Compton-thick AGN and the Synthesis of the Cosmic X-ray
Background: the Suzaku Perspective
Roberto Gilli1,∗), Andrea Comastri1, Cristian Vignali2
and Günther Hasinger3
1INAF - Osservatorio Astronomico di Bologna, Bologna, Italy
2Dipartimento di Astronomia, Università degli Studi di Bologna, Bologna, Italy
3Max-Planck-Institut für extraterrestrische Physik, Garching, Germany
(Received )
We discuss the abundance of Compton-thick AGN as estimated by the most recent
population synthesis models of the cosmic X-ray background. Only a small fraction of these
elusive objects have been detected so far, in line with the model expectations. The advances
expected by the broad band detectors on board Suzaku are briefly reviewed.
§1. Introduction
Despite extensive observational efforts, the population of heavily obscured, Compton-
thick AGN remains elusive, especially at high redshifts, preventing a complete census
of accreting supermassive black holes (SMBHs). While Compton-thick (CT) nuclei
were shown to hide in about half of local Seyfert 2 galaxies (Risaliti et al. 1999,
Guainazzi et al. 2005), observations of heavily obscured objects beyond z ∼ 0.1
are very sparse (see Comastri 2004 for a review) and their abundance can be con-
strained only by indirect arguments. One argument is the comparison between the
mass function of local SMBHs with the one expected if they accreted most of their
mass during past AGN phases (Marconi et al. 2004). Another, and probably more
stringent, argument is the residual emission at 30 keV in the spectrum of the cos-
mic X-ray background (XRB), which is left after removing the contribution from
the better known population of less obscured, Compton-thin AGN. The residual 30
keV XRB emission can indeed be modeled by assuming a population of CT AGN
as large as that of moderately absorbed ones over a broad range of redshifts and
luminosities (see Gilli, Comastri & Hasinger 2007, hereafter GCH07, for a recent
work). In particular, this residual emission is mostly filled by “mildly” CT objects
(defined as those with 1024 < NH < 10
25 cm−2) in which the direct, primary emis-
sion is visible above ∼ 10 keV, rather than by “heavily” CT objects (NH > 10
cm−2), in which only reflected radiation is visible at high energy, and are therefore
significantly less luminous than the formers at 30 keV. As a consequence, the number
of heavily CT AGN is poorly constrained even by population synthesis models and
is generally assumed to be similar (equal in GCH07) to that of mildly CT objects,
as suggested by the results of Risaliti et al. (1999). Because of the strong selection
effects due to absorption, only a small percentage of CT sources have been observed
∗) E-mail: roberto.gilli@oabo.inaf.it
typeset using PTPTEX.cls 〈Ver.0.9〉
http://arxiv.org/abs/0704.1376v1
2 R. Gilli, A. Comastri, C. Vignali and G. Hasinger
Fig. 1. Left : The fractions of CT AGN in the GCH07 baseline model as a function of the 2-10 keV
and 15-200 keV limiting fluxes compared to those observed in the CDFS (Tozzi et al. 2006) and
in the Swift/BAT catalog (Markwardt et al. 2005). The upper Swift/BAT point is corrected
for incompleteness. Note the steep increase expected at fluxes below the current sensitivities
and the identical CT fraction in the two bands at extremely faint fluxes, where all AGN should
be detected. Right : The predicted 20-40 keV AGN counts (see Section 2) normalized to
an Euclidean Universe and compared with those measured by INTEGRAL (Beckmann et al.
2006).
in current X-ray surveys. In the Chandra Deep Field South (CDFS) only about
5% of the detected AGN have been identified as CT candidates (Tozzi et al. 2006).
In the recent INTEGRAL/IBIS and Swift/BAT surveys performed above 10 keV,
where the absorption bias is less effective, a higher fraction is observed (∼ 10− 15%,
Markwardt et al. 2006, Beckmann et al. 2006). This is already remarkable, if one
bears in mind that X-ray surveys above 10 keV are still limited to very bright fluxes
(∼ 10−11 erg cm−2 s−1). As shown in Fig. 1 left, these small observed fractions are
in good agreement with those expected if CT AGN are intrinsically as abundant as
moderately obscured ones (see GCH07), and are predicted to increase dramatically
at fluxes below the current sensitivity limits.
