M. Revnivtsev
Max-Planck-Institute für Astrophysik,
Karl-Schwarzschild-Str. 1, 85740 Garching bei München,
Germany
Space Research Institute, Russian Academy of Sciences,
Profsoyuznaya 84/32, 117810 Moscow, Russia
Received 3 December 2002 / Accepted 11 August 2003
Abstract
We present an analysis of the Galactic bulge emission
observed by the RXTE/PCA during a set of scans over the Galactic Center field,
performed in 1999-2001. The
total exposure time of these observations is close to 700 ks.
We construct the distribution of Galactic ridge emission intensity
and spectral parameters up to Galactic latitudes
.
We show that the intensity distribution of the ridge emission at
could be well described by an exponential model with e-folding
width
.
Best-fit spectral parameters do not show
statistically significant changes over Galactic latitude.
Key words: accretion, accretion disks - black hole physics - instabilities - stars: binaries: general - X-rays: general - X-rays: stars
The observations of the ASCA and Chandra observatories seem to rule out the hypothesis of dominant dim point source contributions to the observed ridge emission (see Ebisawa et al. 2001). Therefore it is believed that it has a diffuse origin.
The discovery by the Tenma satellite of strong 6.7 keV line emission in the spectrum of the Galactic plane made it possible to suggest that the bulk of this Galactic ridge emission in the energy range 1-10 keV is due to an optically thin plasma of temperature of a few keV. Accurate measurements of the ridge spectrum made by ASCA also revealed lines from some other elements, Mg, Si, S, Ar, which also supports the thermal origin of the emission. However, this hypothesis also encounters serious problems. One of the most general problems is connected with the fact that the deduced parameters of the optically thin plasma implies that it is impossible to bound such plasma within Galactic plane (see e.g. Townes 1989) and also it is very hard to provide enough energy for such plasma. There are also serious problems with the approximation of the energy spectra of the ridge emission within the framework of its thermal origin (Tanaka 2000). These complications together with the detection of the presumably non-thermal tail in the spectrum of the Galactic ridge emission (see e.g. Yamasaki et al. 1997; Skibo et al. 1997; Valinia & Marshall 1998) gives rise to the additional interpretation, in which the X-ray line emission was considered to originate through charge-exchange interactions of low-energy cosmic ray heavy ions (e.g. Tanaka et al. 1999; Tanaka 2000), while the hard power-law tail appears as a result of nonthermal bremsstrahlung emission of cosmic ray electrons and protons (e.g. Dogiel et al. 2002).
For the understanding of the origin of the Galactic ridge emission it is
important to know the distribution of its flux and parameters over the
Galaxy. Such a study has been previously carried out using different satellites
(e.g. HEAO1, Worrall et al. 1982; Iwan et al. 1982, GINGA; Yamasaki et al. 1997, RXTE; Valinia & Marshall 1998), but now we could for the first time use relatively uniform
coverage of the central 10
degrees. This became possible because of
the large campaign of RXTE Galactic Center scans, organized by
the RXTE team. In this paper we analyze public
data of RXTE Galactic center scans from March 1999 till July 2001.
For our analysis we used approximately 150 sequences of
3200 Galactic Center observations (each set of Galactic center
scan sequences usually consists of 22 individual observations) performed
from March 1999 till July 2002.
The total exposure of all these observations is approximately 700 ks.
We divided these data
into two parts, depending on the high voltage epoch of PCA, which
determines the energy response of the instrument:
Apr. 1999-May 13. 2000 (Epoch4) and May 14, 2000-July 2001 (Epoch5).
For the data analysis we used a set of standard procedures
of the LHEASOFT 5.2 package. In order to increase our sensitivity for
photons with energy >10 keV we used all three layers of the PCA.
As we are interested in the measurement of low fluxes
we used the "L7_240'' model for the PCA background estimation. This model
includes an instrumental background as well as the Cosmic X-ray
background (CXB) term. The influence of interstellar absorption in
the direction of the Galactic Center could be important at low b. However,
in the subsequent analysis we will restrict ourself to
where the interstellar absorption is negligible in
our bandpass (3-20 keV). Under these conditions,
the extinction of the CXB in the interstellar medium is estimated to
be at most 10% of the HI column density (Dickey & Lockman
1990), thus it can be ignored.
