Broadband X-ray Spectraof AGNsandSynthesisof the X-ray BackgroundYoshihiro Ueda
Kyoto University

X-ray Study of AGN:
major unresolved issues
1.

Origin of the hard X-ray background (E>10 keV)

2.

Number density and evolution of heavily obscured(Compton-thick or CT) AGNs and their contribution
to SMBH growth3.

Nature and origin of obscuring material (“torus”)

4.

Physics of outflow and its impacts to surroundings

5.

Structure of innermost disk (standard disk or RIAF?)

Population Synthesis ofthe X-ray backgroundThe XRB spectrum just gives only an boundary condition

It is easy to construct a “population synthesis model” as there aremany free (poorly constrained) parameters

Key elements

Ajello+ 08

Comoving space number density(Lx, z) or luminosity function
 Absorption distribution function
or NH Function

X-ray Background (XRB)

X-ray broadband spectra
Contribution of Compton thickUnabsorbed AGN

AGNs (log NH >24)
210100
Energy (keV)

Effect of Obscuration

X-ray spectra become “hard” by photoelectric absorption
 Even hard X-rays above 10 keV are suppressed when heavily
Compton thick (NH > a few 1024 cm-2)
 CT AGNs show complex spectra (reflection dominant below
10 keV): need physically self-consitent spectral models !
Ueda+14
log NH = 20.5
21.5 22.5 23.5
NH= 1023

24.5

25.5

NH = 1024

NH = 1025

Photons

Brightman&Nandra+11

Simulated Spectra by Brightman&Nandra+11 model

1

101

10 1
10Energy (keV)110

Absorption distribution in the local universe
Swift/BAT, INTEGRAL with follow-up by Suzaku and otherobservatories: absorbed AGNs dominant Intrinsic CT AGN fraction in total AGNs is ~30% (cf. observed~6-8%) if NH function is extrapolated to NH = 1026 cm-2


Swift/BAT 9-month (Ueda+ 14)“Absorbed”
Compton thin

20

21

2223C-thick

24 25

Log NH

Swift/BAT 70-month (RIcci+, submitted)

Absorbed Fraction (Compton thin AGN)
see Guainazzi’s talkThe absorbed AGN fraction
increases with redshift as~(1+z)0.5-0.6 by keeping theknown anti-correlation with theluminosity
It reflects “averaged” covering
fraction of torus: luminosity andredshift dependent unifiedscheme?

Low Lx (orhigh z)

High Lx (orlow z)

Ueda+ 14

X-ray Baldwin EffectAnti-correlation of the iron-K line equivalent width vsluminosity for type 1 AGNs (Iwasawa&Taniguchi 93)
 Good guality Suzaku spectra confirm this effects in bothtype 1 and type 2 AGNs (Ricci+ 14)
 Consistent with expectation from the luminositydependent unification at LX>1042 erg/s

LFeKα/L10-50

Kawamuro+ 15

Lx (10-50 keV)

AGN X-ray Luminosity Function (Ueda+14)(see also Buchner+14, Miyaji+15, Aird+15)“Multi-cone” X-ray surveys with different depth, width, andenergy bands. Utilize only samples with high identification
completeness (total >97%)
 LF is coupled with the absorption distribution (including CTAGNs), both are simultaneously constrained from the fit.
The LF is coupled with the absorption
distribution, both are simultaneouslyconstrained from the fit.

Number density evolution
AGNs with lower Lx have the number density peak at lower
redshift ( “Downsizing” or “anti-hierarchical evolution”)
 Theoretical models overpredict the space density at high z
and low Lx than observed (or many CT AGN missing there?) Two different modes in AGN fueling? (major merger fortrigger luminous AGNs + another process for low luminosityAGNs? )Ueda+1415
Miyaji+Accretion vs Star Formation History
The overall history of mass accretion rate onto SMBHs andof star forming rate is similar (with a factor of ~500)
 ~74% of total mass was produced by “obscured” accretion
 Discrepancy at z>3 ? (due to CT AGNs??)SFRx0.002 (Madau&Dickinson+14)

Ueda (2015) PJAB 91, 5See also Aird+15

SMBH accretion rate (Ueda+14)

Population Synthesis Model of the XRB (Ueda+14)
XRB = (luminosity function) × (absorption distribution)
 Broad-band spectra determined from local AGNs
 To explain the XRB peak intensity at ~30 keV, the ratio ofCompton thick AGN to Compton thin absorbed AGNs
must be ~1 (0.5-1.6) by assuming that they follow thesame evolution as Compton thin AGNs


Observed XRB spectrum

Integrated spectrumof Compton-thickC-thin AGNs AGNs

Issues in Modeling of the XRB
In population synthesis models of the XRB, the number densityof CT AGNs is strongly coupled with (assumed) broadband
spectra of all AGNs, in particular, Strength of reflection component from “torus” and that fromaccretion disk or another stable hard component (Noda+) ?
 Intrinsic continuum shape: Is “cutoff power law” appropriate? Asharper high E drop is expected from thermal Comptonization
Reynolds, Ueda+ 2015
Mrk 335

inner disc reflection“torus” reflection

Variations of Photon Index

Type 2s have harder spectra (smaller index) than type 1s(though may coupled with reflection components)Comptonizing corona may not be spherical?

Photon Index increases with Eddington ratioMore effective cooling by larger energy density of incident photons?
Brightman+ 2013

Ueda+2014

Type 1
Type 2

Today’s Messages
1.

To fully understand the origin of the XRB andgrowth history of SMBHs, we need to know
everything about AGN phenomena as a function ofluminosity, black hole mass, and/or Eddingtion ratio.
There are still a lot to do!

2.

Broadband X-ray spectra carry key information, notonly to understand accretion physics, but also toconstrain AGN evolution through the XRB model.