Compton thick AGN in the Suzaku era
arXiv:0704.1253v1  [astro-ph]  10 Apr 2007
Compton thick AGN in the Suzaku era
Andrea Comastri1, Roberto Gilli1, Cristian Vignali2, Giorgio Matt3, Fabrizio
Fiore4, Kazushi Iwasawa5
1 INAF – Osservatorio Astronomico di Bologna, Bologna, Italy
2 Dipartimento di Astronomia, Università di Bologna, Bologna, Italy
3 Dipartimento di Fisica, Università di Roma Tre, Roma, Italy
4 INAF – Osservatorio Astronomico di Roma, Monteporzio, Italy
5 MPE – Garching, Germany
(Received )
Suzaku observations of two hard X–ray (> 10 keV) selected nearby Seyfert 2 galaxies
are presented. Both sources were clearly detected with the pin Hard X–ray Detector up
to several tens of keV, allowing for a fairly good characterization of the broad band X–ray
continuum. Both sources are heavily obscured, one of which (NGC 5728) being Compton
thick, while at lower energies the shape and intensity of the scattered/reflected continuum
is very different. Strong iron Kα lines are detected in both sources. There are also hints for
the presence of a broad relativistic iron line in NGC 4992.
§1. Introduction
A fraction as high as 50% of Seyfert 2 galaxies in the nearby Universe are ob-
scured in the X–ray band by column densities of the order of, or larger than the
inverse of the Thomson cross-section (NH ≥ σ
≃ 1.5× 1024 cm−2), hence dubbed
Compton thick (CT). If the optical depth (τ = NHσT ) for Compton scattering does
not exceed values of the order of “a few”, X–ray photons with energies higher than
10–15 keV are able to penetrate the obscuring material and reach the observer.
For higher values of τ , the entire X–ray spectrum is depressed by Compton down
scattering and the X–ray photons are effectively trapped by the obscuring material
irrespective of their energy. The former class of sources (mildly CT) can be effi-
ciently detected by X–ray instruments sensitive above 10 keV, while for the latter
(heavily CT) their nature may be inferred through indirect arguments, such as the
presence of a strong iron Kα line over a flat reflected continuum. The search for
and the characterization of the physical properties of CT AGN is relevant to under-
stand the evolution of accreting Supermassive Black Holes (SMBHs). In particular,
mildly CT AGN are the most promising candidates to explain the residual (i.e. not
yet resolved) spectrum of the X–ray background around its 30 keV peak (Comastri
2004a; Worsley et al. 2005) but only a handful of them are known beyond the local
Universe (see Comastri 2004b for a review). If this were the case, we may be missing
a not negligible fraction of the accretion power in the Universe and of the baryonic
matter locked in SMBH (Marconi et al. 2004). An unbiased census of extremely
obscured AGN would require to survey the hard X–ray sky above 10 keV with good
sensitivity. Such an argument is one of the key scientific drivers of the SimbolX
mission (Ferrando et al. 2006), which will be hopefully launched in the next decade.
typeset using PTPTEX.cls 〈Ver.0.9〉
http://arxiv.org/abs/0704.1253v1
2 A. Comastri et al.
For the time being one has to rely on the observations obtained by the high energy
detectors on board BeppoSAX, INTEGRAL, Swift and, more recently, Suzaku.
Though limited to bright and thus low redshift sources, they have proven to be quite
successful in finding heavily obscured CT AGN. As a first step forward towards a
census of CT AGN we have conceived a program with Suzaku to observe hard X–
ray selected bright AGN from the INTEGRAL/IBIS (Beckmann et al. 2006) and
Swift/BAT (Markwardt et al. 2005) catalogues. The goal of this program is to
discover “new” CT AGN which are likely to be present among the already detected
sources, but not recognized as such due to the poor counting statistics and/or the
lack of information at lower energies. In order to select the most suitable candidates,
we have considered the sources in the above mentioned AGN catalogues with a bright
hard X–ray flux and tentative evidence of intrinsic absorption from observations at
lower energies. For a few of them the column densities are estimated to be close to
the CT threshold. Suzaku observations were obtained for NGC 5728 and NGC 4992.
§2. The Suzaku observations
The reprocessed (v1.2) data were reduced using standard calibration products
available in November 2006. Source spectra are obtained from the Front Illuminated
XIS chips with an extraction radius of ∼3′, while background spectra are extracted
from nearby regions with a larger radius to guarantee good statistics. The effective
exposure time for both sources is of the order of 30 ksec. The pin hard X–ray
source spectra were obtained taking into account both the instrumental background
appropriate for each observation and the cosmic X–ray background. The pin/XIS
intercalibration constant was fixed at 1.16. In the following, we report the basic
results obtained from the analysis of the X–ray spectra of the two sources and refer
to Comastri et al. (2007, in preparation) for a more exhaustive description of the
data analysis and interpretation.
