A&A 483, 425-435 (2008)
DOI: 10.1051/0004-6361:20078691
M. Revnivtsev1,2 - S. Molkov2,1,3 - S. Sazonov1,2
1 - Max-Planck-Institute für Astrophysik,
Karl-Schwarzschild-Str. 1, 85740 Garching bei München,
Germany
2 -
Space Research Institute, Russian Academy of Sciences,
Profsoyuznaya 84/32, 117997 Moscow, Russia
3 -
Centre d'Étude Spatiale des Rayonnements,
31028 Toulouse, France
Received 17 September 2007 / Accepted 19 March 2008
Abstract
We study the cosmic X-ray background (CXB) intensity variations on
large angular scales using the slew data from the RXTE observatory.
We detect intensity variations up to
on angular scales
of 20-40
.
These variations are partly correlated with the local
large-scale structure, which allowed us to estimate the
emissivity of the local Universe in the energy band 2-10 keV at
1038 h75 erg s-1 Mpc-3. The spectral energy
distribution of the large-angular-scale variations is hard and is
compatible with that of the CXB, which implies that normal galaxies
and clusters of galaxies, whose spectra are typically much softer, do
not contribute more than 15% to the total X-ray emissivity of the
local Universe. Most of the observed CXB anisotropy (after exclusion
of point sources with fluxes
10-11 erg s-1 cm-2) can be attributed
to low-luminosity AGNs.
Key words: galaxies: active - galaxies: Seyfert - galaxies: statistics - X-rays: galaxies - X-rays: general
The Universe is a bright source of X-ray emission. The first discovery in X-ray astronomy was that of the brightest source in the X-ray sky - the accreting neutron star Sco X-1 - along with an isotropic cosmic X-ray background (Giacconi et al. 1962), which was later shown to be composed of emission from a large number of discrete extragalactic sources (Giacconi et al. 1979).
Virtually all astrophysical objects produce X-ray emission: starting from
numerous ordinary stars, with typical X-ray (2-10 keV) luminosities
-1030 erg s-1, through accreting white
dwarfs (
erg s-1), neutron stars and black holes
(
erg s-1), to active galactic nuclei
(AGNs) and quasars (
-1046 erg s-1).
The bulk of the cosmic X-ray background (CXB) is created by
AGNs with luminosities 1042-1045 erg s-1 (e.g. Ueda et al. 2003), but a non-negligible contribution to the
CXB must be provided by low-luminosity (
-1042 erg s-1) AGNs, which constitute the majority of
active supermassive black holes in the local Universe (see
e.g. Ho et al. 1997; Elvis et al. 1984), as well as by ordinary galaxies, due to their
populations of X-ray binaries and stars. Unfortunately,
it is difficult to estimate the cumulative luminosities of these
faint extragalactic sources simply by counting them, as is usually
done for powerful AGNs. Indeed, a normal galaxy or a low-luminosity
AGN with an X-ray luminosity
1040 erg s-1 located at a redshift typical of CXB sources (
1-2) will produce an X-ray flux less than 10-17 erg s-1 cm-2, which cannot be detected
by existing X-ray telescopes. However, the cumulative emissivity
of all weak sources can in principle be estimated using measurements
of the CXB (e.g. Boldt 1992).
It has long been understood that the properties of CXB sources are not constant but have evolved with cosmic time (e.g. Silk 1968; Longair 1966; Schmidt 1968; Maccacaro et al. 1991,1983).
In particular, the cumulative X-ray volume emissivity of
AGNs was much higher at redshifts of
1-2 than it is at the present time
(e.g. Barger et al. 2005; Maccacaro et al. 1983), the star-formation rate (and
the associated rate of X-ray production) was similarly higher
(Madau et al. 1996), and so on. It is therefore clear that the integrated
X-ray emissivity of a unit volume or of a unit mass of Universe has
evolved with time, so knowing its local (z=0) value is essential for
understanding the evolution of different populations of X-ray sources,
in particular supermassive black holes in the centers of galaxies.
Even the latest all-sky surveys conducted at energies above 2 keV
(Krivonos et al. 2007b; Revnivtsev et al. 2004; Markwardt et al. 2005), with their sensitivity
10-11-11.5 erg s-1 cm-2, can detect sources with luminosities
1041 erg s-1 only at distances smaller than
10 Mpc, which is not enough to obtain a representative sample of such
objects and to estimate their cumulative luminosity. Stacking of
individually undetectable fluxes from nearby low-luminosity X-ray
sources can hardly help, because the number density of such sources
increases with decreasing luminosity not fast enough (
,
see e.g. Persic et al. 1989; Elvis et al. 1984) to compensate for
the rapidly declining signal-to-noise ratio.
