The Hard X-ray 20-40 keV AGN Luminosity Function

1 MB·English

by  Beckmann, V.

Source of Acquisition
Goddard Space Flight Center
The Hard X-ray 20-40 keV AGN Luminosity Function
V. Beckmannl
NASA Goddard Space Flight Center, Exploration of the Universe Division, Code 661, Greenbelt, MD
20771, USA
beckmannQmilkyway.gsfc.nasa.gov
S. S ~ l d i ~C>. R~. ,S hrader4y5,N . Gehrels4, and N. Produit2
ABSTRACT
We have compiled a complete, significance limited extragalactic sample based on
N
25,000 deg2 to a limiting flux of 3 x lo-'' ergs cm-2 s-l (- 7,000deg2 to a flux limit of
IO-'' ergs cmP2s -l) in the 20 - 40 keV band with INTEGRAL. We have constructed a detailed
exposure map to compensate for effects of non-uniform exposure. The flux-number relation is
best described by a power-law with a slope of Q = 1.66 k 0.11. The integration of the cumu-
lative flux per unit area leads to f20-40keV = 2.6 x lo-'' ergs crn-2 s-l sr-', which is about
1% of the known 20 - 40 keV X-ray background. We present the first luminosity function of
AGN in the 20-40 keV energy range, based on 68 extragalactic objects detected by the imager
IBIS/ISGRI on-board INTEGRAL. The luminosity function shows a smoothly connected two
power-law form, with an index of y1 = 0.9 below, and 72 = 2.2 above the turn-over luminos-
ity of L, = 4.6 x ergs s-'. The emissivity of all INTEGRAL AGNs per unit volume is
W20-40ke"(> 1041 ergss-l) = 2.8 x lo3* ergss-' h;o M ~ c - ~T.h ese results are consistent with
those derived in the 2 - 20keV energy band and do not show a significant contribution by
Compton-thick objects. Because the sample used in this study is truly local (E = 0.022), only
limited conclusions can be drawn for the evolution of AGNs in this energy band. But the objects
explaining the peak in the cosmic X-ray background are likely to be either low luminosity .4GN
(Lx < 1041e rgss-') or of other type, such as intermediate mass black holes, clusters, and star
forming regions.
Subject headings; galaxies: active - gamma rays: observations - X-rays: galaxies - surveys -- galaxies:
Seyfert
1. Introduction
'also with the Joint Center for Astrophysics, De-
partment of Physics, University of Maryland, Baltimore The Galactic X-ray sky is dominated by accret-
County, MD 21250, USA ing binary systems, while the extragalactic sky
21NTEGRAL Science Data Centre, Chemin d' Ecogia shows mainly active galactic nuclei (AGN) and
16, 1290 Versoix, Switzerland
clusters of galaxies. Studying the population of
3also with Observatoire de Geneve, 51 Ch. des Mail-
sources in this energy range has been a challenge
lettes, 1290 Sauverny, Switzerland
ever since the first observations by rocket borne
4NASA Goddard Space Flight Center, Exploration of
the Universe Division, Code 661, Greenbelt, MD 20771, X-ray detectors (Giacconi et al. 1962). At soft
USA X-rays (0.1 - 2.4 keV) the ROSAT All-Sky Survey
5also with Universities Space Research Association, (RASS; Voges et al. 1999) has revealed an extra-
10211 Wincopin Circle, Columbia, MD 21044, USA galactic population of mainly broad line AGNs,
such as type Seyfert 1 and quasars. Between a
1
c
-
few keV and lMeV, no all-sky survey using vations of the ROSAT deep X-ray surveys, which
imaging instruments has been performed to date. showed that 90% of the 0.5 - 2.0 keV CXB can be
In the 2 - 10 keV range surveys based on signif- resolved into AGNs (Miyaji, Hasinger & Schmidt
icant fractions of the sky have been carried out 2000). At higher energies (2 - lOkeV), ASCA
with ASCA (e.g. Ueda et al. 2001), XMM-Newton and Chandra surveys measured the X-ray lumi-
(e.g. Hasinger 2004), and Chandra (e.g. Brandt nosity function (XLF). These studies show that
et al. 2001) and have shown that the dominant in this energy range the CXB can be explained
extragalactic sources are more strongly absorbed by AGNs, but with a higher fraction of absorbed
than those within the RASS energy band, thus (NH> 1022cm-2) objects than in the soft X-rays
more Seyfert 2 type objects are detectable here. (e.g. Ueda et al. 2003). A study based on the
For a summary on the deep X-ray surveys below RXTEsurvey by Sazonov & Revnivtsev (2004) de-
10 keV see Brandt & Hasinger (2005). At higher rived the hard X-ray luminosity function of AGNs.
