Properties of AGN coronae in the NuSTAR era II: hybrid plasma

0.13 MB·

by  A. C. Fabian

Mon.Not.R.Astron.Soc.000,000–000(0000) Printed25January2017 (MNLATEXstylefilev2.2)
Properties of AGN coronae in the NuSTAR era II: hybrid plasma
A.C. Fabian1∗, A. Lohfink1, R. Belmont2,3, J. Malzac2,3 and P. Coppi4
1InstituteofAstronomy,MadingleyRoad,CambridgeCB30HA
2UniversitedeToulouse;UPS-OMP;IRAP;Toulouse,France
7 3CNRS;IRAP;9Av.colonelRoche,BP44346,F-31028Toulousecedex4,France
1 4YaleCenterforAstronomyandAstrophysics,YaleUniversity,NewHaven,Connecticut06520-8121,USA
0
2

n 25January2017
a
J

4 ABSTRACT
2 Thecorona,ahotcloudofelectronsclosetothecentreoftheaccretiondisc,producesthehard

X-raypower-lawcontinuumcommonlyseeninluminousActiveGalacticNuclei(AGN).The
]
E continuumhas a high-energyturnover,typicallyin therangeof oneto several100keV and
issuggestiveofComptonizationbythermalelectrons.WearestudyinghardX-rayspectraof
H
AGNobtainedwithNuSTARaftercorrectionforX-rayreflectionandundertheassumption
.
h thatcoronaearecompact,beingonlyafewgravitationalradiiinsizeasindicatedbyreflection
p and reverberation modelling. Compact coronae raise the possibility that the temperature is
- limitedandindeedcontrolledbyelectron-positronpairproduction,asexploredearlier(Paper
o
I).Hereweexaminehybridplasmasinwhichamixtureofthermalandnonthermalparticles
r
t ispresent.Pairproductionfromthenonthermalcomponentreducesthetemperatureleading
s
a toawidertemperaturerangemoreconsistentwithobservations.
[
Keywords: blackholephysics:accretiondiscs,X-rays:binaries,galaxies

1
v
4
7
7 1 INTRODUCTION blackholehasaverysmallmassofMc=10−8m26R21M⊙,thermal
6 energy
Theprimarypower-law, X-rayemission fromluminous accreting
0
. blackholesoriginatesfromthecorona,whichisahotcloudclose E ≈1042m2R2erg (1)
1 tothecentreoftheaccretiondisc.SoftdiscphotonsareCompton- th 6 1
0
7 upscatteredinthecoronatomakethepower-lawcontinuum.Itof- and crossing time tcross =50m6R1s. The luminosity obtained if
1 ten shows a high-energy turnover somewhere between about 30 all the coronal energy is released on a crossing time Lcross ≈
: keV and 1 MeV, generally interpreted as the temperature of the 2×1040m6R1ergs−1ismuch less than the Eddington luminosity
v corona(oravalueclosetoit).Theprimarycontinuumisusuallythe LEdd, in that Lcross/LEdd ≈10−4R1. Higher luminosities, as are
Xi mostvariablepartofthespectrum,showingrapidchangeswhich, typicallyobserved,requirethatbothcoolingandheatingoccuron
when interpreted using X-ray reflection and reverberation signa- afaster timescale. Asargued by MF01, the energy supplying the
r
a tures, indicate that the corona has a relatively small size of less heatmustbestoredinthecorona,probablyintheformofmagnetic
thanafewtensofgravitationalradii(rg=GM/c2)(Fabian2013; fieldspoweredbytheaccretiondisc.Itisthenunclear,ifonlyheat-
Reis&Miller 2013; Uttleyetal. 2014). The fraction of the bolo- ingandcoolingareinvolved,whatcontrolstheplasmatemperature,
metricpowerpassingthroughthecoronarangesfromabout10to ortemperatures,inthecoronaandhowanystableequilibriumises-
50percent(Vasudevan&Fabian2007). tablished.