However, it has been realized that the CXB can help in estimating the total X-ray emissivity of the local Universe (e.g. Carrera et al. 1995; Scharf et al. 2000; Shafer & Fabian 1983; Warwick et al. 1980; Miyaji et al. 1994; Jahoda et al. 1991; Schwartz 1980). The idea is quite simple - the Universe has large-scale structure characterized at zero redshift by a factor of a few or larger density contrast on scales of tens of Mpc. Since X-ray sources are expected to trace the overall matter distribution, they should exhibit a similar space density contrast. When observed in an X-ray survey, the cumulative emission of all individually undetectable sources located in an overdense region of the local Universe will create an area of enhanced sky surface brightness on top of the nearly isotropic X-ray background emission created at larger distances. The amplitude of this large angular scale enhancement of the CXB intensity can be converted into a value of the cumulative volume emissivity of faint X-ray sources, provided that one knows the distribution of matter density in that local region of the Universe and the so-called bias factor of X-ray sources (i.e. the ratio of fractional X-ray source number density fluctuations to fractional matter density fluctuations).
Recent advances in the understaning of the bias factor of AGNs indicate
that in the nearby Universe ()
it is close to unity (see
e.g. Tegmark & Peebles 1998; Boughn & Crittenden 2004). Arguably the most direct determination
of the bias factor (specifically, a measurement of
the ratio of fractional number density variations of X-ray sources to
fractional number density variations of IRAS infrared-selected
galaxies) was recently made using the INTEGRAL all-sky hard X-ray
survey (Krivonos et al. 2007b), and the AGN bias factor was
confirmed to be close to unity. This implies that to convert CXB intensity variations into an X-ray emissivity of the local Universe we can use any good tracer of mass density
in the local volume, for instance infrared galaxies (see
e.g. Basilakos & Plionis 2006). Supporting evidence that AGNs closely trace the
distribution of galaxies was recently presented by
Li et al. (2006) based on an optically selected sample of narrow-line AGNs from
the Sloan Digital Sky Survey.
According to the current knowledge of the distribution of matter in the local
Universe, even its largest angular scale anisotropy - the dipole
component - continues to grow to distances 100-200 Mpc
(e.g. Basilakos & Plionis 2006; Kocevski et al. 2004; Rowan-Robinson et al. 1990), where
the distribution of matter is not yet well known. Nonetheless, there are
distinct density patterns in the nearest (
30-70 Mpc) Universe
that should be imprinted into the CXB intensity distribution and
could be studied with the available X-ray data.
In this paper we essentially repeat the analysis of large-scale anisotropies of the CXB that was previousely performed based on HEAO1/A2 data (e.g. Boughn et al. 2002; Shafer & Fabian 1983; Scharf et al. 2000; Miyaji et al. 1994; Jahoda et al. 1991), using a newly available dataset from the PCA instrument aboard RXTE and redetermine the X-ray emissivity of the local (z=0) Universe utilizing the advantages of the higher collective area of RXTE/PCA and its smaller field of view (and consequently the reduced confusion noise).
Thoughout the paper we adopt a Hubble constant H0=75 h75 km s-1 Mpc-1 and recalculate all relevant quantities from cited papers to this reference value.
The high level of isotropy of the cosmic X-ray background was noticed already shortly after its discovery (e.g. Schwartz 1970). In order to detect and accurately measure the anisotropy of the CXB caused either by our motion with respect to the outer Universe where the bulk of the CXB is created (the so-called Compton-Getting, or CG effect, Compton & Getting 1935) or by the large-scale structure of the Universe at zero redshift, one should reach an accuracy of measuring the CXB intensity of 0.1-0.5% of its average level.
Indeed, the relative dipole-like (
)
variations of the CXB intensity due to our motion with respect to
the outer Universe radiation field (the cosmic microwave background, CMB)
are expected to be (see e.g. Scharf et al. 2000) on the order of
The X-ray dipole due to large-scale structure in the local Universe,
produced by inhomogeneities in the matter density distribution at
distances
50-100 Mpc, is expected to be
Such accuracy presents a challenge for a vast majority of X-ray
instruments. One of the best instruments suitable for measuring the
tiny large-scale CXB anisotropies was the A2 experiment aboard the HEAO1 observatory (Rothschild et al. 1979). It had a special design of detectors that
made it possible to subtract the internal background of the detectors
with almost absolute accuracy. However, its detectors had relatively
large fields of view (
,
and
)
and therefore seriously suffered from
the confusion noise produced by sources with flux below the detection
threshold.
The field of view of the RXTE/PCA spectrometer is significantly
smaller (0.974 sq deg), while its effective area
is significantly larger (
6400 cm2 of all the detectors combined,
or
1300 cm2 of a single detector), and the model of the PCA detector background developed by the PCA team allows one to subtract it with
an accuracy of at least 2-3% of the CXB intensity (Jahoda et al. 2006).
Therefore, RXTE/PCA is currently the best instrument for
studying large-scale anisotropies of the CXB.
The noise created by point sources with flux just below the
detection threshold
is still an issue for RXTE/PCA. Its influence can be estimated
as follows (see e.g. Warwick et al. 1980). Suppose that the differential
number-flux function of point sources
is isotropic over the sky and
can be described by a power law with a slope of -5/2:
The number of sources in any infinitely small flux bin is subject to
Poisson statistics, hence we can write for the dispersion of the integrated
signal:
The relative uncertainty in the flux from the solid angle will be
.