energies the data become more scarce. The Rossi They showed that the summed output of AGNs in
X-ray Timing Explorer (RXTE)s ky survey in the this energy range is too small to explain the CXB,
3 - 20 keV energy band revealed 100 AGNs, show- and suggested that a comparable X-ray flux may
ing an ever higher fraction of absorbed sources of be produced together by lower luminosity AGNs,
about 60% (Sazonov & Revnivtsev 2004). A sim- non-active galaxies and clusters of galaxies.
ilar study at energies above 20 keV had not been With the on-going observations of the sky by
possible, because a large field of view in combi- INTEGRAL, a sufficient amount of data is now
nation with sufficient sensitivity are required to available to derive the AGN hard X-ray lumi-
study the AGN population on a relevant fraction nosity function. In this paper we present anal-
of the sky. The International Gamma-Ray Astro- ysis of recent observations performed by the IN-
physics Laboratory (INTEGRAL; Winkler et al. TEGRAL satellite, and compare the results with
2003) offers an unprecedented > 20keV collect- previous studies. In Section 2 we describe the
ing area and state-of-the-art detector electronics AGN sample and in Section 3 the methods to de-
and background rejection capabilities. Notably, rive the number-flux distribution of INTEGRAL
the imager IBIS with an operating range from AGNs are presented togethcr with the analysis of
20 - 1OOOkeV and a fully-coded field of view of their distribution. Section 4 shows the local lumi-
10" x 10" enables us now to study a large por- nosity function of AGNs as derived from our data,
tion of the sky. A first catalog of AGNs showed a followed by a discussion of the results in Section
similar fraction of absorbed objects as the RXTE 5.
survey (Beckmann et al. 2006a).
Related to the compilation of AGN surveys in 2. The INTEGRAL AGN Sample
the hard X-rays is the question of what sources
Observations in the X-ray to soft gamma-
form the cosmic X-ray background (CXB). While
ray domain have been performed by the in-
the CXB below 20 keV has been the focus of many
struments on-board the INTEGRAL satellite
studies, the most reliable measurement in the 10
- 500 keV has beer! provided by the High Energy (Winkler et al. 2003). This mission offers the
unique potential to perform simultaneous observa-
Astronomical Observatory (HEAO l),l aunched in
tions over the 2 - 8000 keV energy region. This is
1977 (Marshall et al. 1980). The most precise
achieved by the X-ray monitor (2-30 keV) JEM-
measurement provided by the UCSD/MIT Hard
X-ray and Gamma-Ray instrument (HEAO 1 A- X (Lund et al. 2003), the soft gamma-ray im-
ager (20-1000 keV) ISGRI (Lebrun et al. 2003),
4) shows that the CXB peaks at an energy of
about 30 keV (Gruber et al. 1999). The isotropic and the spectrograph SPI (Vedrenne et al. 2003),
nature of the X-ray background points to an ex- which operates in the 20 - 8000 keV region. Each
tragalactic origin, and as the brightest persistent of these instruments employs the coded-aperture
sources are AGNs, it was suggested earlyon that technique (Caroli et al. 1987). In addition to
those objects are the main source of the CXB (eg. these data an optical monitor (OMC, Mas-Hesse
Zdziarski 1996). In the soft X-rays this concept et al. 2003) provides photometric measurements
has been proven to be correct through the obser- in the V band.
2
r 1
Data used for the analysis presented here were and are not used in our analysis.
all in the public domain by the end of March 2005.
