ASTRO-H White Paper-AGN Reflection

20 Pages·2014·6.5 MB·English

ASTRO-H Space X-ray Observatory
White Paper
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1 AGNReflection
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e C.Reynolds(UniversityofMaryland),Y.Ueda(KyotoUniversity),H.Awaki(EhimeUniversity),
D L.Gallo(SaintMary’sUniversity),P.Gandhi(UniversityofDurham1),
3 Y.Haba(AichiUniversityofEducation),T.Kawamuro(KyotoUniversity),S.LaMassa(YaleUniversity),
A.Lohfink(UniversityofMaryland2),C.Ricci(KyotoUniversity),F.Tazaki(KyotoUniversity3),

E] andA.Zoghbi(UniversityofMaryland4)
H onbehalfoftheASTRO-HScienceWorkingGroup
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h FigureCredit: ESA/V.Beckmann(NASA)
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1AlsoatUniversityofSouthampton
2AlsoatUniversityofCambridge
3AlsoatNationalAstronomicalObservatoryofJapan
4AlsoatUniversityofMichigan
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Abstract
X-rayobservationsprovideapowerfultooltoprobethecentralenginesofactivegalacticnuclei(AGN).
AhardX-raycontinuumisproducedfromdeepwithintheaccretionflowontothesupermassiveblackhole,
andallopticallythickstructuresintheAGN(thedustytorusofAGNunificationschemes,broademission
lineclouds,andtheblackholeaccretiondisk)“lightup”inresponsetoirradiationbythiscontinuum. This
WhitePaperdescribestheprospectsforprobingAGNphysicsusingobservationsoftheseX-rayreflection
signatures. High-resolutionSXSspectroscopyoftheresultingfluorescentironlineintype-2AGNwillgive
us an unprecedented view of the obscuring torus, allowing us to assess its dynamics (through line broad-
ening) and geometry (through the line profile as well as observations of the “Compton shoulder”). The
broad-bandviewobtainedbycombiningalloftheASTRO-Hinstrumentswillfullycharacterizetheshape
of the underlying continuum (which may be heavily absorbed) and reflection/scattering, providing crucial
constraints on models for the Cosmic X-ray Background with a subsequent impact on understanding of
supermassiveblackholeevolution. ASTRO-Hwillalsopermittherelativisticallybroadenedreflectionspec-
trumfromtheinneraccretiondisktoberobustlystudied,evenincomplexsystemswith,forexample,warm
absorptionandcompositesoftexcesses. Finally,theHXIwillallowthedetectionandstudyofreverberation
delaysbetweenthecontinuumandtheComptonreflectionhumpfromtheinnerdisk.
2
CompletelistoftheASTRO-HScienceWorkingGroup
TadayukiTakahashia,KazuhisaMitsudaa,RichardKelleyb,FelixAharonianc,HirokiAkamatsud,FumieAkimotoe,
SteveAllenf,NaohisaAnabukig,LorellaAngelinib,KeithArnaudh,MarcAudardi,HisamitsuAwakij,AyaBambak,
MarshallBautzl,RogerBlandfordf,LauraBrennemanb,GregBrownm,EdwardCackettn,MariaChernyakovac,
MengChiaob,PaoloCoppio,ElisaCostantinid,JelledePlaad,Jan-WillemdenHerderd,ChrisDonep,TadayasuDotania,
KenEbisawaa,MeganEckartb,TeruakiEnotoq,YuichiroEzoer,AndrewFabiann,CarloFerrignoi,AdamFosters,