For the
real number-flux function of extragalactic sources in the energy band
2-10 keV (
,
see e.g. Revnivtsev et al. 2004), this will be
From the above estimates it is clear that in order to reach the
required accuracy of (1-5)
10-3 of the CXB intensity one
needs to exclude sources with fluxes down to
10-11 erg s-1 cm-2 and
average CXB intensity measurements over areas of
1000 deg2.
The sensitivity of the RXTE Slew Survey varies across the sky due
to the non-uniform exposure map. Therefore, strictly speaking the limiting
flux for source detection is not the same in different parts of the sky.
However, more than 60% of the sky is covered with sensitivity better
than 10-11 erg s-1 cm-2 and nearly all of the remaining sky is
covered down to 2.5
10-11 erg s-1 cm-2 (2-10 keV).
Given the sensitivity map of the survey, we can estimate
variations in the X-ray flux that can be resolved into point sources
in a given region of the sky if we assume an isotropic distribution of sources
over the sky with the number flux function described above:
Therefore, despite the non-uniform coverage of the sky with RXTE/PCA slews, the adopted method of data reduction allows for secure sky cleaning, which should not create any significant artificial structures on the map of the CXB intensity measured over large solid angles.
Over its lifetime (Dec. 1995-present), the spectrometer PCA on the
RXTE observatory has collected a lot of data during slews between
pointed observations. The high collective area of the PCA detectors
(1300 cm2 of each of the five) makes it possible to detect
sources down to fluxes
5
1012 erg s-1 cm-2 and this detection limit is
determined solely by the confusion noise (Revnivtsev et al. 2004; Markwardt & Swank 2004).
For our analysis we used all publicly available data of RXTE/PCA scans and
slews from February, 1996 till February 8, 2007. During a significant period
of RXTE operations only some of the five PCA detectors were
switched on (this mode of operations became dominant after the end of Epoch3 of
the PCA voltage, i.e. since March 1999), therefore not all of the PCA detectors have collected sufficient exposure time and sky coverage for
our purposes. For this reason, in our analysis we used only detector #2, which collected 6-6.5 Ms of data between February, 1996
and February 8, 2007 (we consider here only regions away from the
Galactic plane,
,
and >
away from the detected
point sources reported in Revnivtsev et al. 2004).
We analyzed the data of the first layer (which typically enables the highest
signal-to-noise ratio for weak X-ray sources) of the PCU2 in two
energy bands, 3-8 and 8-20 keV, which were converted into
instrumental channels using response matrices (constructed with the HEASOFT 6.0
package) appropriate for each observation. We similarly analyzed
observations of the Crab nebula, 6 background pointings
(see Jahoda et al. 2006) and ``dark'' Earth.
Flux measurements by RXTE/PCA are subject to systematic uncertainties.
Absolute flux measurements (in particular those of the CXB)
depend on how well the effective area of the detector
is known, the knowledge of the effective solid angle of the collimator
and so on. In our analysis we relied on an absolute X-ray flux
calibrator - the Crab nebula. Specifically, our results are obtained
under the assumption that the Crab spectrum is
(which implies a flux in the 2-10 keV energy band
10-8 erg s-1 cm-2).
As we are interested in low X-ray intensity variations over the sky we should take into account all systematic uncertainties that could affect our measurements at the level of a per cent or so.
Our data analysis consists of the following steps:
One of the main systematic uncertainties of relative flux measurement is the accuracy of the PCA background subtraction. The currently standard approach to this consists of modeling the background with the highest possible accuracy using a number of tracers recorded onboard on the 16-s time scale and verifying that ``empty'' sky regions have zero intensity after background subtraction. Although the CXB contribution to the flux measured by RXTE/PCA is eliminated by this procedure, it can be recovered by observations of dark Earth (see e.g. Revnivtsev et al. 2003).
The current version of RXTE/PCA software (LHEASOFT 6.0) provides
an outstandingly accurate model of the instrumental
background (L_7_240CM version of the background), which allows for an
accuracy of background subtraction of a few per cent, i.e. the
root-mean-square amplitude of the unmodeled deviations is
0.02-0.03 cnts/s/PCU/beam in the 3-8 keV energy band, which
corresponds to
3
10-13 erg s-1 cm-2 deg-2 for a
Crab-like spectrum (e.g. Markwardt et al. 2002; Jahoda et al. 2006, see also
Fig. 1), or
2-3% of the
average CXB intensity.
However, as our aim is to measure the CXB intensity with an accuracy
of 0.1-0.5%, we made special corrections to take into account
the unmodeled part of the RXTE/PCA background, taking advantage of
the observational fact that this part appears to be an additive
component to all measured count rates irrespective of the pointing of
a given PCA observation. As an example of such a behavior, we show in
Fig. 1 background subtracted, 4-month-binned count
rates averaged over the whole sky, over the cone of
half-opening
angle around
and over the 6 RXTE/PCA
pointings that were used to calibrate the PCA background model. The
similarity of the patterns of the unmodeled parts of the background
in these three cases is apparent.