A compact corona in a luminous accreting black hole sys-
Herewefollowuponourrecentpaper(F15),whichexplored
tem must be a dynamic structure since the heating and cooling
theproperties(temperatureandcompactness)ofcoronaededuced
timescalesforthehotelectronsarelessthanthelightcrossingtime
in the light of NuSTAR observations. The coronal plasma is as-
ofthecorona(Fabianetal.2015,PaperI,F15).Thereisnotenough
sumedtobethermalandthetemperatureatthemaximumallowed
timeforanysimpleequilibriumtobeestablished.Thiscanbefur-
byelectron-positronpairproduction: heatingpushes thetempera-
ther considered by looking at the thermal energy content of the
tureupwarduntilitisbalancedbythecreationofpairs–thepair
corona,whichissmall(Merloni&Fabian2001,MF01).Aspheri-
thermostat(Svensson1984;Zdziarski1985).NuSTARobservations
calcoronawithtypicalpropertiesofunitThomsondepth,tT=1, ofActiveGalacticNuclei(AGN)showtheyavoidtheregionwhere
temperature kT =50keV and radius 10R1rg above a 106m6M⊙ pairproductionrunsaway,indicatingthattheprocessmayplaya
role.HoweverthereisasignificantnumberofAGNforwhichthe
temperatureismuchsmallerthanexpectedonthebasisofpurely
thermal pair production (e.g., Balokovic´etal. 2015; Ursinietal.
∗ E-mail:[email protected] 2016).
(cid:13)c 0000RAS
2 A.C. Fabianet al
Hereweaimtotackletheissueofalowtemperaturecorona
e−−e−
byconsideringhybridcoronaecontainingamixtureofthermaland coupling
1000
nonthermalparticles.Weassumethatthecoronaishighlymagne- slab
tizedandpowered bydissipationof magnetic energy. Such dissi- hemisphere
pation is often intermittent in both space and time. In a compact sphere at 0.5h
100
coronatheheatingandcoolingaresointensethatenergeticparti-
clesmaynothavetimetothermalizebeforeinverseComptoncool-
ingreducestheirenergy(seetheelectron-electronequilibrationline ℓh
10
inFig.1,whichisadaptedfromF15:theelectron-protonlinelies
about 3 orders of magnitude lower). Only a small fraction of the
electronsneedstartfromenergiesofanMeVormoreforthemto
1
emithardphotonswhichcollideandcreateelectron-positronpairs.
Ifthepairsareenergeticenoughthenyetmorepairsareproduced
andarunawaysituationcanoccur.Meanwhilethecooledpairscan,
beforeannihilating,sharetheavailableenergyleadingtoareduc- 10 100 1000
tion in the mean energy per particle and thus temperature of the kT [keV]
thermalpopulation,whichmaybecomposedmostlyofpairs.This
can result in a relatively cool thermal particle population with a Figure1.Electrontemperature(kT)–compactness(ℓ)distributionforNuS-
temperaturewellbelow100keV,alow-levelhardtailandapossi- TAR observed AGN (blue points) as first shown in Fabianetal. (2015).
blebroadannihilationlineat511keV.Unlessverysensitivehard Thee–ecouplinglinefromGhisellini,Haardt&Fabian(1993)isincluded:
X-rayobservationsareobtained,theemittedcontinuumappearsto electronsintheregionabovethedashedlinedonothavetimetothermally
originatefromalow-temperaturethermalplasmaincapableofpair equilibratewitheachotherbeforecoolingoccurs.Pairlines,aboveandto
therightofwhichthereisrunawaypairproduction,takenfromSternetal.
production. Paradoxically, what appear to be thelowest tempera-
(1995)arealsoshown.Theslablinehasbeenextrapolatedslightlytohigher
tureobjectscanbethemostpair-dominated.
ℓ.Opencirclesareusedforobjectswherethetemperaturemeasurementisa
Hybrid models for the corona were introduced over two lowerlimit.Thetemperaturesshownaretheobservedvaluesreportedfrom
decadesago(Zdziarski,Lightman&Maciolek-Niedzwiecki1993; spectralfits.