§2. Uncertainties on the number of Compton-thick AGN
Since the overall abundance of CT AGN is estimated by subtracting from the
XRB spectrum the contribution of Compton-thin sources, it is imperative to estimate
the latter at the best of present knowledge. The modeling presented in GCH07
took into account a detailed characterization of the average AGN X-ray spectra,
including dispersion, and cosmological evolution, but is nonetheless worth exploring
the parameter space to some extent and check how different assumptions may affect
the estimated CT number. In the baseline model presented by GCH07, a Gaussian
distribution in the AGN primary continuum was considered, with average spectral
Compton-thick AGN and the Synthesis of the Cosmic X-ray Background 3
INTEGRAL 
JEM-X
INTEGRAL
IBIS/SPI
Chandra
INTEGRAL 
JEM-X
INTEGRAL
IBIS/SPI
Chandra
INTEGRAL 
JEM-X
INTEGRAL
IBIS/SPI
Chandra
Fig. 2. The spectrum of the X-ray background. Most of the datapoints are described in GCH07.
Here we add the recent 5-200 keV measurement by INTEGRAL (Churazov et al. 2007) and
the 1-7 keV measurement by Chandra (Hickox & Markevitch 2006). Model curves based on
GCH07 are also plotted: the upper magenta curves show the total contribution from AGN (plus
galaxy clusters), according to different assumptions on the average AGN spectral slope and
number of CT objects as labeled (see text); the lower black curves represent the corresponding
CT contribution.
slope and dispersion of 〈Γ 〉 = 1.9 and σΓ = 0.2, respectively, in agreement with the
observed distributions (Mateos et al. 2005). In Fig. 2 we show the effects of assuming
an average spectral index 〈Γ 〉 = 1.8 with the same dispersion. A sligthly harder
average spectral powerlaw is in principle sufficient to saturate the XRB emission
with Compton-thin AGN, leaving little room for CT sources. Indeed, when adding
as many CT AGN as in the baseline model, the 30 keV XRB emission measured
by HEAO-1, and recently confirmed within 10% by INTEGRAL (Churazov et al.
2007), is exceeded if 〈Γ 〉 = 1.8. Furthermore, the baseline model appears to be in
much better agreement with other observational constraints, such as the spectral
distributions observed in different AGN samples and the observed numbers of CT
AGN (see GCH07). The model predictions have been further compared with the
AGN counts in the 20-40 keV band recently estimated by Beckmann et al. (2006).
The situation (Fig. 1 right) is similar to that shown in Fig.2 for the XRB, although
the constraints are less stringent. While a model with 〈Γ 〉 = 1.8 would imply a small
number of CT AGN, the baseline model provides a good match to the data with a
relative ratio of one between Compton-thick and Compton-thin AGN at all redshifts.
4 R. Gilli, A. Comastri, C. Vignali and G. Hasinger
This assumed ratio appears more in line with current observations both in the local
(Risaliti et al. 1999) and in the distant Universe (Martinez-Sansigre et al. 2006).
§3. The Suzaku perspective
To date, about 40 local AGN have been shown to be CT through X-ray obser-
vations (Comastri 2004) and their number is expected to increase significantly in
the next future. Indeed, CT AGN candidates in current INTEGRAL and Swift
surveys can be easily flagged as such if their X-ray flux above 10 keV is much larger
than their soft X-ray flux, which is often available from archival X-ray data. Like
BeppoSAX in the past years, Suzaku is now carrying on board detectors which are
sensitive in the broad 0.5-50 keV band and are therefore the ideal instruments to
determine the X-ray spectral energy distribution of bright, nearby CT objects. In
EAO-1 we have obtained Suzaku observations of 2 CT candidates selected from the
INTEGRAL and Swift AGN catalogs (two further candidates will be observed in
EAO-2). One of the two objects indeed proved to be CT, while the other turned out
to be heavily obscured but still Compton thin (see Comastri et al. this volume, for a
more detailed discussion). Suzaku observations of additional CT candidates selected
above 10 keV are being performed by other groups (e.g. Ueda et al., this volume).