If we assume that the true intensity averaged over the whole sky is zero
at relatively high Galactic latitudes
(to eliminate the
contribution from the Galactic ridge X-ray emission, see Sect. 2.6)
and also excluding
regions around detected point sources,
the 4-month-averaged measurements of sky intensity provide us an
estimate of the unmodeled part of the RXTE/PCA background, which we
add to the model-predicted rates of the PCA background.
In our subsequent analysis we only used data obtained later than 108 RXTE seconds (i.e. after March 3rd, 1997), because only during this period more than 3 background poinings were used to calibrate the PCA background model, which apparently has an effect on the accuracy of the background subtraction (see Fig. 1).
The effect of the additional background subtraction is demonstrated in the lower panel of Fig. 1. We see that the two-step background subtracted sky intensity from some sky region is much more stable than that obtained after just the first (standard) step and that the variations of the inferred intensity are almost compatible with the statistical uncertainties of the measurements.
![]() |
Figure 1:
Upper panel: background subracted intensities of sky, averaged
over 4-month intervals. Crosses denote all-sky averaged values, open
circles the intensity integrated over a ![]() ![]() ![]() |
Open with DEXTER |
Another important issue in X-ray flux measurement is the parameters of the detector. The efficiency of the PCA detectors slightly changed during the large time span considered here (1996-2007) for different reasons, including slow drift of the energy scale of the detectors and a slight change of the gas purity or pressure within the signal volumes (see e.g. Jahoda et al. 2006).
This small (5%) drift of the detector effective area manifests
itself in the measured count rates from the Crab, a constant X-ray source
which has been regularly observed by RXTE (see Fig. 2). We
made a special correction for this effect, modeling it by a piecewise
function.
In order to check the stability of the corrected count rates we
studied the detector count rate during dark-Earth observations. By
construction, the PCA background model eliminates the count rate
during off-bright-source observations, automatically subtracting
the average CXB flux contribution. Observations of dark Earth
therefore allow one to see this oversubtracted part of the PCA background, which is essentially the all-sky average CXB flux (see Revnivtsev et al. 2003). The count rates measured during dark-Earth
observations, corrected for all the above effects, are shown
in Fig. 2. We see that the resulting dark-Earth count
rate (=-CXB flux) is stable to within 0.5-1%.
![]() |
Figure 2:
Upper panel: background subtracted count rate
of the Crab nebula measured in the energy band 3-8 keV with RXTE/PCA
over its lifetime. The count rate is not corrected for the deatime
fraction which is typically ![]() |
Open with DEXTER |
Similar corrections were done in the energy channel 8-20 keV.
After all the corrections, the CXB intensity measured within relatively
large sky areas (so that the Poisson variance of the number of unresolved
sources within a given area does not influence the result) exhibits
definite variations over the sky. We studied the time dependence of
these variations to check for any remaining systematic effects in our
data. The difference in the CXB intensity between various sky regions
revealed no systematic variations (
cnts/s/PCU, see Fig. 3).
As a byproduct of the analysis described above we have estimated the
CXB intensity in two broad energy channels (=
). The
averaged CXB fluxes measured by PCA in the bands 3-8 keV and 8-20 keV
are 0.847
0.008 mCrab/beam and 1.43
0.05 mCrab/beam,
respectively. Note that in making these estimates we adopted that the
average deadtime fraction during Crab observations is
4% higher
than that during empty sky/dark Earth observations (see e.g. Jahoda et al. 2006).
For a power-law spectral shape of the CXB with a photon index
the flux measured by PCA in the energy band 3-8 keV
corresponds to a flux at 2-10 keV of
10-11 erg s-1 cm-2 beam-1. Taking into account
that the effective solid angle of the PCA spectrometer is
0.974 sq deg, we find that the average CXB intensity
10-11 erg s-1 cm-2 deg-2.
![]() |
Figure 3:
Differences of the count rates integrated over a
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
Open with DEXTER |
Our Galaxy is a bright emitter in soft (e.g. McCammon & Sanders 1990), standard (e.g. Revnivtsev et al. 2006; Worrall et al. 1982) and hard X-rays (e.g. Krivonos et al. 2007a). Due to the large angular scale of this emission, it can contribute to the large-scale anisotropy of the CXB and therefore this contribution should be estimated and subtracted or masked.
The operating energy band of RXTE/PCA (3-20 keV) precludes a detection of the soft X-ray background of the Galaxy, hence only the Galactic ridge X-ray emission (GRXE) is important for our study.
It was recently shown that most likely the GRXE is a superposition of weak Galactic sources, namely cataclysmic variables and coronally active stars, and that its surface brightness closely follows the near-infrared (NIR) surface brightness of the Galaxy (Revnivtsev et al. 2006). Therefore, one can estimate the ridge X-ray intensity in a given direction by rescaling the NIR map of the Galaxy.