According to the
latest calibration information the systematic uncertainty of the
flux obtained with the help of the background model used is considered
to be within 1-1.5%
(see RXTE GOF web page. http://heasarc.gsfc.nasa.gov/docs/xte/xhp_proc_analysis.html). Therefore
in our analysis we included 1.5% (of the background count rate
in the considered energy range) uncertainty in the measurements.
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Figure 1: Map of the Galactic Center field, reconstructed from the RXTE/PCA scans in 1999-2000. Circles represent regions where the contribution of point sources dominates (see text). |
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Figure 2: Exposure map of the Galactic scan observations during period March 1999-May 2000 (high voltage Epoch4). |
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The map of the Galactic center region, reconstructed from scans performed during Epoch4 (March 1999-May 2000) is presented in Fig. 1. The map represents the flux measured by PCA in the direction to which the center of the PCA field of view was pointed. Any point source on the map contributes to a sky region around it in accordance with the response of the RXTE/PCA collimator. Combining information from available X-ray catalogs of bright sources (Wood et al. 1984; Liu et al. 2000; Liu et al. 2001; Voges et al. 1999; Sugizaki et al. 2001) with the analysis of the obtained map, we have selected a set of sources that can significantly contribute to the observed flux in the region of the scans. A list of the selected sources is presented in Table 1. Note that in our analysis the identification of sources in crowded regions, like in the immediate vicinity of the Galactic Center, is quite complicated and therefore might not be exact.
Table 1: List of point sources, areas around which were excluded from the analysis.
In order to separate the contribution of bright point sources from the
Galactic
ridge emission
we excluded areas with a radius of 1.35
around them - such a
radius ensures that even the brightest sources (such as GX 5-1)
do not contribute more than a few cnts/s/PCU to the
surrounding points.
Upper limits on the unaccounted sources within the field of the scans could
be estimated as 1-2 cnts/s/PCU (
0.5-1 mCrab) at Galactic
latitudes
.
Assuming a Crab-like spectrum of the sources
this upper limit corresponds to a flux
erg/s/cm2.
The effective field of view of the RXTE/PCA spectrometer is about 1
deg2. According to the
luminosity function of the Galactic X-ray sources, measured e.g. by
ASCA (Ueda et al. 1999; Sugizaki et al. 2001), RXTE/ASM (Grimm et al. 2002),
CHANDRA (Ebisawa et al. 2001) at Galactic Latitudes
|b|<0.5 the density of sources with a flux higher than
erg/s/cm2 (i.e. compatible with our rejection limit) is of the order
of 0.1 deg-1 and this value drops with increasing |b|
(e.g. Grimm et al. 2002). The
contribution of weaker point sources to the ridge emission does not exceed
approximately 10% (e.g. Ebisawa et al. 2001). Therefore, due to our
limited sensitivity to weak point sources the obtained brightness profiles
and spectra could slightly suffer from their influence and an additional
"noise'' of
10% could appear.
After the rejection of regions affected by point sources
we have hardly any data
within
.
Besides, a large number of weak point sources
(see e.g. Sugizaki et al. 2001) in this region could strongly contaminate
the ridge emission observed by the RXTE/PCA. Therefore we will not try
to study the ridge emission at these latitudes in detail.
During Epoch4 and Epoch5 the PCA detectors had significantly different response functions, therefore we will analyze data obtained during these periods separately.
Measured profiles at latitudes higher than 2
show
a quite weak dependence on longitude, see Fig. 3.
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Figure 3: Distribution of the 3-20 keV intensity of the Galactic ridge emission with longitude. The regions over which the brightness is averaged are 2 < |b| < 4 for the upper plot, and 4 <|b|<10 for the lower plot. |
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We constructed profiles of the brightness of the Galactic ridge emission, averaged over all longitudes (l) in our scan field. The intensity profiles in three energy bands are presented in Fig. 4.
The latitude distribution of the
Galactic ridge emission at
can be well described by an
exponential model of the form
.
The PCA collimator collects X-rays from approximately
1 sq.deg, and therefore the measured profile of the ridge emission
is in reality a convolution of the sky distribution with the response of
the PCA collimator. Therefore in our approximation of the observed profiles
we folded the model profile with the response of the PCA collimator.
The measured profiles of the X-ray intensity of the ridge
emission and the best fit exponential models (folded with the
collimator response) are presented in Fig. 4.
Note that there are indications that an additional
narrow component is present at low latitudes,
.