Ghisellini,Haardt&Fabian 1993) providing a better explanation
forthehardX-rayspectraofluminousaccretingblackholesthan
purelynonthermal(Svensson1994)orthermal(Fabian1994)mod- 1 ℓnth/ℓh=0.0
els. Comparison of hybrid models with AGN spectra has been ℓnth/ℓh=0.1
cJoahrrniesodnoeuttaol.n1N99G7C),4u1s5in1g(ZOdSzSiaErsdkait,aJforhonmsotnhe&CMomagpdtozniaGrzam19m96a-; 10−1 ℓℓnntthh//ℓℓhh==00..23
Ray Observatory to conclude that the nonthermal contribution in
NGC4151islessthan15percent.AstudyofstackedAGNspec- 10−2
trafromOSSEbyGondeketal.(1996)wasunabletodistinguish
dτdp
between the possibilities. More recent work on NGC4151 using
INTEGRALdataLubinskietal.(2010)showsthatthermalmodels 10−3
fitthespectra,whichextendupto200keV,initsbrightstateand
thatthesourceappearsharderinitslowstate.Thereisnonewin- 10−4
formation on any possible hard tail or annhilation feature; OSSE
resultsremain the most sensitive at those energies. Integral spec-
trainthe2–200keVbandof28brightSeyfertgalaxies,combined 10−5
with lower energy data from other instruments, are characterized 10−3 10−2 10−1 1 10
bythermalComponizationwithameantemperaturekT ∼50keV p
(Lubin´skietal.2016)andatailtohighervalues.Wefoundasimi-
lardistributionfromNuSTARdata(F15,Fig.1). Figure2.Lepton distributions foranincreasing sequence ofℓnth/ℓh and
ℓh/ℓs=1atℓh=100,wherepisthedimensionlessleptonmomentum(bg )
Hybrid models have been successfully applied to the spec- and ddtp =R×s T×ddnp withRbeingtheradiusofthespheremodelledby
tra of X-ray Black Hole Binaries (BHB) such as Cyg X-1 Belm,s TtheThomsoncross-section,andntheleptonnumberdensity.As
where high energy tails are clearly seen (Gierlinskietal. 1999; the non-thermal fraction increases the peak of the distribution moves to
McConnelletal. 2001; Parkeretal. 2015), for other sources see lowerenergies.
also (Groveetal. 1998; Kalemcietal. 2016; Wardzinskietal.
2002;Droulansetal.2010).Inthispaper,weconcentrateonAGN,
conclusions inSection3.Detailedspectralcomparison withindi-
sincehigh-frequency,X-rayreverberationresultsindicatetheloca-
vidualobjectsislefttoalaterpaper.
tion and size of the corona. Such reverberation has not yet been
detectedinBHB.
OurgoalhereistocomparetherangeofNuSTARresultswith
2 HYBRIDPLASMAS
thepairthermostatoperatinginhybridplasma.Wearenotatthis
stageattemptingtoreproducespectraofindividualobjects.Wede- The excellent hard X-ray sensitivity of NuSTAR has allowed de-
fine our parameters and discuss hybrid plasmas in more detail in tailedspectral modelling of the reflection component and precise
Section2.WecomputeandshowkT−ℓdiagramsforvariouslev- measurementsofthespectralturnoverinthecontinuumspectrum
elsofthenonthermalfractionalcontribution.Finallywediscussour ofmanyX-raybrightActiveGalacticNuclei(AGN)tobemade.
(cid:13)c 0000RAS,MNRAS000,000–000
PropertiesofAGNcoronaeII 3
1000 1000
h 100 h 100
ℓ ℓ
ℓnth/ℓh=0.0 ℓnth/ℓh=0.0
ℓnth/ℓh=0.01 ℓnth/ℓh=0.01
ℓnth/ℓh=0.09 ℓnth/ℓh=0.09
ℓnth/ℓh=0.17 ℓnth/ℓh=0.17
ℓnth/ℓh=0.23 ℓnth/ℓh=0.23
ℓnth/ℓh=0.29 ℓnth/ℓh=0.29
ℓnth/ℓh=0.33 ℓnth/ℓh=0.33
10 10
10 100 10 100
kT [keV] kT [keV]
Figure3.kT−ℓdistributionsforanincreasingsequenceofℓnth/ℓhandtwodifferentℓh/ℓsvalues:1(leftpanel),0.1(rightpanel).Asthenon-thermalfraction
increasestheequilibriumtemperatureislower.Similarly,ageometrywithmoresoftphotonsenteringthecoronaleadstolowerequilibriumtemperatures.
