Issue |
A&A
Volume 512, March-April 2010
|
|
---|---|---|
Article Number | A25 | |
Number of page(s) | 9 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/200912555 | |
Published online | 24 March 2010 |
XMM-Newton unveils the
complex iron K
region of Mrk 279
E. Costantini1 - J. S. Kaastra1,2 - K. Korista3 - J. Ebrero1 - N. Arav4 - G. Kriss5 - K. C. Steenbrugge6
1 - SRON National Institute for Space Research, Sorbonnelaan 2,
3584 CA Utrecht, The Netherlands
2 - Astronomical Institute, Utrecht University, PO Box 80000,
3508 TA Utrecht, The Netherlands
3 - Department of Physics, Western Michigan University, Kalamazoo, MI
49008, USA
4 - Department of Physics, Virginia Tech, Blacksburg, VA 24061, USA
5 - Space Telescope Science Institute, 3700 San Martin Drive,
Baltimore, MD 21218, USA
6 - University of Oxford, St John's College Research Centre, Oxford,
OX1 3JP, UK
Received 22 May 2009 / Accepted 14 January 2010
Abstract
We present the results of a 160 ks-long
XMM-Newton observation of the Seyfert 1
galaxy Mrk 279. The spectrum shows evidence of both
broad and narrow emission features. The Fe K
line may be equally well explained by a
single broad Gaussian (
km s-1)
or by two components: an unresolved core plus a very broad profile
(
km s-1).
For the first time we quantified, via the ``locally
optimally emitting cloud'' model, the contribution of the broad line
region (BLR) to the absolute luminosity of the broad component of the
Fe K
at 6.4 keV. We find that the contribution of the BLR is only
3%. In the
two-line component scenario, we also evaluated the contribution of the
highly ionized gas component, which produces the Fe XXVI
line in the iron K region. This contribution to the narrow core of the
Fe K
line is marginal <0.1%. Most of the
luminosity of the unresolved, component of Fe K
may come from the obscuring torus, while the very-broad associated
component may come from the accretion disk. However, models of
reflection by cold gas are difficult to test because
of the limited energy band. The Fe XXVI
line at 6.9 keV is consistent to be produced in a high column
density (
cm-2),
extremely ionized (log
)
gas. This gas may be a highly
ionized outer layer of the torus.
Key words: galaxies: individual: Mrk 279 - galaxies: Seyfert - quasars: absorption lines - quasars: emission lines - X-rays: galaxies
1 Introduction
The X-ray spectrum of active galactic nuclei (AGN) bears the signature
of different environments in the vicinity of the
supermassive black hole. In particular, the emission features detected
in an X-ray spectrum
are believed to arise in different environments, with dramatically
distinct physical characteristics of temperature
and density. Narrow emission line profiles (especially from C, N, O
He-like and H-like ions), not significantly variable on a short
time-scale, may form in the far
narrow-line region (e.g. Pounds
et al. 2004). The broad symmetric line profiles, the
more and more often detected in AGN
spectra (Kaastra
et al. 2002; Steenbrugge et al. 2005; Costantini
et al. 2007; Ogle et al. 2004,
hereinafter C07), are consistent to be produced, at least in one
case, within the broad line region (C07), which is
5.6L1/242
light days away from the central black hole (L is
the luminosity of C IV
C IV in units of
1042 erg s-1,
Peterson 1993). Emission
from the accretion disk, located in the proximity of the black hole,
has been extensively studied via the iron K
line at 6.4 keV. For this
line, relativistic effects result in a skewed and asymmetric line
profile (e.g. Wilms
et al. 2001; Tanaka et al. 1995; Young et al.
2005).
Recently, relativistic profiles have been reported also for other
lines, in particular O VIII (Kaastra
et al. 2002; Branduardi-Raymont et al. 2001;
Steenbrugge
et al. 2009; Ogle et al. 2004).
However, a contribution to the line emission from other AGN regions
(NLR, BLR and molecular
torus) add up to these relativistic profiles. A narrow and not variable
component, probably originating far from the black hole, has been
disentangled from the Fe K
line
(Yaqoob & Padmanabhan 2004;
Jiménez-Bailón et al. 2005).
This component is predicted in the unification model (Antonucci & Miller 1985)
as produced in a high-column
density, cold region, about 1 pc away from the black hole
(e.g. Krolik
et al. 1994; Krolik & Kallman 1987) and
then scattered into our line
of sight (e.g. Ghisellini
et al. 1994). An unresolved Fe K
line and the associated reflection seems indeed to be ubiquitous
in bright type 1 objects (Nandra
et al. 2007).
As the FWHM of the narrow core is
compatible with the width of the UV/optical broad lines (Yaqoob & Padmanabhan 2004),
a contribution to the broad component from the BLR is in principle
possible. However, considering individual sources, no evident
correlation between the FWHM of neither the
hydrogen H
or H
,
supposed to be entirely formed in the BLR, has been found (Nandra 2006;
Sulentic
et al. 1998). These results are based on optical and
X-ray data not simultaneously collected. In
NGC 7213,
using simultaneous observations, the H
and the resolved iron line shared the same value of the FWHM
(Bianchi et al. 2008).
