1. Risolvere il Background X al suo picco
Andrea Comastri (INAF-OABologna)
Roberto Gilli (INAF-OABologna)
K. Iwasawa (INAF-OABologna)
C. Vignali (Bologna University)
Georgantopoulos (INAF-OABologna)
P. Ranalli (Bologna University)
F. Fiore (INAF-OARoma)
M. Brusa (MPE)
… many others …

2. Peak Energy
~ 29 keV HEAO1-PDS
~ 22 keV Swift
NHXM, November 13, 2009

3. INTEGRAL & Swift (> 10 keV) --> a few (1-2)%
Deep “soft” X-ray surveys have resolved some 80% of the XRB (below a few keV ) but “only” 50% > 6 keV
INTEGRAL & Swift (> 10 keV)
--> a few (1-2)%
Wealth of X-ray and multi-wavelength follow-up available -> possible to estimate the “size” and “average spectrum” of the “missing” population
Compton Thick AGN ?
NHXM, November 13, 2009

4. Modelling
XRB flux “uncertain” , so use the more robust constraints below 10 keV (eg. source counts) to lock the properties (eg obs/unobs ratio, NH dist.) of the Compton Thin (log NH < 24) AGN
Estimate the Compton Thin/Unobs. luminosity dependent ratio by comparing hard vs. soft XLF.
Absorption distribution from X-ray counts.
Include continuum slope dispersion.
Add Compton thick AGN to fit the 30 keV bump …
Verify assumptions/make predictions on Compton Thick AGN

5. NHXM, November 13, 2009

6. ~ 4 at low Log LX < 44 Luminosities ~ 1 at high Log LX > 44
Mildly (log NH =24-25)
(NGC 6240, Circinus)
Heavily (log NH >25)
(NGC1068)
~ 4 at low
Log LX < 44
Luminosities
~ 1 at high
Log LX > 44
luminosities
NHXM, November 13, 2009

7. Why bother ?
The missing population may provide an important contribution to the BH mass function SMBH accretion rate, AGN vs. galaxy co-evolution…
Different prescriptions for the details of luminosity and redshift dependence of fOBS
Interesting for physics but little impact on models
Every model has a lot of parameters which can be tuned and strong degeneracies especially at high energies where the resolved fraction is tiny . Claims on the properties of the missing population should be treated with caution
High energy cut-offs (~100 keV) and radio-loud (Blazars)

8. Compton Thick AGN contribution in Gilli+07

9. Compton Thick in nearby Universe
QSO2/QSO = 0.3
QSO2/QSO = 0.5
Treister+09
Decrease the fraction of obscured
AGN at high luminosity wrt Gilli+07
At the expenses of a worse fit to the
XRB spectrum at high energies
10% contribution from “flat” spectrum
radio loud AGN (Blazars ?)
<z> ~ 0 @ 10-11
<z> ~ 1 @ 10-15
NHXM, November 13, 2009

10. Compton Thick AGN at high redshift
Several evidences
Daddi+07
Fiore+08,09
Alexander+08 …
for a large population
of CT AGN at z ~
“Contamination” by SF
galaxies or just mildly obscured
may be an issue
If nearby CT are “rare” while
abundant at high z a stronger
Cosmological evolution is implied
Vignali+09 in prep

11. Scattering fraction < 0.5 %
“Buried” AGN ?
Ueda+07
Scattering fraction < 0.5 %
NGC4992 Comastri+09
“Scattering” fraction is very low ( < 0.5 %) more examples in Eguchi+09
not related to Compton Thickness but rather to Reflection component
“Buried” AGN (see also Winter+08)
NHXM, November 13, 2009

12. 4 X
NHXM, November 13, 2009

14. Local Black Holes and AGN relics
Marconi et al have shown that local BHs are relics of AGN activity by comparing:
the local BH mass function (from galaxy L/σ functions and MBH-Lbul/MBH-σe)
the relic BHMF (from AGN luminosity function and continuity equation)
Importance of AGN LF: even
the hard (2-10 keV) XLF does not sample the whole AGN population.
Heavily obscured Compton-thick AGN are missing ...
Hard-X LF
Soft-X LF
Correction for CT
from XRB models
L = ε dM/dt c2
L = λ LE
Qso LF
NHXM, November 13, 2009

15. Radio loud and -ray background
Giommi+06
Comastri+96
Ajello+09
Draper+09

16. High energy cut-off 100-400 keV
Radio loud AGN / Blazars contribute some 10% to the soft XRB
Normalizing to 1 keV for a range of slopes ( ) a break or cut-off must be present around 1 MeV otherwise the
soft -ray background would be exceeded
High energy cut-off keV
NHXM, November 13, 2009

17. More peaked
Narrower redshift distr.?
Different sources ?
Difference spectrum
CT from GCH07
EC = 200 keV

18. Residuals assuming Swift
= 10 kT ~ 8(1+z) keV
Residuals assuming Swift
best fit
= 10 kT ~ 10(1+z) keV
Saturated Comptonization
= 10 kT ~ 10(1+z) keV
Compton Thick
in Gilli+07
z = 2 z ~ 0.6
z = 3 z ~ 0.8

19. 60% to 80%
of the hard keV
XRB will be resolved

20. Observational Strategy
A spectral survey of local CT Seyfert 2 galaxies previously discovered by BeppoSAX, INTEGRAL Swift and Suzaku. This should be accompanied by a volume limited survey of IR-bright or OIII-bright or … galaxies which have not shown strong X-ray emission below 10 keV, to search for highly CT objects.
A spectral survey of moderately obscured luminous QSOs discovered by XMM, Chandra up to z~1 (High luminosity)
Deep observations to search for faint mildly CT AGN + serendipitous survey (high redshift)
A survey of candidate CT AGN selected using their infrared emission (Spitzer/Herschel).
NHXM, November 13, 2009

21. Polarimetric and Energetic Radiation Observatory a Legacy for Astrophysics
PEROLA