Eventually, once these programs are put together to get sufficient object statistics,
the fraction of CT AGN in the local Universe will be determined to better accuracy.
In particular, new mildly CT AGN should be revealed in significant numbers, and
a few spectra of heavily CT AGN, which went undetected by BeppoSAX, may be
also obtained.
Acknowledgements
We are grateful to Mike Revnivtsev and Volker Beckmann for providing the
INTEGRAL XRB spectrum and 20-40 keV AGN counts, respectively. RG, AC and
CV acknowledge financial support from the Italian Space Agency (ASI) under the
contract ASI–INAF I/023/05/0.
References
1) V. Beckmann, et al., ApJ 652 (2006), 126.
2) E. Churazov, et al., A&A (2007), in press (astro-ph/0608250).
3) A. Comastri, Supermassive Black Holes in the Distant Universe (A.J. Barger, Dordrecht,
2004), p. 245.
4) R. Gilli, A. Comastri and G. Hasinger, A&A 463 (2007), 79. (GCH07)
5) M. Guainazzi, et al., A&A 444 (2005), 119
6) R. Hickox & M. Markevitch, ApJ 645 (2006), 95.
7) A. Marconi, et al., MNRAS 351 (2004), 169.
8) C.B. Markwardt, et al., ApJ 633 (2005), L77.
9) A. Martinez-Sansigre, et al., MNRAS 370 (2006), 1479
10) S. Mateos, et al., A&A 444 (2005), 79.
11) G. Risaliti, R. Maiolino and M. Salvati, ApJ 522 (1999), 157.
12) P. Tozzi, et al., A&A 451 (2006), 457.
http://arxiv.org/abs/astro-ph/0608250
	Introduction
	Uncertainties on the number of Compton-thick AGN
	The Suzaku perspective

  We discuss the abundance of Compton-thick AGN as estimated by the most recent
population synthesis models of the cosmic X-ray background. Only a small
fraction of these elusive objects have been detected so far, in line with the
model expectations. The advances expected by the broad band detectors on board
Suzaku are briefly reviewed.

Introduction
Despite extensive observational efforts, the population of heavily obscured, Compton-
thick AGN remains elusive, especially at high redshifts, preventing a complete census
of accreting supermassive black holes (SMBHs). While Compton-thick (CT) nuclei
were shown to hide in about half of local Seyfert 2 galaxies (Risaliti et al. 1999,
Guainazzi et al. 2005), observations of heavily obscured objects beyond z ∼ 0.1
are very sparse (see Comastri 2004 for a review) and their abundance can be con-
strained only by indirect arguments. One argument is the comparison between the
mass function of local SMBHs with the one expected if they accreted most of their
mass during past AGN phases (Marconi et al. 2004). Another, and probably more
stringent, argument is the residual emission at 30 keV in the spectrum of the cos-
mic X-ray background (XRB), which is left after removing the contribution from
the better known population of less obscured, Compton-thin AGN. The residual 30
keV XRB emission can indeed be modeled by assuming a population of CT AGN
as large as that of moderately absorbed ones over a broad range of redshifts and
luminosities (see Gilli, Comastri & Hasinger 2007, hereafter GCH07, for a recent
work). In particular, this residual emission is mostly filled by “mildly” CT objects
(defined as those with 1024 < NH < 10
25 cm−2) in which the direct, primary emis-
sion is visible above ∼ 10 keV, rather than by “heavily” CT objects (NH > 10
cm−2), in which only reflected radiation is visible at high energy, and are therefore
significantly less luminous than the formers at 30 keV. As a consequence, the number
of heavily CT AGN is poorly constrained even by population synthesis models and
is generally assumed to be similar (equal in GCH07) to that of mildly CT objects,
as suggested by the results of Risaliti et al. (1999). Because of the strong selection