Inordertounderstandwhatsetsthosetemperatures,itisuse-
ful to show these measurements in the form of a temperature-
100
compactness, kT−ℓ, diagram (Fig. 1; adapted from F15), where
kT istheelectrontemperatureandℓisthedimensionlesscompact-
nessparameter
L s 10
ℓ= RmeTc3 (2) νFν
withL being theluminosity, R theradius of thesource (assumed
spherical), s T theThomson crosssectionandme themassof the 1 ℓnth/ℓh=0.0
electron.
ℓnth/ℓh=0.1
Coronaeshould liealongoneofthelinesinthekT−ℓplot, ℓnth/ℓh=0.2
which in Fig. 1 represent the expectation for a purely thermal ℓnth/ℓh=0.3
plasma,whosetemperatureisregulatedbypairs.Thistakesplace 0.1
1 10 100 1000
asthecoronaisheatedanddrivendiagonallyupwardsfromleftto Energy [keV]
rightintheplot(i.e.increasingbothLandkT)untilpairsstartto
formfromenergeticcollisions.Thisthenreducesandfinallystabi- Figure4.Simulatedspectraforℓhof10,100,1000(dashed,solid,dotted)
lizesthetemperatureattheline. andℓh/ℓsof1foranincreasingsequenceofℓnth/ℓh.
Sourcesshouldnotbeseentotherightoftheline,sincecatas-
trophicpairproductionwouldensue,rapidlydrivingdownthetem-
results are obtained using EQPAIR1 (Coppi 1999).) BELM si-
perature.Ifasourceisconfirmedtooccurandremainintheforbid-
multaneouslysolvesthecoupledkineticequationsforleptonsand
denregion,thenthebasisofthemodelneedstobere-considered,
photons ina(magnetized,) uniform, isotropic medium. Theequi-
withsomethingotherthanastaticcompactcoronabeingrelevant.
libriumenergydistributionsof theComptonizing leptonsarecal-
Jettedemissionwouldbemorelikelyinthiscase.Objectswellto
culatedself-consistently,takingintoaccountthecouplingbetween
theleftofthelineposetheproblemofwhatstabilizestheminsuch
particlesandradiation.Radiationtransferisdealtwithusingasim-
ahighlydynamicsituation?
pleescapeprobabilityformalism.Thecodetakesintoaccount all
Weproposeherethathybridcoronaecansolvethisproblem.
therelevantradiationprocessessuchasself-absorbedsynchrotron
In our hybrid corona scenario, we assume that part of the coro-
radiation,Comptonscattering,self-absorbedbremsstrahlungradi-
nal heating power, Lnth, creates a nonthermal power-law electron ation, pair production/annihilation, coulomb collisions, and uses
spectrumwithinthesource,withcompactnessparameterℓnth.The prescriptions for particle heating/acceleration. In the simulations
thermalheatingoftheelectronpopulationℓth andtheinjectionof showninthispaper theeffectsof cyclo-synchrotron radiationare
soft photons ℓs continues as in a standard, thermal scenario. The
neglected (magnetic fieldisset to0G). EQPAIRisvery similar
total heating is then given by ℓh=ℓth+ℓnth and the nonthermal (andgivesidenticalresults)butassumesthelowenergycomponent
forpatcitciaolndiesptthhust ,ℓnpthla/yℓsh.aTshterornagtioroℓlhe/iℓns,dtoetgeertmheinriwngiththtehepTowhoemr-lsaown oftheelectrondistributionisaperfectMaxwellian,whileourelec-
trondistributionsarecalculatedaccordingtothefullkineticequa-
slopeoftheemergingX-rayradiation,withhighvaluesleadingto
aflatspectrumandlowvaluestoasteepone.Increasingt ,which
canbearesultofpairproduction,tendstohardenthespectrum. 1 An excellent discussion of hybrid plasmas and their possible appli-
We use the BELM code (Belmont,Malzac&Marcowith cation to BHB is given in the unpublished paper by Paolo Coppi at
2008)forsimulatingemissionfromhybridplasmas.(Comparable http://www.astro.yale.edu/coppi/eqpair/eqpap4.ps
(cid:13)c 0000RAS,MNRAS000,000–000
4 A.C. Fabianet al
1000
ℓnth/ℓh=0.0
ℓnth/ℓh=0.01
1000 ℓnth/ℓh=0.09
ℓnth/ℓh=0.17
ℓnth/ℓh=0.23
ℓnth/ℓh=0.29
h 100 100 ℓnth/ℓh=0.33
ℓ ℓnth/ℓh=0.0 ℓh
ℓnth/ℓh=0.01
ℓnth/ℓh=0.09
10
ℓnth/ℓh=0.17
ℓnth/ℓh=0.23
ℓnth/ℓh=0.29
ℓnth/ℓh=0.33 1
10
0.95 1.0 10 100 1000
PairFraction(2×τe+) kT [keV]
τtotal
Figure6.NuSTARdataoverplottedonthetheoreticalpredictionsforkT−ℓ
Figure5.Pairfractionforanincreasingsequenceofℓnth/ℓhandℓh/ℓs=1. distributionsforanincreasingsequenceofℓnth/ℓhwithℓh/ℓs=1.Thedata
Whilethecoronacanbeconsideredpairdominatedforallcases,thepair havebeencorrectedforgravitationalredshift.
