspectrumofNGC 1052. The0.5–10keV continuumis well
modeledbyapower-lawcontinuummodifiedbyGalacticandintrinsicabsorptionandasoft,thermalcomponentbelow1
keV.BroadFeKαlineisalsodetected.
MRK335: MRK 335 was monitored by XMM with 200 ks exposure. Its spectra show warm absorber and blurred reflection
(Galloetal.2013).
UGC05025: Galloetal. (2005) fitted the Chandra spectrum of UGC 05025 (MRK 705) with a primary power-law and an
additionalbrokenpower-lawrepresentingthesoftexcess.
MRK1298: Giustinietal.(2011)foundmassiveionizedabsorbersalongthelineofsightofMRK1298(PG1126-041).
IC5063: LaMassaetal.(2011)fittedtheChandraspectrumofIC5063withadoubleabsorbedpower-law.
ESO383-G035: Milleretal.(2008)compiledalltheavailablelong-exposure,high-qualitydataforESO383-G035(MCG-6-30-
15):522ksofChandraHETGS,282ksofXMMPN/RGSand253ksofSuzakuXIS/PINdata.Avariablepartial-covering
zoneplusabsorbedlow-ionizationreflection,distantfromthesource,providesacompletedescriptionofthevariableX-ray
spectrum.
NGC1097: WiththehighresolutionobservationofChandra,Nemmenetal.(2006)eliminatedthenearbyULXandsurrounding
star-formingringofNGC1097,andpresentedthespectrumofthenucleusregion.
IC4329A: Steenbruggeetal. (2005) detected seven distinct absorbing systems in the high-resolution X-ray spectrum of IC
4329AtakenwithXMM.
NGC4051: Steenbruggeetal. (2009) fitted the Chandra LETGS high-resolutionspectrum of NGC 4051 with multiple warm
absorbermodels.
NGC1566,NGC4941: Kawamuroetal. (2013) presented broad band (0.5–195 keV) X-ray spectra of NGC 1566 and NGC
4941observedwithbothSuzakuandSwiftBAT.