This includes data from orbit revolutions 19 - 137 3. Number-Flux Distribution of INTE-
and revolutions 142 - 149. Data before revolution GRAL AGNs
19 have been excluded as the instruments settings
In order to compute the AGN number-flux re-
changed frequently and therefore the data from
lation counts of AGNs it is necessary to have a
this period are not suitable to be included into a
complete and unbiased sample, and to know the
homogeneous survey.
characteristics of the survey used to extract the
The list of sources was derived from the analysis
data. Because of the in-homogeneous character of
as described in Beckmann et al. (2006a). Addi-
the survey, we had to apply a significance limit
tional observations performed later led to further
rather than a flux limit to define a complete sam-
source detections within the survey area. We ex-
ple. The task is to find a significance limit which
tracted spectra at those positions from the data
ensures that all objects above the given limit have
following the same procedure. It is understood
been found. To test for completeness, the V,/Va-
that most of those objects did not result in a sig-
test has been applied.
nificant detection in the data set used here, but it
ensures completeness of the sample at a moderate The V,/V,-test is a simple method developed
by Avni & Bahcall (1980) based on the V/Vma,
significance limit.
test of Schmidt (1968). V , stands for the vol-
The analysis was performed using the Offline va
ume, which is enclosed by the object, and is
Science Analysis (OSA) software version 5.0 dis-
the accessible volume, in which the object could
tributed by the ISDC (Courvoisier et al. 2003a).
have been found (e.g. due to a flux limit of a
We applied the same method for IBIS/ISGRI and
survey). Avni & Bahcall showed that different
SPI analysis as described in Beckmann et al.
survey areas with different flux limits in various
(2006a). The analysis of the INTEGRALIIBIS
energy bands can be combined by the V,/V,-test.
data is based on a cross-correlation procedure be-
In the case of no evolution (Ve/Va) = 0.5 is ex-
tween the recorded image on the detector plane
pected. This evolutionary test is applicable only
and a decoding array derived from the mask pat-
to samples with a well-defined significance limit
tern (Goldwurm et al. 2003). The ISGRI spectra
down to which all objects have been found. It can
have been extracted from the count rate and vari-
therefore also be used to test the completeness of
ance mosaic images at the position of the source,
a sample. We performed a series of V,/V,-tests
which in all cases corresponds to the brightest
to the INTEGRAL .4GN sample, assuming com-
pixel in the 20 - 40 keV band.
pleteness limits in the range of 1c up to 60 IS-
The significances listed in Tab. 1 have been de-
GRI 20 - 40 keV significance. For a significance
rived by using the OSA software, and refer to the
limit below the true completeness limit of the sam-
count rate and count rate error for ISGRI in the
ple one expects the V,/V,-tests to derive a value
20 .- 40 keV energy band. The luminosities listed (Ve/l’a) < (Ve/Va)true, where (Ve/Va)tTue is the
are the observed luminosities in this energy band.
true test result for a complete sample. Above the
The absorption listed is the intrinsic absorption
completeness limit the (Ve/Va) values should be
in units of 10’2cm-2 as measured in soft X-rays
distributed around (V, /Va)true within the statis-
by various missions as referenced. We also include
tical uncertainties. An example of this method to
the most important reference for the INTEGRAL
determine the significance limit can be found in
data of the particular source in the last column of
Beckmann et al. (2006a).
Table 1. The extracted images and source results
It appears that the sample becomes complete
are available in electronic form’.
at a significance cutoff of approximately 50, which
In order to provide a complete list of AGNs
inclu*de s 38 AGNs. The average value is (Ve/Va) =
detected by INTEGRAL, we included also those
0.45 0.05. This is consistent with the expected
sources which are not covered by the data used
value of 0.5 which reflects no evolution and an even
for our study. Those sources are marked in Tab. 1
distribution in the local universe.