fractionincreasesfurtherforahighernon-thermalcontribution.
tionsinBELM.Non-thermalparticlesareinjectedwithapower- temperature, since the cutoff energy is 2–3 times higher than the
lawindexof2.5rangingfromg1=1.3tog2=1000forthepurpose gastemperaturedepending ontheopticaldepthofthecorona(i.e
ofthispaper. whetherthinorthick)andalsoonthegeometryofthecoronaand
Theelectrontemperatureisobtainedbyfittingthethermalpart itsillumination(seee.g.Petruccietal.2001).
of the lepton distribution with a Maxwellian. While this approx-
imationbecomes lessaccurate as thenon-thermal contribution to
the lepton distribution increases (Fig. 2), it still characterizes the
generalbehaviorofthetemperaturewell.
3 DISCUSSION
Figs3showthekT−ℓdistributionaslinesatconstantvalues
ofℓnth/ℓhandfortwodifferentvalues(1and0.1)ofℓh/ℓs.HardX- Wefindthatcurrentdataareconsistentwiththepairthermostatop-
rayspectraforafixedratioofℓth=100andanincreasingsequence eratinganddeterminingthetemperatureofcompactAGNcoronae,
ofℓnth/ℓhareshowninFig.4.Weseethatasthenonthermalpower providedthatahybridplasmaisinvolved.Formostsources,only
isincreased from zero, then the temperature of the thermal com- asmalladditionofnonthermalplasmaisrequired.Thewiderange
ponent drops from around several hundred keV to below 20keV. oftemperaturesatafixedvalueofℓh canbeaccountedforwitha
The drop in spectral turnover is apparent in the spectra as is the non-thermalfractionrangingupto30percent.Objectsmeasured
riseofthehardX-rayfluxandtheannihilationlineatthehighest to have the lowest temperatures require the greatest nonthermal
nonthermalfractions. fraction.Theyshouldthenhavethestrongesthighenergytailand
Toassureourselvesthattheobserveddecreaseinelectrontem- broad annihilationlines. Suchfeaturesaregenerally undetectable
peratureisduetopairsweplotthepairfractioninFig5.Itisclear withpresentinstrumentationinAGN.
thatthecoronaearepairdominated. Wealsoacknowledgethatthesimpleonezoneapproachinthe
Finally,inFig.6weshowtheNuSTARdatapointsfromFig.1 current modelling is probably too simplistic to capture the likely
againinthekT−ℓdiagramwithourhybridcoronaresultsoverlaid. geometry of the corona and introduces a small uncertainty in the
Afew new datapointshavebeen addedfromveryrecent results, predictedtemperatures.
seeTable1fordetails.Thenewvalueswereobtainedinasimilar In future work, we shall explore the location of individual
fashiontoFabianetal.(2015).IncontrasttoFig.1,wecorrected sourcesinkT−ℓh−ℓh/ℓsspace,whichplaysaroleindetermining
thedatapointsforgravitationalredshiftingassumingthemeasured the spectral index as well as the continuum cutoff. We shall also
coronalsizeandheightgiveninTable1or10rgifnomeasurement include Stellar Mass Black Hole Binaries (BHB) and investigate
exists. A 10 to 30 per cent nonthermal contribution appears suf- theeffectofmagneticfieldsactingthroughthesynchrotronboiler
ficienttoaccount formost of theseobjectsand particularlythose (see e,g. Malzac&Belmont (2009); Veledina,Vurm&Poutanen
withwellconstrainedtemperatures. (2011))oncurrentspectralresults.