RyuichiFujimotot,YasushiFukazawau,StefanFunkf,AkihiroFuruzawae,MassimilianoGaleazziv,LuigiGallow,
PoshakGandhip,MatteoGuainazzix,YoshitoHabay,KenjiHamaguchih,IsamuHatsukadez,TakayukiHayashia,
KatsuhiroHayashia,KiyoshiHayashidag,JunkoHiragaaa,AnnHornschemeierb,AkioHoshinoab,JohnHughesac,
UnaHwangad,RyoIizukaa,YoshiyukiInouea,HajimeInouea,KazunoriIshibashie,ManabuIshidaa,KumiIshikawaq,
YoshitakaIshisakir,MasayukiItoae,NaokoIyomotoaf,JelleKaastrad,TimothyKallmanb,TuneyoshiKamaef,
JunKataokaag,SatoruKatsudaa,JunichiroKatsutau,MadokaKawaharadaa,NobuyukiKawaiah,DmitryKhangulyana,
CarolineKilbourneb,MasashiKimuraai,ShunjiKitamotoab,TetsuKitayamaaj,TakayoshiKohmuraak,
MotohideKokubuna,SaoriKonamir,KatsujiKoyamaal,HansKrimmb,AyaKubotaam,HideyoKuniedae,
StephanieLaMassao,PhilippeLaurentan,Franc¸oisLebrunan,MauriceLeuteneggerb,OlivierLimousinan,
MichaelLoewensteinb,KnoxLongao,DavidLumbap,GrzegorzMadejskif,YoshitomoMaedaa,KazuoMakishimaaa,
MaximMarkevitchb,HironoriMatsumotoe,KyokoMatsushitaaq,DanMcCammonar,BrianMcNamaraas,JonMillerat,
EricMillerl,ShinMineshigeau,IkuyukiMitsuishie,TakuyaMiyazawae,TsunefumiMizunou,KojiMoriz,
HideyukiMorie,KojiMukaib,HiroshiMurakamiav,ToshioMurakamit,RichardMushotzkyh,RyoNaginog,
TakaoNakagawaa,HiroshiNakajimag,TakeshiNakamoriaw,ShinyaNakashimaa,KazuhiroNakazawaaa,
MasayoshiNobukawaal,HirofumiNodaq,MasaharuNomachiax,SteveO’Dellay,HirokazuOdakaa,TakayaOhashir,
MasanoriOhnou,TakashiOkajimab,NaomiOtaaz,MasanobuOzakia,FritsPaerelsba,Ste´phanePaltanii,ArvindParmarx,
RobertPetreb,CiroPinton,MartinPohli,F.ScottPorterb,KatjaPottschmidtb,BrianRamseyay,RubensReisat,
ChristopherReynoldsh,ClaudioRicciau,HelenRusselln,SamarSafi-Harbbb,ShinyaSaitoa,HiroakiSameshimaa,
GoroSatoag,KosukeSatoaq,RieSatoa,MakotoSawadak,PeterSerlemitsosb,HiromiSetabc,AuroraSimionescua,
RandallSmiths,YangSoongb,ŁukaszStawarza,YasuharuSugawarabd,SatoshiSugitaj,AndrewSzymkowiako,
HiroyasuTajimae,HiromitsuTakahashiu,HiroakiTakahashig,YohTakeia,ToruTamagawaq,TakayukiTamuraa,
KeisukeTamurae,TakaakiTanakaal,YasuoTanakaa,YasuyukiTanakau,MakotoTashirobc,YuzuruTawarae,
YukikatsuTeradabc,YuichiTerashimaj,FrancescoTombesib,HiroshiTomidaai,YohkoTsuboibd,MasahiroTsujimotoa,
HiroshiTsunemig,TakeshiTsurual,HiroyukiUchidaal,YasunobuUchiyamaab,HidekiUchiyamabe,YoshihiroUedaau,
ShutaroUedag,ShiroUenoai,ShinichiroUnobf,MegUrryo,EugenioUrsinov,CordeVriesd,ShinWatanabea,
NorbertWernerf,DanWilkinsw,ShinyaYamadar,HiroyaYamaguchib,KazutakaYamaokae,NorikoYamasakia,
MakotoYamauchiz,ShigeoYamauchiaz,TahirYaqoobb,YoichiYatsuah,DaisukeYonetokut,AtsumasaYoshidak,
TakayukiYuasaq,IrinaZhuravlevaf,AbderahmenZoghbih,andJohnZuHoneb
aInstituteofSpaceandAstronauticalScience(ISAS),JapanAerospaceExplorationAgency(JAXA),Kanagawa252-5210,Japan
bNASA/GoddardSpaceFlightCenter,MD20771,USA
cAstronomyandAstrophysicsSection,DublinInstituteforAdvancedStudies,Dublin2,Ireland
dSRONNetherlandsInstituteforSpaceResearch,Utrecht,TheNetherlands