A pure number flux distribution (i.e. logN,s
http://heasarc.gsfc.nasa.gov/docs/integral/inthp-archive.html
J
versus logs) for the sample presented here would is detectable with a 50 detection significance in the
not give meaningful results, because of the differ- 20-40 keV energy band. The resulting correlation
ent exposure times across the survey, and there- is shown in Fig. 4.
fore the varying sensitivity within the survey. An
uncorrected number flux distribution for INTE- 3.1. The Slope of the Number-Flux Dis-
GRAL AGNs has been shown in Beckmann et tribution
al. (2006b). To correct for the different expo-
We applied a maximum-likelihood (ML) algo-
sure times it is necessary to count the number
rithm to our empirical number-flux distribution
of AGNs per unit sky area. Thus the number
to obtain a power-law approximation of the form
of AGNs above a given flux have to be counted
N(> S) = K . S-". Our approach was based on
and divided by the sky area in which they are de-
the formalism derived by Murdoch, Crawford and
tectable throughout the survey. We therefore first
Jauncey (1973), also following the implementation
determine-d the exposure time in 64,620 sky ele-
of Piccinotti et a1 (1982). The htter involved mod-
ments of 0.63deg2 size within our survey. In
ification of the basic ML incorporated to facilitate
each sky bin, the exposure is the sum of each indi-
handling of individual source flux-measurement
vidual exposure multiplied by the fraction of the
errors. The ML method also involves the applica-
coded field of view in this particular direction. The
tion of a Kolmogorov-Smirnov (K-S) test to eval-
dead time and the good time intervals (GTI) are
uate the goodness of fit, as detailed in Murdoch,
not taken into account but the dead time is fairly
Crawford and Jauncey (1973). Once the slope is
constant (around 20%) and GTI gaps are very rare
determined, a chi-square minimization is used to
in IBIS/ISGRI data. Figure 1 shows the exposure
deterimine the amplitude K.
map in Galactic coordinates for this survey. We
For this analysis, we used a sub-sample of 38
excluded those fields with an exposure time less
sources for which the statistical significance of our
than 2 ks, resulting in 47,868 sky elements with
flux determinations was at a level of 50 or greater.
a total coverage of 9.89sr. The flux limit for a
The dimmest source among this sub-sample was
given significance limit should be a function of the
fx = 5.6 x ergs cm-' s-', and the brightest
square root of the exposure t,ime, if no systemat.ic
was fx = 3.2 x 10-'0ergscm-2ss-'. We derived
effects apply, but this assumption cannot be made
a ML probability distribution, which can be ap-
here. The nature of coded-mask imaging leads to
proximated by a Gaussian, with our best fit pa-
accumulated systematic effects at longer exposure
rameters of a = 1.66 f 0.11. A normalization
times. In order to achieve a correlation between
of K = 0.44 sr-' ( ergs cmP2s -')" was then
the exposure time and the flux limit, we therefore
obtained by performing a least-squares fit, with
used an empirical approach. We correlated the ex-
the slope fixed to the ML value.
posure time of each object with an equivalent flux
correspoding to 5u significance based on the flux
4. The Local Luminosity Function of AGNs
and significance of each sample object. By using
at 20 - 40 keV
only the AGNs of our survey we are additionally
assured to consider effects based on the spectral
The complete sample of INTEGRAL AGNs
r
slope (for AGNs 21 2). The correlation was then with a detection significance 2 5a also allows us
fit by a smooth polynomial (Fig. 2). This function
to derive the density of these objects in the local
was then used to estimate the flux limit of each
Universe as a function of their luminosity. In or-
survey field. It has to be noted that the individ-
der to derive the density of objects above a given
ual flux limit of each survey field is not important,
luminosity, one has to determine for each source
but only the correct distribution of those flux lim-
in a complete sample the space volume in which
its. The total area in the survey for a given flux
this source could have been found considering both
limit is shown in Figure 3.
the flux limit of each survey field and the apparent
Based on the flux limits for all survey fields, we brightness of the object. We have again used the
are now able to construct the number flux distri- correlation between exposure time and flux limit
I bution for the INTEGRAL AGN, determining for as discussed in the previous section in order to as-
each source flux the total area in which the source sign a 5a flux limit to each survey field. Then the
4
maximum redshift zmaZa t which an object with 2.23f0.15, and L, = (4.6f2.0) x h;: ergs s-'.
flux fx would have been detectable in each sky The errors have been estimated on the basis of the
element was used to compute the total accessible uncertainties of the density determination and of
volume the flux measurement. These values are consis-
tent with values derived from the 2 - 10 keV XLF
N - .