However, due to the discrete nature of the GRXE one should not expect to see
this emission in regions of extremely low surface number density of stellar
objects, such as the Galactic poles. Indeed, if we assume the volume
emissivity distribution of Galactic X-ray emission to be
erg s-1 pc-3 with the exponential
scale hight
pc (Revnivtsev et al. 2006), the
flux from a solid angle
in the direction of the
Galactic poles (
)
will be
However, in reality the GRXE flux from these directions should be even
lower because sources creating the GRXE are expected to be quite rare
there. Indeed, in the direction of the Galactic poles the mass of
stars in the solid angle
is (here we assume for simplicity an
exponential distribution of stellar density in the Galactic disk,
,
the stellar density in the Solar vicinity being
/pc3, Robin et al. 2003)
After all the filterings we constructed an X-ray intensity profile of the sky
along the Galactic latitude, by averaging measured intensities over
(Fig. 4).
At latitudes
no dependence of the sky surface
brightness on latitude exceeding the involved uncertainties is seen,
hence we mask the wide
region from our subsequent analysis.
![]() |
Figure 4:
Upper panel: latitudinal (at
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
Open with DEXTER |
We note that the signal previously seen at
by
HEAO1/A2 (Iwan et al. 1982) may be created by unresolved Galactic and
extragalactic sources for which the HEAO1/A2 instrument was less
sensitive than PCA due to the wider field of view of the former and
consequently higher confusion noise. This is supported by the
fact that Iwan et al. (1982) claimed the Galactic background emission to be
present at
at a level comparable to that at
,
while the latest studies show that there is virtually
no GRXE emission from the Galactic anticenter region (Revnivtsev et al. 2006).
The Universe is highly homogeneous on large scales (100-200 Mpc, e.g. Wu et al. 1999). Due to this uniformity and because most of the CXB originates at relatively large distances from
us (at redshifts
1-2, e.g. Hasinger et al. 2005; Ueda et al. 2003), the CXB is very
isotropic. However, at smaller distances the Universe becomes
inhomogeneous and this should cause some anisotropies of the CXB intensity. Since the CXB is composed of discrete sources, these anisotropies should be studied on angular scales sufficiently
large that the Poisson variations of the number of sources within the
area of study are unimportant, as described by Eq. (1).
The dipole component of the CXB intensity caused by the local large-scale
structure saturates at distances -300 Mpc
(e.g. Scharf et al. 2000; Kocevski et al. 2004). However, higher order harmonics
might be created by more distant structures. This fact can
significantly complicate the comparison of the CXB anisotropies with
the structures in the nearby Universe known from sky surveys
(e.g. the well-known IRAS PSCz survey in the infrared
band, Saunders et al. 2000), because typically such structures are
well probed only out to distances
100-200 Mpc.
In order to study the amplitude of CXB anisotropies that may be created
by more distant structures we analyzed the distribution of
galaxies resulting in the semi-analytic modeling of formation of
galaxies in the Millennium simulation
(De Lucia & Blaizot 2007). It was shown (e.g. Wang et al. 2007) that the distribution of
galaxies in this simulation has statistical properties
that resemble those of the real populations of galaxies in the local
Universe. Our examination of the distribution of simulated galaxies
showed that the matter density structures at distances >100-200 Mpc create
fluctuations of the CXB intensity comparable in amplitude with those
created by the more nearby mass concentrations. Therefore, one should
anticipate only a partial correlation of the CXB intensity map with
that of the nearest mass concentrations.
After implementing all the masks and corrections (see the previous section), we constructed maps of sky intensity (with the average CXB intensity subtracted as a consequence of the PCA background subtraction) in the energy bands 3-8 keV (Fig. 5) and 8-20 keV.
The simplest all-sky anisotropy that can be studied using these maps is the CXB dipole component.
A simple determination of the dipole component of the CXB angular
distribution - its approximation by the function
(where
is the angle between a given direction and
the direction of the dipole) - yields the amplitude
.
The obtained
parameters of the CXB dipole in the 3-8 keV energy band are presented
in Table 1.
Several effects can produce a dipole component of the cosmic X-ray background. First, it may arise from some primordial anisotropy of the Universe at high redshifts.
Second, the observed CXB dipole must contain a contribution from the Doppler effect associated
with our motion with respect to the reference frame in which the bulk
of the CXB emission is produced (-2).
![]() |
Figure 5:
Adaptively smoothed map of the CXB intensity in the 3-8 keV
energy band, in Galactic coordinates. Plotted is the X-ray intensity averaged
over cones with half opening angles 20-40![]() ![]() |
Open with DEXTER |
Finally, relatively nearby (z<0.1-0.5) mass concentrations can impose their imprints on the CXB map and, in particular, create a non-negligible dipole component. According to our current understanding of the distribution of matter density in the nearby Universe, it indeed contains a dipole component that causes a bulk motion of the local group of galaxies, which in turn can be seen via measurements of the dipole component in the map of the cosmic microwave background (see e.g. Rowan-Robinson et al. 1990).
As dipole anisotropy is a very wide feature on the sky, it is very hard to disentangle in the CXB intensity map the different components of the total dipole signal described above.