eDepartmentofPhysics,NagoyaUniversity,Aichi338-8570,Japan
fKavliInstituteforParticleAstrophysicsandCosmology,StanfordUniversity,CA94305,USA
gDepartmentofEarthandSpaceScience,OsakaUniversity,Osaka560-0043,Japan
hDepartmentofAstronomy,UniversityofMaryland,MD20742,USA
iUniversite´deGene`ve,Gene`ve4,Switzerland
jDepartmentofPhysics,EhimeUniversity,Ehime790-8577,Japan
kDepartmentofPhysicsandMathematics,AoyamaGakuinUniversity,Kanagawa229-8558,Japan
lKavliInstituteforAstrophysicsandSpaceResearch,MassachusettsInstituteofTechnology,MA02139,USA
mLawrenceLivermoreNationalLaboratory,CA94550,USA
nInstituteofAstronomy,CambridgeUniversity,CB30HA,UK
oYaleCenterforAstronomyandAstrophysics,YaleUniversity,CT06520-8121,USA
pDepartmentofPhysics,UniversityofDurham,DH13LE,UK
qRIKEN,Saitama351-0198,Japan
rDepartmentofPhysics,TokyoMetropolitanUniversity,Tokyo192-0397,Japan
sHarvard-SmithsonianCenterforAstrophysics,MA02138,USA
3
tFacultyofMathematicsandPhysics,KanazawaUniversity,Ishikawa920-1192,Japan
uDepartmentofPhysicalScience,HiroshimaUniversity,Hiroshima739-8526,Japan
vPhysicsDepartment,UniversityofMiami,FL33124,USA
wDepartmentofAstronomyandPhysics,SaintMary’sUniversity,NovaScotiaB3H3C3,Canada
xEuropeanSpaceAgency(ESA),EuropeanSpaceAstronomyCentre(ESAC),Madrid,Spain
yDepartmentofPhysicsandAstronomy,AichiUniversityofEducation,Aichi448-8543,Japan
zDepartmentofAppliedPhysics,UniversityofMiyazaki,Miyazaki889-2192,Japan
aaDepartmentofPhysics,UniversityofTokyo,Tokyo113-0033,Japan
abDepartmentofPhysics,RikkyoUniversity,Tokyo171-8501,Japan
acDepartmentofPhysicsandAstronomy,RutgersUniversity,NJ08854-8019,USA
adDepartmentofPhysicsandAstronomy,JohnsHopkinsUniversity,MD21218,USA
aeFacultyofHumanDevelopment,KobeUniversity,Hyogo657-8501,Japan
afKyushuUniversity,Fukuoka819-0395,Japan
agResearchInstituteforScienceandEngineering,WasedaUniversity,Tokyo169-8555,Japan
ahDepartmentofPhysics,TokyoInstituteofTechnology,Tokyo152-8551,Japan
aiTsukubaSpaceCenter(TKSC),JapanAerospaceExplorationAgency(JAXA),Ibaraki305-8505,Japan
ajDepartmentofPhysics,TohoUniversity,Chiba274-8510,Japan
akDepartmentofPhysics,TokyoUniversityofScience,Chiba278-8510,Japan
alDepartmentofPhysics,KyotoUniversity,Kyoto606-8502,Japan
amDepartmentofElectronicInformationSystems,ShibauraInstituteofTechnology,Saitama337-8570,Japan
anIRFU/Serviced’Astrophysique,CEASaclay,91191Gif-sur-YvetteCedex,France
aoSpaceTelescopeScienceInstitute,MD21218,USA
apEuropeanSpaceAgency(ESA),EuropeanSpaceResearchandTechnologyCentre(ESTEC),2200AGNoordwijk,TheNetherlands
aqDepartmentofPhysics,TokyoUniversityofScience,Tokyo162-8601,Japan
arDepartmentofPhysics,UniversityofWisconsin,WI53706,USA
asUniversityofWaterloo,OntarioN2L3G1,Canada
atDepartmentofAstronomy,UniversityofMichigan,MI48109,USA
auDepartmentofAstronomy,KyotoUniversity,Kyoto606-8502,Japan
avDepartmentofInformationScience,FacultyofLiberalArts,TohokuGakuinUniversity,Miyagi981-3193,Japan