of AGNs as shown by e.g. Ueda et al. (2003)
and La F'ranca et al. (2005). For example the
work by Ueda et al. (2003) reveals for a pure
with N being the number of sky elements in which
density evolution model the same values for A,
the object would have been detectable and 0, the
71, and 72, but a higher L, = 1.29 x 1044ergs-1.
solid angle covered by sky element i. We applied a
This ratio of 2.8 in the turnover luminosity can
cosmology with HO = 70 km s-' Mpc-l (h70 = l),
be easily explained by the different energy bands
k = 0 (flat Universe), Rmatter = 0.3, and A0 = 0.7, applied. A single power law with photon index
although a A0 = 0 and qo = 0.5 cosmology does of I' = 1.9 in the range 2 - 40 keV would lead to
not change the results significantly because of the
L(2-lokev)/L(zo-40kev) = 2.8, assuming no intrin-
low redshifts in our sample. Figure 5 shows the
sic absorption. Thus it appears that the local lu-
cumulative luminosity function for 38 INTEGRAL
minosity function of AGNs in the 20 - 40 keV band
detected (2 5~7)A GNs in the 20 40 keV energy
- can simply be extrapolated from the 2 - 10 keV
band. Here the density 4 describes the number
range. This has, of course, no implications for
of objects per Mpc3 above a given luminosity Lx:
the XLF at higher redshifts. The values are also
K
q5 = VaT: with K being the number of objects consistent with the luminosity function for AGNs
i= 1 in the 3 - 20 keV band as derived by Sazonov &
with luminosities > Lx. Blazars have been ex-
Revnivtsev (2004) from the RXTE all-sky survey.
cluded because their emission is not isotropic. The
Information about intrinsic absorption is avail-
redshifts in the sample range from z = 0.001 to
able for 34 of the 38 objects (89%) from soft X-ray
z = 0.129 with an average redshift of Z = 0.022.
observations. This enables us to derive the lumi-
Thus the luminosity function is truly a local one.
nosity function for absorbed (NH 2 1022cm-2)
Figure 6 shows the luminosity function in differen-
and unabsorbed sources, as shown in Figure 7.
tial form, indicating tha.t the sample indeed does
The absorbed sources have a higher density than
not show luminosity bins with large incomplete-
the unabsorbed sources at low luminosities, while
ness compared to the rest of the sample. The
this trend is inverted at high luminosities. The lu-
errors are based on the number of objects con-
minosity where both -4GY types have similar den-
tributing to each value. The differential XLF also
shows, like the cumulative one, a turnover around sities is about L(20-40 keV) = 3 x erg s-l. This
tendency is also evident when comparing the frac-
Lx = (5 - IO) x io43e rgss-'.
tion of absorbed -4GNs with the luminosity in the
Because our study is based solely on low red-
three luminosity bins depicted in Figure 8. The
shift objects, we are not able to constrain models
luminosity bins have been chosen so that an equal
involving evolution with redshift. Nevertheless we
number of objects are contained in each bin. The
can compare the XLF presented here with model
position of the data point along the luminosity axis
predictions from previous investigations. XLFs
indicates the average luminosity in this bin, while
are often fit by a smoothly connected two power-
the error bars in luminosity indicate the range of
law function of the form (Maccacaro et al. 1991)
luminosities covered.
Based on the luminosity function, the contribu-
tion of the AGNs to the total X-ray emissivity W
can be estimated (Sazonov & Revnivtsev 2004).
This can be done by simply multiplying the XLF
by the luminosity in each bin and integrating
We fit this function using a least-squares
over the range of luminosities (lo4' ergs sP1 <
method applying the Levenberg-Marquardt algo-
rithm. The best fit values we obtained are A = L20-40keV < 1045.5e rgs s-I). This results in
W20-40kev(> 1041e rgss-') = 2.8~10~~ergshs+-ol
(2.7f0.5) x hpo MPC-~7,1 = 0.93*0.15,72 =
5
.