One promising approach would be to study the
dependence of the amplitude of CXB intensity variations on
energy. Indeed, as was already mentioned in
Sect. 2.1, the amplitude of the Compton-Getting
dipole component (caused by our motion with respect to
the CXB reference frame) has a strong dependence on the slope of the
CXB spectrum in the considered energy band:
,
where
is the characteristic power-law photon
index of the spectrum at energy E. This means that in hard X-rays
(50-200 keV), where the CXB spectrum has a much steeper slope
-2.6 than at energies below
20 keV
(
), the amplitude of the CG effect must be considerably
higher and have a specific energy dependence. On the other hand,
the spectrum of the CXB dipole component created by large-scale structures
in the nearby Universe is expected to be similar to that of the cumulative
emission spectrum of local X-ray sources (mainly AGNs),
which resembles the CXB spectrum blueshifted by a factor of
2.5(Sazonov et al. 2008). Therefore, the relative
amplitude of this dipole component must also be approximately constant
below
20 keV and growing at higher energies, but the energy
dependence is expected to be different from that of the CG dipole.
Unfortunately, within the RXTE energy band we cannot distinguish the CG dipole from the large-scale structure one. Therefore, in our analysis we estimated the CG dipole based on the known parameters of our motion with respect to the cosmic microwave background (Lineweaver et al. 1996).
In Table 1 we present the measured CXB dipole in the energy band 3-8 keV and its value after subtraction of the CG component. We note that the obtained dipole amplitude and orientation are in good agreement with the values determined from the analysis of HEAO1/A2 data (Scharf et al. 2000).
Table 1: Parameters of the CXB dipole component in the energy band 3-8 keV determined from RXTE/PCA data.
In order to compare the CXB intensity variations with the expected variations due to the nearby mass concentrations, we made use of the IRAS PSCz survey of galaxies (Saunders et al. 2000). The underlying assumption of our analysis below is that X-ray emitting AGNs trace the underlying mass distribution approximately as well as IRAS galaxies, i.e. the relative bias factor between the two classes of objects is close to unity. As discussed in the Introduction, this has been shown to be the case at least on large distance scales.
The difference in the distribution of nearby (<100-200 Mpc) galaxies in
a pair of directions will cause a difference in the CXB intensities in these directions. Suppose that the X-ray emissivity per unit number density of galaxies is constant (
), whereas the space density of galaxies
(
)
varies with direction and with distance from us (R).
Let the density of galaxies in one direction be
and in another
.
Then the difference of the CXB fluxes
created by the density variations, measured within a solid angle
in these directions will be
We estimated the space density of IRAS galaxies with luminosities
>
from the PSCz catalog
by taking into account the dependence of the minimum detectable galaxy
luminosity on distance and the finite sky coverage of the survey.
In doing that we assumed that the density of galaxies averaged
over the entire sky does not depend on distance from us.
As we are interested in relative galaxy densities
,
we have made the most straightforward correction
for the distance dependence of the minimum detectable galaxy
luminosity: compared the number of detected galaxies within a given
solid angle
at given distance R with that averaged over the
whole sky:
We only considered density variations out to a radius of
150 h75-1 Mpc.
![]() |
Figure 6:
Space density of IRAS galaxies calculated in two directions of the
sky within cones with a half-opening angle of 25![]() |
Open with DEXTER |
Figure 6 shows the radially integrated difference
between the IRAS galaxy number
densities in two particular directions, determined within cones with
a half-opening angle of 25
.
Figure 7 shows an
all-sky map of relative galaxy number density variations
,
calculated in similar
cones. It can be seen that mass concentrations in the nearby Universe create a clear excess in the
upper-right part of the map, which resembles the
pecularity on the CXB intensity map (Fig. 5). Due to the
much smaller contrast of R* values in the southern Galactic
hemisphere, its correlation with the CXB intensity map provides very
little information in addition to the northern
hemisphere. Therefore, in studing the infrared-X-ray correlation we only
considered the northern hemisphere.
![]() |
Figure 7:
Radially integrated relative space density of galaxies
(
![]() ![]() |
Open with DEXTER |
In Fig. 8 we show a scatter plot of relative variations of the CXB intensity (3-8 keV) with respect to relative number density variations of nearby galaxies across the sky. There is marginal evidence of a correlation between these two quantities.
Since we have filtered out sources with X-ray flux larger
than
from the all-sky map, the linear part of this
correlation provides us an estimate of the cumulative emissivity of
sources with luminosities
,
where
is the typical sensitivity of the
survey, and
is the characteristic distance at which
inhomogeneities in the galaxy distribution are probed (see
e.g. Fig. 6). We note that after we added new RXTE
data, the typical sensitivity
of our survey
has slightly improved with respect to the value reported by
Revnivtsev et al. (2004) and is now
10-11 erg s-1 cm-2 for Crab-like spectra in the energy band 2-10 keV. This translates into a limiting luminosity
1-3
1042 erg s-1 for
-50 Mpc, the distances at which most of the local
anisotropy is created (see Fig. 6).
Thus we find from the linear part of the correlation shown in Fig. 8 that the cumulative emissivity of local sources with
is
1038 h75 erg s-1 Mpc-3 in the energy band 2-10 keV.