awDepartmentofPhysics,FacultyofScience,YamagataUniversity,Yamagata990-8560,Japan
axLaboratoryofNuclearStudies,OsakaUniversity,Osaka560-0043,Japan
ayNASA/MarshallSpaceFlightCenter,AL35812,USA
azDepartmentofPhysics,FacultyofScience,NaraWomen’sUniversity,Nara630-8506,Japan
baDepartmentofAstronomy,ColumbiaUniversity,NY10027,USA
bbDepartmentofPhysicsandAstronomy,UniversityofManitoba,MBR3T2N2,Canada
bcDepartmentofPhysics,SaitamaUniversity,Saitama338-8570,Japan
bdDepartmentofPhysics,ChuoUniversity,Tokyo112-8551,Japan
beScienceEducation,FacultyofEducation,ShizuokaUniversity,Shizuoka422-8529,Japan
bfFacultyofSocialandInformationSciences,NihonFukushiUniversity,Aichi475-0012,Japan
4
Contents
1 GeneralIntroduction 6
2 MappingthestructureofobscuredAGN 8
2.1 TheX-rayViewofType-2AGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2 ASTRO-H/SXSstudiesoftype-2AGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3 BroadbandContinuumstudiesofType-2AGNwithASTRO-H . . . . . . . . . . . . . . . . . 10
3 TheNatureofCompton-ThickAGNs 11
3.1 TheImportanceofCompton-ThickAGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.2 TheComptonShoulderasaProbeoftheObscurer . . . . . . . . . . . . . . . . . . . . . . . 12
3.2.1 BroadbandContinuumStudiesofCTAGN. . . . . . . . . . . . . . . . . . . . . . . . 13
4 ProbesoftheRelativisticAccretionDiskandSMBH 13
4.1 IntotheHeartoftheAGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2 DisentanglingtheAccretionDiskSpectrumFromOtherComplexities . . . . . . . . . . . . . 15
4.3 X-rayReverberationfromtheInnerDisk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5 SummaryofTopScience 18
5
1 General Introduction
Afteroverfourdecadesofstudy,activegalacticnuclei(AGN)continuetobeofgreatinteresttoastrophysicists.
With the realization that supermassive black holes (SMBHs) are ubiquitous, being present at the center of
essentially every galaxy, a tremendous amount of attention is focused on the co-evolution of SMBHs and
galaxies. ThetightcorrelationbetweenthemassoftheSMBHinagalacticcenterandthevelocitydispersion
of the stellar budge (Gebhardt et al. , 2000; Ferrarese & Merritt , 2000) found in the local universe strongly
suggests that SMBHs and host galaxies co-evolved. In the extreme (probably realized in massive galaxies),
a SMBH can dramatically suppress baryonic cooling and star formation in the galaxy, all-but truncating any
furthergrowthofthegalaxy. AGNarealsointerestingintheirownright,providingoneofthemostaccessible
laboratoriesforstudyingtheextremesofphysicsfoundclosetothehorizonofablackhole(BH).
Figure1:TheanatomyofanAGNasaccordingtothestandardunifiedmodel.FigurefromUrry&Padovani (1995)
AnystudyofAGNmuststartbycharacterizingtheir“engineeringblueprint”,i.e.,thepropertiesoftheSMBH
(mass and spin), the distribution of circumnuclear matter, the (inward and outward) flow of matter, and the
formstakenbytheliberatedenergyasitleavestheAGN.OurcurrentunderstandingissummarizedinFigure1.