=
5. Discussion A turnover j the XLF at 4.6 x ergss-'
is observed (Fig. 6). Below this luminosity also
The number flux distribution (Fig. 4) shows the fraction of absorbed AGNs starts to be larger
a slope of cy = 1.66 f 0.11. In the local Uni- than that of the unabsorbed ones, although the
verse with no evolutionland isotropic distribution effect is significant only on a la level (Fig. 7).
of AGN and assuming Eucledian geometry, the ex- Both effects have been seen also in the 2 - 10 keV
pected value is cy = 1.5 Even though the difference (Ueda et al. 2003; La Franca et al. 2005) and in
is only significant on a 1.5a level, this might indi- the 3-20 keV band (Sazonov & Revnivtsev 2004).
cate that the area density at the low flux end of the This implies that we do detect a similar source
distribution has been slightly overcorrected. One population as at lower energies. If a large pop-
has to keep in mind that only a few sources derived ulation of absorbed AGN-s is dominating the cos-
from a small area of the sky are constraining the mic X-ray background at 30 keV as indicated by
low flux end. Krivonos et al. (2005) studied the HEAO 1 A-4 measurements (Gruber et al. 1999),
extragalactic source counts as observed by INTE- and the source population is the same through-
GRAL in the 20-50 keV energy band in the Coma out the Universe those objects would have to
region. Based on 12 source detections they deter- have luminosities L(20--40 keV) < ergs s-l , as
mine a surface density of (1.4 6 0.5) x deg-2 might be indicated by the larger fraction of ab-
above a threshold of lo-" ergs cm-2 s-l in the sorbed sources toward lower luminosities (Fig. 7).
20 - 50 keV energy band, where we get a consis- Even though it has to be taken into account that
tent value of (1.2 f 0.2) x deg2. Comparing the low luminosity end of the XLF is based only
the total flux of all the objects in the AGN sam-
on a small number of objects, below this lumi-
ple (f20-40ke~ = 2.6 x lO-'Oergs cm-2 s-l sr-l) nosity the distribution between active and nor-
with the flux of the X-ray background as pre-
mal galaxies becomes blurred. Additionally ob-
sented by Gruber et al. (1999) shows that the
jects like Ultra-luminous X-ray sources (ULX)
INTEGRAL AGN account only for about 1% of
and star-forming galaxies could provide the nec-
the expected value. This is expected when tak-
essary emission. One interesting case in this cat-
ing into account the high flux limit of our sample:
egory is the detection of NGC 4395 with a lu-
La Franca et al. (2005) have shown that objects
minosity of L(20-40ke~) = 1.4 x 1040ergss-1,
with f2-1okev > lo-" ergs cm-2 s-l contribute consistent with measurements by XMM-Newton
less than 1%t o the CXB. This flux limit translates
which showed L(2-1OkeV) = 1.5 x 1040ergss-1
to the faintest flux in our sample of f20--40kev = (Vaughan et al. 2005). The central engine of this
r
5.6 x ergs cmT2s -' for a = 1.9 power law galaxy has been classified as an i'ultra-luminousll
spectrum.
source, possibly associatied with an intermediate
We compared the emissivity per unit volume of mass black hole with MBH = (4'6, x 104Ma
our objects W20-40kev(> 104'ergss-') = 2.8 x (McHardy et al. 2005). In addition, NGC 4395
ergss-' h30 M ~ c w- i~th that found in the 3- harbors a ULX with L(&*OkcV) = lo3' ergs s-l at
20 keV band. Assuming an average power law of a distance of 2.9 kpc from the center of the galaxy
r
= 1.9, the extrapolated value is W3-20keV(> (Colbert & Ptak 2002).
1041 ergs s-l) = 6.7 x ergs s-l h;o M ~ c - ~ , Another scenario is that the population of X-
which is a factor of 1.3 larger than the value
ray emitting sources depends on redshift, i.e. that
measured by RXTE (Sazonov & Revnivtsev 2004)
there is an evolution of population in time, indi-
and consistent within the la error. Extrapolating
cating that the fraction of absorbed sources might
our result down to 2-10 keV shows an emissiv-
be higher at larger redshifts, alt,hough it should
ity which is lower by a factor of 1.2 compared
be noted that the latter effect, is not clearly de-
to the one derived from the HEAO-1 all-sky map tectable in the 2 - 10keV range. The fraction
(Miyaji et al. 1994) and also within the statistical
of absorbed sources seems to depend on lumi-
uncertainties.
nosity (Ueda et al. 2003; Treister & Urry 2005),
The luminosity function derived from the IN- as seen also in the 20 - 40keV band (Fig. 8).