Energy (keV)
5.5 6 6.5 7 7.5
Energy (keV)
iron Kα line
iron Kβ line
CT obscured
power law
unobscured
reflection
Fig. 1. Left panel: The unfolded broad band spectrum of NGC 5728 with the various components
used to model the continuum and the iron lines. Right panel: A zoom on the ”iron band”
showing a strong Kα line at ∼ 6.4 keV and a less prominent Kβ line (∼ 7 keV) on top of the
underlying continuum (upper line) made by the sum of a CT obscured power law (middle line)
and an unobscured reflected component (lower line).
Compton Thick AGN 3
105 20
Energy (keV)
4.5 5 5.5 6 6.5 7
Energy (keV)
Fig. 2. Left panel: The unfolded broad band spectrum of NGC 4992. The reflection dominated
absorbed continuum, the relativistic disk line and a weak unobscured reflected component are
reported. Right panel: The residuals vs. the best fit continuum in the 4.5–7.5 keV band.
2.1. NGC 5728
The Suzaku spectrum of NGC 5728 is shown in Fig. 1 (left panel). The source
is clearly detected by the pin detector up to about 50 keV. The primary X–ray
continuum is absorbed by Compton thick gas (NH ≃ 2.1 ± 0.2 × 10
24 cm−2). The
power law slope has been fixed at Γ = 1.9 due to the narrow energy range (20–40
keV) over which the continuum is free from obscuration effects. At lower energy, the
continuum can be represented by a two component model: a flat one responsible for
most of the X–ray flux in the ∼ 2–6 keV energy range and a steep one taking over
below 2 keV. The former may be ascribed to reflection of cold material presumably
from the inner wall of the torus, while the latter has a power law shape and can
be identified as primary emission scattered by off–nuclear gas into the line of sight,
or unresolved soft X–ray emission lines, as commonly observed in Seyfert 2 galaxies
(Guainazzi & Bianchi 2007). The scattered/reflected flux accounts for 1–2 % of the
total unabsorbed flux (∼ 5× 10−11 erg s−1 cm−2) in the 2–10 keV band. The 2–50
keV unabsorbed luminosity is 2.3 × 1043 erg −1, typical of a bright Seyfert galaxy.
It is interesting to note that the hard (> 10 keV) X–ray flux as measured by the
pin detector is consistent within 20% with the Swift/BAT measurement in the
overlapping energy range. A zoom of the ∼ 5–8 keV unfolded spectrum is shown in
Fig. 1 (right panel). The iron line complex is best fitted with two gaussian lines: a
strong (EW ≃ 1.0±0.3 keV) iron Kα line at ∼ 6.4 keV and a Kβ (EW ∼ 130±70 eV)
at ∼ 7 keV. The relative ratio is consistent with that expected from cold neutral gas.
The addition of a Compton shoulder parameterized by a Gaussian profile centered at
6.3 keV and σ = 40 eV (Matt 2002), though not statistically required, accounts for
some 10% of the Kα line flux, in reasonably good agreement with the value expected
for reflection from Compton thick matter.
2.2. NGC 4992
The Seyfert 2 galaxy NGC 4992 is detected by Suzaku up to about 30 keV with
a flux consistent (within 10%) with that reported by INTEGRAL. The continuum
(a power law with Γ=1.9) is heavily obscured (NH ∼ 4.5 ± 0.5 × 10
23 cm−2) but
4 A. Comastri et al.
not Compton thick. The high energy spectrum is best fitted by adding a strong,
absorbed, disk reflection component to the primary power law. The quality of the
data is not such to tightly constrain the intensity of the reflection component. The
90% lower limit (R > 5) indicate a reflection dominated spectrum which is similar to
that reported by Miniutti et al. (2007) from the analysis of the XMM-Newton data
of IRAS 13197-1627. The source is extremely weak below 3–4 keV. The addition
of an unabsorbed reflection spectrum only marginally improves the fit (Fig. 2, left
panel). A zoom of the residuals in the 4.5–7.5 keV range, wrt the best fit continuum
model, is shown in Fig. 2 (right panel). The shape of the residuals suggests the
presence of a broad line. Indeed the best fit to the line emission is obtained with
a diskline model. Leaving only the line flux and the disk inclination angle as free
parameters, the line equivalent width is ∼ 750± 200 eV and the inclination angle is
< 40 degrees (at 90% confidence). The best fit EW is consistent with a reflection
dominated nature of the broad band spectrum. The absorption corrected 2–50 keV
luminosity is ∼ 6× 1043 erg s−1.
2.3. Epilogue
Relatively shallow Suzaku observations of two hard X–ray selected (with INTE-
GRAL/IBIS and Swift/BAT) nearby Seyfert 2 galaxies have revelead a wealth of
spectral complexity in their X–ray spectra. The good sensitivity over a broad X–
ray energy range makes Suzaku very efficient to study the most obscured sources in
the nearby Universe and will eventually allow us to establish the AGN absorption
distribution at high column densities.
Acknowledgements
We thank G. Miniutti for extremely useful discussions. Support from the Italian
Space Agency (ASI) under the contract ASI-INAF I/023/05/0 is acknowledged.
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