At its heart, an AGN consists of a SMBH fed by an accretion disk. Powerful winds can be produced from
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the accretion disk which probably produce the optical broad line region (BLR) with characteristic velocities
of 2,000–20,000kms−1. These broad optical lines are the defining characteristic of the classical unabsorbed
(type-1)AGN.Furtherout,weknowthatmanyAGNpossessacoldanddustystructure(the“dustytorus”)that
can,dependinguponviewingangle,obscuretheBLRandthecentralaccretiondisk,leadingtoaclassification
as an obscured (type-2) AGN. The nature (and even the location) of this important structure is unclear. The
torusmaybethereservoirfortheaccretiondisk,beingfedbycoldgasflowsfromtheISMofthegalaxy—in
other words, it is gas on the way in. Alternatively, it may be the colder/outer regions of the disk wind — gas
on the way out. Both possibilities may be realized across the AGN population. In either case, “the torus” is
the interface of the AGN with the rest of the galaxy, and understanding its basic nature is crucial if we are to
understandtheAGN-galaxylink.
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Figure2:ExampleofanX-rayreflectionspectrum,assumingaslabofmatterirradiatedbyapower-lawspectrumwithaphotonindex
Γ=2(reddashedline).Thesurfaceoftheslabisassumedtohaveanionizationparameterofξ=1ergcms−1.Figureproducedusing
thexillvercodeofGarciaetal. (2013)
X-ray spectroscopy is a powerful tool for determining the engineering blueprint of an AGN. X-rays are
produceddeepwithintheheartoftheAGN,withinafewgravitationalradii(r ≡GM/c2,where M isthemass
g
of the SMBH) of the black hole. Any optically-thick structures in the AGN (the accretion disk, BLR clouds,
and the torus) will then be irradiated by the X-ray continuum and respond by producing an “X-ray reflection
spectrum”. AnexampleofanX-rayreflectionspectrumisshowninFigure2. TheclassicsignaturesofX-ray
reflection are a strong iron-Kα emission line (at energies 6.4–6.97keV depending upon the ionization state
of the reflector) and a broad hard X-ray hump peaking at 20keV where Compton reflection dominates over
photoelectricabsorption(atlowerenergies)andComptonrecoillosses(athigherenergies). Ifthereflectorhas
amoderate-to-highionizationparameter,asisthecasefortheinneraccretiondisk,thereflectionspectrumalso
possessesaforestofsoftX-rayradiative-recombinationfeatures.
TheironlineisacrucialdiagnosticofAGNstructure;ironisanabundantelementandpossessstrongemis-
sion/absorptionfeaturesthatarerelativelyisolatedfromotherspectralcomplexities. TheironlineistoAGN/X-
rayastronomerswhattheCOrotationallinesaretostar-formation/mm-waveastronomers. Detailedstudiesof
reflectionspectraallowustostudythedistributionandkinematicsofgasflowingin,outandaroundtheSMBH;
withthistool,wecanstarttodeterminetheblueprintoftheAGNcentralengine.
This White Paper (WP) discusses the impact of ASTRO-H on studies of AGN structure using X-ray reflec-
tion. The WP is divided into two parts. We start (Sections 2 and 3) by discussing the ability of ASTRO-H to
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mapoutthestructuresthataredistantfromtheSMBH.Section2discussestheimpactofASTRO-Honstudies
of photoionized winds and the torus in general type-2 AGN, whereas Section 3 focuses on the particularly
interesting case of Compton-Thick AGN. The superior spectral resolution of the ASTRO-H/SXS and the su-
perior bandbass achieved by combining SXS and HXI data will transform our knowledge of these systems.