TEGRAL 20 - 40 keV AGN sample appears to be But some studies come to the conclusion that
consistent with the XLF in the 2 - 20 keV range. there is no evolution of A r ~(U eda et al. 2004;
6
4
Treister & Urry 2005), while others find the frac- are more likely to belong to the Galaxy: the Sec-
tion of absorbed sources increasing with redshift ond IBIS/ISGRI Soft Gamma-Ray Survey Cata-
(La Franca et al. 2005). The latter also find that log (Bird et al. 2006) lists 55 new sources detected
a combination of effects (the fraction of absorbed by INTEGRAL, of which 93% are located within
AGN decreases with the intrinsic X-ray luminos- -10" < b < f10". Among those sources 3 are
ity, and increases with the redshift) can be ex- listed as extragalactic sources, 18 are of Galactic
plained by a luminosity-dependent density evo- origin, and 29 have not been identified yet.
lution model. They further show that the lu- In addition, those objects which have been
minosity function o-f AGNs like those presented demonstrated to be AGNs, are no more likely to
here peaks at z - 0.7 while high luminosity be Compton-thick objects than the overall AGN
AGN peak at z 2. Unified models also pre- population studied here. Only four AGNs (NGC
dict, depending on the applied model, a frac- 1068, NGC 4945, MRK 3, Circinus galaxy) de-
tion of 0.6 - 0.7 for high flux low redshift AGN tected by INTEGRAL have been proven to be
(Treister & Urry 2005). Worsley et al. (2005) Compton thick objects so far, and none of them
examined Chandra and XMM-Newton deep fields showed absorbtion of NH > 5 x 1024cm-2. In
and come to the conclusion that the missing CXB order to clarify this point, observations at soft X-
component i-s formed by highly obscured AGNs rays of those objects without information about
at redshifts 0.5 - 1.5 with column densities of intrinsic absorption are required for all INTE-
the order of fx = - cm-2. Evidence for GRAL detected AGNs (Tab. 1). Up to now 23
this scenario is also found in a study of Chandra % of the INTEGRAL AGN are missing absorp-
and Spitzer data (Polletta et al. 2006). Combin- tion information. A first indication of what the
ing multiwavelength data, this work estimates a absorption in these sources might be, can be de-
surface density of 25 AGNdegF2 in the infrared rived from comparison of the INTEGRAL fluxes
in the 0.6deg2 ChandralSWIRE field, and only with ROSAT All-Sky Survey (RASS) Faint Source
33% of them are detected in the X-rays down Catalogue data (Voges et al. 2000). In order to
to f0.3-8 kev = ergs cm-2 s-I. The work do so we assumed a simple power law with photon
also indicates a higher abundance of luminous and index r = 2.0 between the ROSAT 0.1 - 2.4keV
Compton-thick AGN at higher redshifts (z >> 0.5). band and the INTEGRAL 20 - 40 keV range and
This source population would be missed by the fit the absorption. In the six cases where no detec-
study presented here, because of the low redshifts tion was achieved in the RASS, an upper limit of
(E = 0.022) of the INTEGRAL AGNs. f(0.1--2 4k&) 5 10-~~ergscms--' ~ has been as-
Several studies (Ueda et al. 2004; Treister & Urry 2005bumed, resulting in a lower limit for the absorp-
propose that the absorbed AGNs needed to ex- tion NH > (5 - 11) x cm-2. In Fig. 9 we show
plain the CXB should be Compton thick, and the distribution of intrinsic absorption. It has to
therefore would have been missed at 2 - 10 keV. be pointed out that the estimated values can only
This argument does not hold for the INTEGRAL give an idea about the distribution of intrinsic ab-
observations, where the impact of absorption is sorption and should not be taken literally, as the
much less severe than at lower energies. The ef- spectral slope between the measurements is un-
fect on the measured flux of a source with pho- known and the observations are not simultaneous.