On the other hand, the cumulative X-ray emissivity of nearby AGNs with
higher luminosities (
erg s-1) has been measured fairly
accurately by directly counting sources
(e.g. Sazonov & Revnivtsev 2004; Piccinotti et al. 1982; Ueda et al. 2003). Taking into account
that in the 3-20 keV band the cumulative emission of such AGNs is
characterized by a spectral slope similar to
that of the CXB, i.e.
(Sazonov et al. 2008),
we can convert the emissivity in the 3-20 keV band reported by
Sazonov & Revnivtsev (2004) to 2-10 keV
:
1038 h75 erg s-1 Mpc-3.
Adding up the contributions of sources with luminosities below 1042 erg s-1 and AGNs with luminosities over
1042 erg s-1, estimated above, we find that the total X-ray
emissivity of the local Universe in the energy band 2-10 keV is
1038 h75 erg s-1 Mpc-3.
In reality, the liming luminosity depends on the distance to sources and
also on the direction in the sky (due to the non-uniform exposure map of the
RXTE Slew Survey). Therefore, to do a more accurate estimate of the
X-ray volume emissivity of the nearby Universe, ideally it would be better not
to exclude bright sources from the analysis and correlate total
(due to both weak and bright sources) flux variations with the
integrated density contrast of galaxies. However, bright sources in
the real sky would introduce strong Poisson-like noise in the sought
correlation. To circumvent this problem and in an attempt to obtain a
more accurate estimate of the total local X-ray emissivity, we
evaluated the contribution of bright sources (which are luminous AGNs
with
erg s-1) by integrating their known
luminosity function rather then by summing up the observed fluxes of
really detected sources.
To this end, we adopted the shape of the luminosity function of
bright (
erg s-1) AGNs as we have
measured it using the RXTE Slew Survey (Sazonov & Revnivtsev 2004),
whereas the normalization of the luminosity function was taken to be
proportional to the matter density as traced by IRAS
galaxies, with the constraint that the total emissivity in the
2-10 keV energy band averaged over the local Universe is
1038 h75 erg s-1 Mpc-3.
Thus, we calculated the contribution of bright sources according to the formula:
![]() |
Figure 8:
Distribution of CXB variations measured by RXTE/PCA in the
energy band 3-8 keV in beams of size 1932 sq deg (cones with
a ![]() ![]() ![]() ![]() |
Open with DEXTER |
Here we would like to mention one more time that the exact values of X-ray emissivity
of the local Universe obtained here are affected by the variability of our
sensitivity for point sources removal over the sky (see e.g. Sect. 2.1).
However, even adopting the most conservative
flux limit for point sources removal we obtain the value
of the local Universe X-ray emissivity within the quoted 1 confidence interval.
The simple estimates presented in Sect. 2.1 suggest
that, due to the descrete nature of the CXB, variations of the CXB flux on
different angular scales should have different energy spectra. Indeed,
on small angular scales (e.g. 1 sq deg), Poisson variations of the
number of unresolved sources (with flux below the detection threshold)
dominate and one should expect that a few sources within the
field of view of the detector will provide a dominant contribution to
such variations. Therefore, the count rate variations will have
spectral hardness typical of such (
erg s-1 cm-2) sources
and will have the amplitude give by Eq. (1).
After filtering our bright sources and averaging the resulting count rates
over areas of the sky 1 sq deg, one should expect that the
spectrum of the variations will have a shape typical for the total
emissivity of the local Universe (Sazonov et al. 2008), since the
majority of the CXB flux variations on such angular scales are
produced by large-scale mass structures in the nearby Universe (note that
we have subtracted the estimated contributiuon of the CG dipole).
As we show in Fig. 9, the slope of the scatter
plot of flux variations (with respect to the average CXB intensity)
measured in PCA (1 sq deg) beams in the 3-8 and 8-20 keV energy bands is
0.31
0.02, which corresponds to the hardness ratio of a power-law
spectrum with a photon index
0.1. For comparison, the
slope of the scatter plot of flux variations measured
over larger solid angles (
2000 sq deg) is 0.42
0.07, which
corresponds to the
hardness ratio of a power law with a photon index
0.2.
Therefore, we do find an indication that unresolved sources with flux
below the RXTE detection threshold have softer spectra than the CXB
(see a similar result obtained using GINGA data in Butcher et al. 1997).
![]() |
Figure 9:
Scatter plot of count rates in the energy bands 3-8 and
8-20 keV measured within the field of view of RXTE/PCA during its
observations of the 6 background pointings at different times
( upper plot), and of fluxes averaged over different wide (![]() |
Open with DEXTER |
Another important conclusion that can be drawn from this RXTE/PCA measurement
of the spectrum of large-angular-scale variations is that the total
X-ray emission of the local Universe is characterized by spectral hardness
similar to that of the CXB. This in particular implies that
emission from non-active galaxies and clusters of galaxies, which
is typically much softer than the CXB, does not significantly
contribute to the total X-ray emissivity of the local
Universe. Assuming that the cumulative spectral energy distribution of
non-active galaxies and low-luminosity (
erg s-1) clusters of galaxies is characterized by a hardness
ratio (8-20 keV over 3-8 keV) that coresponds to a power law with a
photon index
in the energy band 3-20 keV, we can
estimate that such objects contribute less than 15% of the total
emissivity of the local Universe. This is consistent with our
estimates presented below that are based on the statistics of
local non-active galaxies and clusters of galaxies.