Asalreadymentioned, anunderstandingofAGNwindsandthetorusiscrucialifwewishtotrulyunderstand
how the AGN is fueled and how it feeds back on its host galaxy. The second part of this WP (Section 4)
discusses the ability of ASTRO-H to study X-ray reflection from the inner accretion disk, where relativistic
Doppler and gravitational redshifts can strongly skew the observed spectrum. Reflection from the inner disk
is a broad-band phenomenon, resulting in a soft excess (as the radiative recombination lines are smeared into
a pseudo-continuum), the broadened iron line, and the Compton reflection hump. ASTRO-H will be the first
observatorycapableofsimultaneouslyobservingandcharacterizingindetailallthreeaspectsofdiskreflection.
Thisenergyreachrepresentsanimportantadvance.
2 Mapping the structure of obscured AGN
2.1 TheX-rayViewofType-2AGN
The basic nature of dusty tori in AGN remains unclear. We are ignorant about many of their most basic
characteristics, including their location, size, total mass, clumpiness, dynamics, and relation to the accretion
diskandBLR.Eventhe“definition”ofthetorusisambiguous,anditisnotentirelyclearwhetheritisdistinct
structure or just a smooth continuation from the BLR. A particularly curious fact is that they must typically
subtend a large solid angle as seen from the SMBH in order to produce the observed fraction of type-2 AGN.
Theoretical studies suggest that it is not easy to stably sustain such a cold but large scale-height (h/r ∼ 1)
structure. This implies that the torus is not just static structure butmay be a sequence of dynamic phenomena
like nuclear starburst (e.g., Hopkins et al., 2012; Wada, 2012). The total mass of a torus may be an important
parameterreflectingtheevolutionalstageofthegalaxy(Kawakatu&Wada,2008).
Highqualitybroad-bandX-rayspectragiveuniqueinsightsintotheanatomyofanAGN.HardX-rayshave
strong penetrating power against obscuration, enabling us to study the properties of the obscuring matter and
surroundinggas. Type-2AGNareparticularlywellsuitedforastudyofAGNstructure;(1)informationonthe
line-of-sightmaterialcanbeobtainedthroughX-rayabsorption,(2)photoionizedgaslocatedoutsidethetorus
(inthenarrowlineregion;NLR)canbeobservedinemissionandscatteringlinesthatgivesdefiniteprobeson
itsphysicalstate,(3)theequivalentwidthsofiron-Klinesfromtheobscuringtorusbecomelargethankstothe
attenuatedcontinuumlevelandhencearemorereadilycharacterized,and(4)thehighinclinationanglesmake
iteasiertomeasureKeplerianmotionoflineemittingmatter.
The typical X-ray spectrum of a type-2 AGN is characterized by the combination of a heavily absorbed
primary power-law, Compton reflection with fluorescence lines from the cold torus (with additional reflection
possible from the BLR and accretion disk), and soft X-ray emission dominated by the emission lines from
highly ionized gas (Turner et al., 1997). The soft emission lines can provide diagnostic tools to reveal the
temperature,density,andionizationstateofthegassurroundingthesupermassiveblackhole.
Asanillustration,Figure3(leftpanel)showstheSuzakuspectrumoftheprototypicalSeyfert-2galaxyMrk3
(z = 0.0135), one of the brightest objects among the class (Cappi et al., 1999). Despite having an absorption
column that is marginally Compton-Thick, we can see still see the direct (absorbed) power-law continuum at
high-energies. Wealsoseethatsomefractionofthiscontinuumisscatteredaroundthethetorus. AcoldX-ray
reflectioncomponentfrom,presumably,Compton-Thickregionsofthetorusoutofourlineofsightproducesa
strong6.4keVironfluorescencelineandcontributessignificantlytothecontinuumabove5keV.Thesoft-band
isverycomplex(Sakoetal.,2000;Pounds&Page,2005;Awakietal.,2008),requiringphotoionizedemitters
withatleastthreedifferentionizationparametersaswellasacollisionallyionizedthermalplasmacomponent
(seefigurecaption).