r
ton index = 2 for Compton thick absorption Nevertheless apparently none of the RASS detec-
(NH = loz4c m-2) is only a 5% decrease in flux tions and non-detections requires an intrinsic ab-
(40% for NH = 1025cm-2). It is therefore un- sorption of NH > 2 x 1023cm-2. Therefore it
likely that many Compton-thick objects have been appears unlikely that a significant fraction of IN-
missed by the INTEGRAL studies performed to TEGRAL AGNs will show an intrinsic absorption
date. One possibility would be, that they are NH > loz4 A similar result in the 3-20 keV
among the newly detected sources found by IN- band let Sazonov & Revnivtsev (2004) to the con-
TEGRAL. The fraction of unidentified objects clusion that, the missing emission to explain the
among those sources is of the order of 50%. It CXB is not produced in 'normal' AGNs, but that
should be pointed out though, that most of these a comparable X-ray flux might be produced to-
sources are located close to the Galactic plane and gether by low luminosity AGNs, non-active galax-
7
ies and clusters of galaxies. by Worsley et al. (2005), which would imply an
evolution towards stronger absorption with red-
Most investigations have been focused so far
shift, or the missing emission has to be due to low
on the X-rays below 20 keV, and INTEGRAL
luminosity AGNs, clusters of galaxies, non-active
can add substantial information to the nature of
galaxies or ultra-luminous X-ray sources.
bright AGNs in the local Universe. Considering
the expected composition of the hard X-ray back- Over the life time of the INTEGRAL mission
ground, it does not currently appear that the pop- we expect to detect of the order of 200 AGNs.
ulation detected by INTEGRAL can explain the Combining these data with the studies based on
peak at 30 keV, as Compton thick AGNs are ap- SwZfi/BAT, operating in a similar energy band as
parently less abundant than expected. But the IBIS/ISGRI, will further constrain the hard X-ray
sample presented here might be still too small to luminosity function of AGNs. But we will still be
constrain tlhe fraction of obscured sources, and limited to relatively the high flux, low luminosity
the missing Compton thick AGN could be de- and low redshift end of the distribution, which will
tected when studying sources with f(20-40keV) < be inadequate to explain the cosmic X-ray back-
IO-” ergs cmP2 s-’. ground at E > 20keV. Future missions such as
EXIST or NuStar will be required to answer the
6. Conclusions question of what dominates the Universe in the
hard X-rays.
A statistically complete extragalactic sample
derived from the INTEGRAL public data archive
VB would like to thank Olaf Wucknitz for pro-
comprises 58 low redshift Seyfert galaxies ((2) = viding software to handle the A0 > 0 cosmology.
0.022 f 0.003) and 8 blazars in the hard X-ray
This research has made use of the NASA/IPAC
domain. Two galaxy clusters are also detected,
Extragalactic Database (NED) which is operated
but no star-burst galaxy has been as yet. This
by the Jet Propulsion Laboratory, of data ob-
INTEGRAL AGN sample is thus the largest one
tained from the High Energy Astrophysics Science
presented so far.
Archive Research Center (HEASARC), provided
The number flux distribution shows a slope
by NASA‘s Goddard Space Flight Center, and of
of (Y = 1.66 f- 0.11. Because of the high flux the SIMBAD Astronomical Database which is op-
limit of our sample the objects account in to-
erated by the Centre de Donn6es astronomiques
tal for less than 1% of the 20 - 40keV cosmic
de Strasbourg. This research has made use of
X-ray background. The emissivity of all AGNs
the Tartarus (Version 3.1) database, created by
io4’
per unit volume ~20--40k~V(> ergss-l) = Paul O’Neill and Kirpal Nandra at Imperial Col-
2.8 x lo3*e rgs s-l h$o M ~ c -ap~p ears to be con-
lege London, and Jane Turner at NASA/GSFC.
sistent with the background measurements in the
Tartarus is supported by funding from PPARC,
2-10 keV energy band.
and NPLSA grants N4G5-7385 and NAG5-7067.
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~~
This 2-column preprint was prepared with the AAS MT&i
macros v5.2.
10