effects due to absorption, only a small percentage of CT sources have been observed
∗) E-mail: roberto.gilli@oabo.inaf.it
typeset using PTPTEX.cls 〈Ver.0.9〉
http://arxiv.org/abs/0704.1376v1
2 R. Gilli, A. Comastri, C. Vignali and G. Hasinger
Fig. 1. Left : The fractions of CT AGN in the GCH07 baseline model as a function of the 2-10 keV
and 15-200 keV limiting fluxes compared to those observed in the CDFS (Tozzi et al. 2006) and
in the Swift/BAT catalog (Markwardt et al. 2005). The upper Swift/BAT point is corrected
for incompleteness. Note the steep increase expected at fluxes below the current sensitivities
and the identical CT fraction in the two bands at extremely faint fluxes, where all AGN should
be detected. Right : The predicted 20-40 keV AGN counts (see Section 2) normalized to
an Euclidean Universe and compared with those measured by INTEGRAL (Beckmann et al.
2006).
in current X-ray surveys. In the Chandra Deep Field South (CDFS) only about
5% of the detected AGN have been identified as CT candidates (Tozzi et al. 2006).
In the recent INTEGRAL/IBIS and Swift/BAT surveys performed above 10 keV,
where the absorption bias is less effective, a higher fraction is observed (∼ 10− 15%,
Markwardt et al. 2006, Beckmann et al. 2006). This is already remarkable, if one
bears in mind that X-ray surveys above 10 keV are still limited to very bright fluxes
(∼ 10−11 erg cm−2 s−1). As shown in Fig. 1 left, these small observed fractions are
in good agreement with those expected if CT AGN are intrinsically as abundant as
moderately obscured ones (see GCH07), and are predicted to increase dramatically
at fluxes below the current sensitivity limits.
§2. Uncertainties on the number of Compton-thick AGN
Since the overall abundance of CT AGN is estimated by subtracting from the
XRB spectrum the contribution of Compton-thin sources, it is imperative to estimate
the latter at the best of present knowledge. The modeling presented in GCH07
took into account a detailed characterization of the average AGN X-ray spectra,
including dispersion, and cosmological evolution, but is nonetheless worth exploring
the parameter space to some extent and check how different assumptions may affect
the estimated CT number. In the baseline model presented by GCH07, a Gaussian
distribution in the AGN primary continuum was considered, with average spectral
Compton-thick AGN and the Synthesis of the Cosmic X-ray Background 3
INTEGRAL 
JEM-X
INTEGRAL
IBIS/SPI
Chandra
INTEGRAL 
JEM-X
INTEGRAL
IBIS/SPI
Chandra
INTEGRAL 
JEM-X
INTEGRAL
IBIS/SPI
Chandra
Fig. 2. The spectrum of the X-ray background. Most of the datapoints are described in GCH07.
Here we add the recent 5-200 keV measurement by INTEGRAL (Churazov et al. 2007) and
the 1-7 keV measurement by Chandra (Hickox & Markevitch 2006). Model curves based on
GCH07 are also plotted: the upper magenta curves show the total contribution from AGN (plus
galaxy clusters), according to different assumptions on the average AGN spectral slope and
number of CT objects as labeled (see text); the lower black curves represent the corresponding
CT contribution.
slope and dispersion of 〈Γ 〉 = 1.9 and σΓ = 0.2, respectively, in agreement with the
observed distributions (Mateos et al. 2005). In Fig. 2 we show the effects of assuming
an average spectral index 〈Γ 〉 = 1.8 with the same dispersion. A sligthly harder
average spectral powerlaw is in principle sufficient to saturate the XRB emission
with Compton-thin AGN, leaving little room for CT sources. Indeed, when adding
as many CT AGN as in the baseline model, the 30 keV XRB emission measured
by HEAO-1, and recently confirmed within 10% by INTEGRAL (Churazov et al.