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Figure 3: Left panel : The spectrum of Mrk 3 as observed by the Suzaku/XIS in 2005. The continuum (direct/absorbed an
scattered/unabsorbed) is shown as dotted orange line. A strong cold reflection component is marked as a solid orange line. The
complex soft spectrum is described as coming from three different photoionized plasmas (light blue; logξ = 0.01, N =1.0×1020
H
cm−2, green; logξ = 1.8, N = 1.4×1021 cm−2, blue; logξ = 2.9, N = 2.0×1022 cm−2) and an optically-thin thermal plasma (ma-
H H
genta; apec in XSPEC) with temperature kT of 1.0 keV. Right panel : The simulated SXS spectrum of Mrk 3 for a 200ksec
exposure based on the spectral model for Mrk 3. We employ the redistribution matrix function (RMF) with 5eV resolution, the
currentbest-estimated(fileah sxs 5ev basefilt 20100712.rmf), andtheancillaryresponsefile(ARF)forthepointsource(file
sxt-s 120210 ts02um of intallpxl.arf).
2.2 ASTRO-H/SXSstudiesoftype-2AGN
The superior resolution of the ASTRO-H/SXS will permit a giant leap forward in the study of these line-rich
AGN.Forexample,Figure3(rightpanel)showsasimulationofa200ksSXSobservationofMrk3. Comparing
withtheSuzaku/XISspectrum,wecanseethattheforrestofsoftX-rayemissionlinesisresolved,allowingus
toemploythefullmachineryoflineratiodiagnosticsfordensity,temperature,andexcitationmechanism. The
Kα-tripletlinesfromhelium-likeionsareparticularlyuseful(Gabriel&Jordan,1969;Porquet&Dubau,2000).
WhilethesesoftX-raylineshavealreadybeenseenatsuchresolutionsbythegratinginstrumentsonChandra
and XMM-Newton, ASTRO-H/SXS provides significantly more sensitivity at soft energies (permitting the full
spectralresolutiontoberealizedinrealisticexposuretimes)and, simultaneously, unprecedentedresolutionof
theironlinecomplex.
Theabilitytoresolveandcharacterizethevelocityprofileofthefluorescentironlinewillpermitthefirststudy
ofthedynamicsoftheX-rayreflectingtorus. Atthesametime,theresolutionoftheSXSwillgreatlyfacilitate
ourabilitytocentroidtheKαlineaswellasdetecttheKβline—bothoftheseobservablesaresensitivetothe
ionization state of the gas. Putting this information together will permit us to determine the location (r), size
(∆r) and density (n) of the torus material — the velocity broadening ∆v gives us the location from Keplerian
√
arguments(∆v ≈ GM/r),thedominantionizationstatesofirontellustheionizationparameter(ξ = L/nr2,
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whereL istheionizingluminosity)andthecombinationofthesetwoyieldsthedensity. Comparingthedensity
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withthelineofsightcolumndensity(whichcanbemeasuredfromthecontinuumabsorption,providedthatthe
AGNisCompton-thin)givesthesizeofthetorus∆r.
Whiledetaileddynamicalandionizationmodelswillbeneededtointerprettherealresults,wecanobtaina
first look at the impact of ASTRO-H/SXS results by assuming that the torus has a single ionization parameter
andtheformofadisk-likestructureinKeplerianmotionabouttheSMBH.Forthisexercise,weusetheXSTAR
photoionizationcodetomodelthescatteredandemittedspectrumofatoruswithadensityofn = 1013 cm−3,a
columndensityof N = 1023cm−2 irradiatedbyaSeyfert-likeX-raysource(luminosityof1044 ergs−1 (0.01–
H
10 keV) and a power-law spectrum of Γ = 2.0). We adopt three different distances for the torus, 1 pc, 0.1
pc, and 0.01 pc, which we designate the “far torus”, “near torus”, and “BLR” cases, respectively. These tori
spectra are then convolved with the Doppler broadening appropriate for a disk in Keplerian motion (using the
9
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nts s k−1 0.5 nts s k−1 0.5 nts s k−1 0.5
u u u
d co0.2 d co0.2 d co0.2
alize0.1 alize0.1 alize0.1
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Energy (keV) Energy (keV) Energy (keV)
Figure4: SimulatedspectraofNGC4388withtheSXS,assuming300ksexposure. Threecasesshownarethefartorus(left),near
torus(middle)andBLR(right).
disklinemodelwithanemissivityprofiler−2)assuminganinclinationof45◦.