Wehavenotaccountedfortheeffectoflightbendingoncoro-
nalfluxasthisdependsontheinclinationandotherparameters.It
couldincreasetheintrinsicvalueofℓh byafactoroftwo,oreven
moreifthecoronaliesclosetotheblackhole.Iftheemissionsite
ACKNOWLEDGEMENTS
withinthecoronaisverylocalizedthentheobservationallyinferred
valueofℓhcouldbeunderestimatedbyafactorofafew. Wethanktherefereeforhelpfulcomments.ACFandALacknowl-
Wenotethatmostofthespectralfitsusedindeterminingthe edge support from ERC Advanced Grant FEEDBACK 340442.
coronal temperaturesassume apower-lawcontinuum withanex- R. Belmont and J. Malzac acknowledge financial support from
ponentialhigh-energycutoffasanapproximationtoafullComp- theFrenchNational ResearchAgency (CHAOSprojectANR-12-
tonizationspectrum.Thisintroducesasystematicuncertaintyinthe BS05-0009)
(cid:13)c 0000RAS,MNRAS000,000–000
PropertiesofAGNcoronaeII 5
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6 A.C. Fabianet al
Table1.Thetargets andproperties oftheactive galactic nuclei withcut-off constraints resulting fromobservations withNuSTAR.Thereferences tothe
individualworksaregivenintheright-mostcolumn.
Source z log(M) rco Fx Ecut kT ℓ (1+zgrav) G Data References
[M⊙] [rG] [keV]
CenA 0.002 7.65+0.11 10 34.34 >1000 >333 11+4 1.12 1.80±0.01 XMM/NU 1−2
−0.14 −3
NGC4593 0.009 5.45+0.13 10 0.50 90+40 45+20 612+242 1.12 1.59+0.03 XMM/NU 3−4
−0.15 −20 −22 −158 −0.02
Mrk766 0.013 6.8+0.05 3.4 0.88 >441 >147 244+34 1.48 2.22+0.02 XMM/NU 5−6
−0.06 −27 −0.03
3C120 0.033 7.75±0.04 10 2.23 305+142 153+71 153+18 1.12 1.70+0.10 XMM/NU 7−8
−74 −76 −16 −0.03
PG1211+143 0.081 8.16+0.11 10 0.32 >124 >41 63+28 1.12 2.51±0.2 NU 9−10
−0.16 −14
4C74.26 0.104 9.6±0.5 7 1.17 183+51 92+26 20+42 1.18 1.81±0.03 Swift/NU 11−12
−35 −42 −13
3C273 0.158 8.84+0.16 10 1.71 52+2 26+1 365+79 1.12 1.66±0.01 NU 13−14
−0.11 −2 −9 −43
PG1247+267 2.038 8.3+0.17 4 0.02 89+134 45+56 2480+1023 1.38 2.35+0.09 XMM/NU 15−16
−0.15 −34 −26 −803 −0.08
53+2 XX XX XMM/NU 17−18
−2
Fxisthe0.1-200keVX-rayfluxin10−10ergcm−2s−1.
References:1:Fuerstetal.(2015),2:Neumayeretal.(2007),3:Ursinietal.(2016),4:Petersonetal.(2005),5:Buissonetal.,inprep.,6:
Bentz,Walsh&Barth(2010),Grieretal.(2013),7:Lohfinketal.,inprep.,8:Grieretal.(2012),9:Zoghbietal.(2015),10:Petersonetal.(2004),11:
Lohfinketal.,submitted,12:Woo&Urry(2002),13:Madsenetal.(2015),14:Petersonetal.(2004),15:Lanzuisietal.(2016),16:Treveseetal.(2014)
(cid:13)c 0000RAS,MNRAS000,000–000