A similar
narrow component was also found by Valinia & Marshall (1998). But in
our case strong
contamination by point sources in the Galactic plane does not
allow us to study this component in detail. Our obtained profiles
at
are consistent with the results of Valinia & Marshall
(1998), but their model of the spatial variation of the
intensity of the ridge emission
(a Gaussian with FWHM long Galactic latitude
)
can
no longer describe the profile of the diffuse emission at higher
latitudes. One should use the exponential model instead.
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Figure 4:
The profile of the brightness of the Galactic ridge
emission with latitude in three energy bands constructed using
March 1999-May 2000 data (Epoch4, left) and May 2000-July 2001 data
(Epoch5, right). The solid line shows the convolution
of the exponential model with the response of the RXTE/PCA collimator.
There is a clear indication of the presence of an additional
component within
![]() ![]() ![]() ![]() ![]() |
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The parameters of the approximations of the observed profiles at
in the two energy
bands 3-20 keV and 10-20 keV are presented in Table 2.
The presence of the Galactic ridge emission in 20-30 keV energy band
of PCA is not statistically significant in the analyzed data.
Table 2: Approximation of observed brightness profiles of Galactic ridge emission in two energy bands.
The spectrum of the Galactic ridge emission is known to be quite rich, full of different lines of different elements (see e.g. Kaneda et al. 1997). Unfortunately most of these lines lie in energy bands lower than 3-3.5 keV, i.e. below our bandpass. In our energy range we can see only the blend of Fe lines at energies around 6-7 keV.
As was shown before (e.g. Yamasaki et al. 1997; Valinia & Marshall 1998), the 3-20 keV
spectrum of the Galactic ridge emission could be relatively well described
by a single power law
with a Gaussian line at energy 6.7 keV.
The quite large exposure time of the collected data (approximately 200 ks
for each epoch after subtraction of contaminated regions) allows us to make
a spectral approximation of the observed ridge emission at different latitudes.
Below we will use a power law with a Gaussian line
model for the approximation of the the ridge emission collected over
the whole scan field with
,
as well as for emission collected
over individual 1
-width
strips along the Galactic plane.
Best fit parameters of the approximation of the Galactic ridge spectrum
averaged over the whole scan field
with
are presented in Table 3. Dependences of
best fit parameters on Galactic latitude are presented in Fig. 6.
Table 3:
Spectral approximation of data collected at
.
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Figure 5:
PCA spectrum of the Galactic ridge emission collected over
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We analyzed the data of RXTE scans over Galactic center regions
performed in 1999-2001. After subtraction of regions contaminated by
bright pointed sources we constructed the intensity profile
of the Galactic ridge emission and its spectral parameters
across the galactic plane within
.
We show
that the intensity profile at
could be well described
by an exponential model (
)
with e-folding size
.
A spectral approximation of data collected over 1-deg strips along
the galactic plane does not show statistically significant changes
of best fit parameters both of the continuum and of the Fe line. The averaged
spectrum of the ridge emission observed by the PCA could be approximated
by a power law with a slope
and a Gaussian line at
energy
6.7 keV with
equivalent width
eV. These results are compatible with
previously published results of GINGA (Yamasaki et al. 1997) and RXTE
(Valinia & Marshall 1998).
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Figure 6: Latitude distribution of parameters of spectral approximation of the Galactic ridge emission (combined data of Epoch4 and 5). |
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Figure 7: Distribution of observed brightness of the Galactic ridge emission (crosses and open circles), the CO line intensity (short dashed line) and number of supernova remnants within 1 deg strips along the galactic plane within the field of scan of RXTE. The Y-axis has units cnts/s/PCU for the observed Galactic ridge X-ray emission (3-20 keV), K km s-1 deg for CO line. |
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Following Markevitch et al. (1993) and Yamauchi & Koyama (1993) it is interesting to compare the observed profile of the ridge emission with the distribution of the CO line flux (e.g. Dame et al. 1987) as a tracer of molecular gas in the Galaxy and also with the density of SNR (Green 2001) as a tracer of supernova explosions. The constructed distributions within the field of scans of RXTE/PCA are presented in Fig. 7. It is clearly seen that CO line intensity drops much more abruptly than the intensity of the ridge emission, while the distribution of SNR more closely resembles the profile of the ridge X-ray emission. Unfortunately, poor statistics of SNR at high latitudes does not allow us to make any solid conclusions.
Acknowledgements
The Author is grateful to R. Sunyaev, M. Gilfanov and E. Churazov for valuable discussions. This research has made use of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center.