Figure 4 shows a simulated 300ks SXS spectra in the iron-K band for NGC 4388, the brightest obscured
Compton-thin AGN with a 14–195keV flux of F = 2.8×10−10ergs−1cm−2 (Baumgartner et al., 2013). It is
clearlyseenthatdetailsofthe6.4keVlineprofileareresolved,inthiscaserevealingthedouble-peakstructure
characteristic of the (assumed) keplerian disk. We can use the centroid and Kα/Kβ line ratios to constrain the
dominantionizationstateoftheiron; fortheneartoruscase, weobtaintherangeFeII–FeVI,consistentwith
theXSTARmodelthatisdominatedbyFeII.Inthissimulatedcase, wecanalsoconstraintheinnerradiusof
thetorustobetterthan3%,andmeasuremeanbulkmotionwithaprecisionof< 100kms−1.
Whiletheparametersofthissimulationareclearlysimplistic, ithighlightsthefactthatASTRO-Hwillopen
anewwindowonAGNtori.
2.3 BroadbandContinuumstudiesofType-2AGNwithASTRO-H
The Cosmic X-ray Background (CXB) results from the integrated X-ray emissions of all the AGN in the
Universe, and the spectrum, normalization and spatial fluctuations of the CXB are important constraints on
models for the cosmological evolution of SMBHs. While it is spectrally smooth, the CXB has a substantially
harderspectrumthananyknownunabsorbedAGNand,infact,absorbedAGNarecrucialcomponentsanyCXB
model. AgoodknowledgeofthereflectionandcontinuumparametersofAGNisfundamentalforthecreation
of such models. Especially important but ill-constrained are the cut-off energies for the power-law continua
from AGN resulting from the finite temperatures of the X-ray emitting coronae. This is another area where
ASTRO-Hwillmakeamajorimpact,thistimeduetoitsabilitytosimultaneouslydeterminetheparametersof
theprimarycontinuum,scatteringofthatcontinuum,reflection,andthecontinuumcut-off.
Again, we illustrate this with the example of NGC 4388. A broad-band study of this source performed
combiningXMM-NewtonEPIC/PN,INTEGRALIBIS/ISGRIandSwift/BATdatashowsanabsorbedcontinuum
with a photon index of Γ = 1.79+0.07 and moderate levels of reflection (Ricci et al. in prep). However, even
−0.08
withthisarrayofdata,thehigh-energycutoffofthepower-lawcouldnotbedetermined.
To appreciate the importance of the high-energy cut-off, it is necessary to consider the physical processes
responsible for producing the primary X-ray continuum. The primary X-ray continuum is thought to be pro-
duced through Comptonization of the thermal optical/UV photons from the accretion disk by hot electrons in
a magnetically energized corona (e.g., Haardt & Maraschi, 1993). The most basic parameters that we need to
know about the corona if we wish to understand its nature and origin are its temperature kT and optical depth
τ. Now, the resulting spectrum is approximated as an exponentially cut-off power-law, with a cut-off energy
at E ∼ 3kT. If we only have knowledge of the photon index of the power-law, we can only constrain the
C
combinationkT ×τ(proportionaltotheso-calledComptony-parameter). Onlyifwemeasurethehigh-energy
cutoffwecanbreakthedegeneracybetweenthesefundamentalcoronalparameters.
Very recent NuSTAR results are already demonstrating the dramatic impact that a focusing hard X-ray tele-
scopeishavingonourabilitiestomeasurecut-offenergiesinAGNspectra(seestudyofIC4329A;Brenneman
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