2007), is exceeded if 〈Γ 〉 = 1.8. Furthermore, the baseline model appears to be in
much better agreement with other observational constraints, such as the spectral
distributions observed in different AGN samples and the observed numbers of CT
AGN (see GCH07). The model predictions have been further compared with the
AGN counts in the 20-40 keV band recently estimated by Beckmann et al. (2006).
The situation (Fig. 1 right) is similar to that shown in Fig.2 for the XRB, although
the constraints are less stringent. While a model with 〈Γ 〉 = 1.8 would imply a small
number of CT AGN, the baseline model provides a good match to the data with a
relative ratio of one between Compton-thick and Compton-thin AGN at all redshifts.
4 R. Gilli, A. Comastri, C. Vignali and G. Hasinger
This assumed ratio appears more in line with current observations both in the local
(Risaliti et al. 1999) and in the distant Universe (Martinez-Sansigre et al. 2006).
§3. The Suzaku perspective
To date, about 40 local AGN have been shown to be CT through X-ray obser-
vations (Comastri 2004) and their number is expected to increase significantly in
the next future. Indeed, CT AGN candidates in current INTEGRAL and Swift
surveys can be easily flagged as such if their X-ray flux above 10 keV is much larger
than their soft X-ray flux, which is often available from archival X-ray data. Like
BeppoSAX in the past years, Suzaku is now carrying on board detectors which are
sensitive in the broad 0.5-50 keV band and are therefore the ideal instruments to
determine the X-ray spectral energy distribution of bright, nearby CT objects. In
EAO-1 we have obtained Suzaku observations of 2 CT candidates selected from the
INTEGRAL and Swift AGN catalogs (two further candidates will be observed in
EAO-2). One of the two objects indeed proved to be CT, while the other turned out
to be heavily obscured but still Compton thin (see Comastri et al. this volume, for a
more detailed discussion). Suzaku observations of additional CT candidates selected
above 10 keV are being performed by other groups (e.g. Ueda et al., this volume).
Eventually, once these programs are put together to get sufficient object statistics,
the fraction of CT AGN in the local Universe will be determined to better accuracy.
In particular, new mildly CT AGN should be revealed in significant numbers, and
a few spectra of heavily CT AGN, which went undetected by BeppoSAX, may be
also obtained.
Acknowledgements
We are grateful to Mike Revnivtsev and Volker Beckmann for providing the
INTEGRAL XRB spectrum and 20-40 keV AGN counts, respectively. RG, AC and
CV acknowledge financial support from the Italian Space Agency (ASI) under the
contract ASI–INAF I/023/05/0.
References
1) V. Beckmann, et al., ApJ 652 (2006), 126.
2) E. Churazov, et al., A&A (2007), in press (astro-ph/0608250).
3) A. Comastri, Supermassive Black Holes in the Distant Universe (A.J. Barger, Dordrecht,
2004), p. 245.
4) R. Gilli, A. Comastri and G. Hasinger, A&A 463 (2007), 79. (GCH07)
5) M. Guainazzi, et al., A&A 444 (2005), 119
6) R. Hickox & M. Markevitch, ApJ 645 (2006), 95.
7) A. Marconi, et al., MNRAS 351 (2004), 169.
8) C.B. Markwardt, et al., ApJ 633 (2005), L77.
9) A. Martinez-Sansigre, et al., MNRAS 370 (2006), 1479
10) S. Mateos, et al., A&A 444 (2005), 79.
11) G. Risaliti, R. Maiolino and M. Salvati, ApJ 522 (1999), 157.
12) P. Tozzi, et al., A&A 451 (2006), 457.
http://arxiv.org/abs/astro-ph/0608250
	Introduction
	Uncertainties on the number of Compton-thick AGN
	The Suzaku perspective