As we summarize in Table 2 and detail below, it seems
possible to explain the total emissivity of sources with luminosities
,
estimated above from the correlation
of the CXB anisotropies with the local large-scale structure, as a
superposition of contributions from known types of sources:
low-luminosity AGNs, clusters of galaxies, normal and starforming galaxies.
AGNs with X-ray luminosities below 1042 erg s-1, which we
refer to below as low-luminosity AGNs (LLAGNs) are known to be abundant
in the local Universe (e.g. Ho et al. 1997). Elvis et al. (1984) used a
sample of optically selected LLAGNs to estimate the X-ray luminosity
function of such objects by converting their measured H luminosities to X-ray luminosities under the assumption of linear correlation
between these two quantities. These authors concluded that the
cumulative emissivity of LLAGNs is
3
1038 h75 erg s-1 Mpc-3, i.e. comparable to that of AGNs with higher luminosities.
The X-ray emission from clusters of galaxies is typically fairly soft,
especially from low-luminosity (low-mass) clusters. Therefore, the cumulative
emissivity of clusters of galaxies must strongly depend on the energy
band of the study. We consider here the standard X-ray energy band
2-10 keV, where the cumulative emissivity of clusters of galaxies can
be estimated from their soft X-ray luminosity function
(e.g. Mullis et al. 2004; Böhringer et al. 2002) using the well-known intracluster
gas temperature-luminosity relation (e.g. Markevitch 1998). We
find that clusters with luminosities below 1042 erg s-1 in the energy band 2-10 keV contribute together
1037 h75 erg s-1 Mpc-3 to the local X-ray emissivity.
The contribution of normal and starforming galaxies to the X-ray
emissivity in the energy band 2-10 keV mainly comes from low-mass and
high-mass X-ray binaries (LMXBs and HXMBs), respectively. The shape of
the luminosity function of LMXBs is approximately constant while its
normalization is proportional to the galaxy mass
(e.g. Gilfanov 2004; Fabbiano 1986). Therefore, the cumulative emissivity of
normal galaxies can be estimated from the known value of
integrated stellar mass density (
103,
i.e.
Mpc-3) in the
low-redshift Universe (Cole et al. 2001; Kochanek et al. 2001). This gives a value of
(2-4)
1037 h75 erg s-1 Mpc-3.
The emissivity of starforming galaxies in the energy band 2-10 keV
(due to HMXBs) can be estimated either via the luminosity function of
nearby starforming
galaxies (e.g. Ranalli et al. 2005), or by converting the average star
formation rate in the local Universe (e.g. Gallego et al. 1995) to an
X-ray volume emissivity using known statistical properties of HMXBs in
starforming galaxies (e.g. Gilfanov et al. 2004). Both types of
estimate suggest that the emissivity is approximately
(3-4)
1037 h75 erg s-1 Mpc-3.
Table 2:
Emissivities (2-10 keV) of different types of low-luminosity objects
(
erg s-1) in the local Universe in units of
1038 h75 erg s-1 Mpc-3.
Using data collected by the PCA instrument during slews of the RXTE
observatory between its pointed observations in 1996-2007 we have
obtained an all-sky map of the cosmic X-ray background
intensity. Through a careful data analysis we measured the CXB intensity variations with an accuracy 0.5%.
On small angular scales we detected variations with 7% FWHM, which
are most likely caused by Poisson variations of the number of weak sources
below the detection threshold in the field of view of the detector.
Upon application of different filters, we detected statistically
significant variations of the CXB intensity on the 20-40
angular
scales with an amplitude
2%. Part of these variations is
correlated with mass concentrations in the nearby (D<150 Mpc)
Universe, which allowed us to make an estimate of the total emissivity of
low-luminosity (
) sources. The obtained value is in
agreement with those determined previously based on HEAO1/A2 data
(Miyaji et al. 1994; Jahoda et al. 1991). We note that these authors studied
correlations on smaller angular scales and had to deal with
complications due to the strong influence of Poisson statistics and clustering
of sources.
The spectral hardness ratio of large-angular-scale variations of the CXB intensity is compatible with that of the average CXB spectrum. This allowed us to put an upper limit on the combined contribution of soft X-ray sources (normal and starforming galaxies, and low-mass clusters of galaxies) to the total emissivity of the local Universe in the energy band 2-10 keV: <15%. Most of the observed CXB anisotropy can be attributed to low-luminosity AGNs.
Acknowledgements
M.R. thank Gerard Lemson for assistance in the usage of the results of the Millennium simulations. This research made use of data obtained from the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center. This work was supported by DFG-Schwerpunktprogramme SPP 1177.