Mrk 279 is a bright Seyfert 1 galaxy (
erg cm-2 s-1,
this study), extensively studied in X-rays (C07 and
references therein). C07, using Chandra-LETGS data
of this source, for the first time quantified the contribution of the
BLR to the soft X-ray spectrum. Indeed, thanks to the simultaneous
observation of the broad lines in the UV band by HST-STIS and FUSE (Gabel et al. 2005) the
ionization structure of the BLR has been determined using the ``locally
optimally emitting clouds'' model (LOC, Baldwin
et al. 1995). This allowed to infer the X-ray broad
lines luminosities, that were then contrasted with the LETGS data. C07
found that the broad lines observed in the soft X-ray spectrum (where
the O VII triplet was the most
prominent one) were consistent to be entirely formed in the BLR. The
average peak of production of the X-ray lines is possibly
10 times
closer in with respect to the UV lines, implying a larger Keplerian
width for the X-ray lines. Unfortunately, the limited LETGS energy band
prevented C07 to study the iron K
region. Here we present a detailed study of the X-ray spectrum of
Mrk 279 as observed by XMM-Newton, focusing
mainly on the emission components and the 6.4-7 keV region.
The paper is organized as follows. Section 2 is
devoted to the spectral analysis of the data. In Sect. 3 we
model the emission
spectrum evaluating, for each component, the contribution to the
Fe K
line. In Sect. 4 we discuss our results and in
Sect. 5 we present the conclusions. The cosmological
parameters used are: H0 = 70
km s-1 Mpc-1,
,
and
.
The abundances were set to solar following
Anders & Grevesse (1989)
prescriptions. The redshift is 0.0305 (Scott
et al. 2004). The quoted errors are 68% confidence
errors (
), unless otherwise stated.
Table 1: XMM- Newton observation log.
2 The data analysis
The observation of Mrk 279 was spread over 3 orbits on
November 15-19, 2005. In Table 1 we report the
observation log.
The data were processed with the standard SAS pipeline
(SAS 7.0) and filtered for any background
flares. The Epic-PN exposure time in small-window mode reduced the
exposure time from the original 160 ks to 110 ks.
After the background filtering we obtained a net exposure time of 75 ks.
The light curve of the three time intervals is displayed in
Fig. 1
in the soft (0.3-2 keV) and hard (2-10 keV)
band. The maximal variation is about 36% in both energy bands.
To increase the statistics, we combined the Epic-PN data after
verifying that the physical parameters of the modeling were the same
for the three separate data sets. Epic-MOS1 camera was set in timing
mode. Due to still existing energy gain uncertainties in this mode, we
did not analyze these data further. Epic-MOS2 were collected in imaging
mode that resulted in about 61 ks of net
observation, after filtering out short high background episodes. For
both Epic-PN and MOS2 we imposed at least 20 counts per channel to
allow the
use of the
minimization.
The RGS total useful exposure time is about 110 ks.
Each RGS data set has been rebinned by a factor 5 which
resulted in a bin size of 0.07 Å
and signal-to-noise ratio of 10 for each set of data that were
simultaneously fitted. The spectral analysis was carried out using the
fitting package SPEX
(ver 2.0).
In the following we describe first the underlying continuum, then the emission line spectrum and finally the absorption features which furrow the spectrum.
![]() |
Figure 1: Epic-PN light curves in the 0.3-2 and 2-10 keV band. The maximal variation is about 36%. |
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Table 2: Best-fit continuum parameters for a phenomenological model.
2.1 The continuum
The continuum has been first determined using Epic-PN. A single
powerlaw fit, modified by Galactic absorption is
unacceptable (
,
where
is the number of degrees of freedom). This fit produces systematic
residuals both at soft and hard energies, in addition to narrow
features both in absorption and emission. We then added
a black body component, modified by coherent Compton scattering
(Kaastra & Barr 1989).
The reduced
is again unacceptable (
). The residuals show that a
broad band component is still missing. We then substituted the
simple powerlaw model with a broken power law, obtaining an acceptable
description of the continuum. The reduced
,
considering only the best-fit continuum, without narrow
emission/absorption features, is
.
The parameters of this phenomenological interpretation of the continuum
are listed in Table 2
and the fit shown in Fig. 2. The parameters
in Table 2
and reduced
refer to the total best fit, including emission and
absorption features, as described below (Sects. 2.2, 2.3).
For comparison, we analyzed also MOS2 data, with the caveat
that deviation from the Epic-PN best fit may occur,
especially at the low (E < 0.5 keV)
and high energy (E > 8 keV) ends of
the band, and at the instrumental gold edge
around 2 keV.
The final MOS2 best-fit parameters are shown in Table 2. The
parameters agree well with the Epic-PN fit, albeit with some
differences, which we ascribe to the cross calibration mismatch between
the MOS and PN cameras.
![]() |
Figure 2:
Best fit for the Epic-PN data. The model includes a modified black body
(dotted line), a broken powerlaw (dashed
line), both absorbed by ionized gas, a
broad Fe K |
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2.2 The emission line spectrum
We see evidence of a range of broad and narrow emission lines both in the RGS and in the Epic spectra. Here we refer as narrow lines the ones which are unresolved by Epic-PN (i.e. FWHM < 7000 km s-1), but may be resolved by RGS. Then the broad lines ( FWHM > 7000 km s-1) are instead resolved also by Epic. Finally, the very broad lines are the ones with
In our previous study broad emission from a blend of the
O VII triplet lines was very
significantly detected (at ,
C07). We therefore looked for such evidence in the present RGS data. We
fix the wavelength of the line centroid (
Å) to the values
found in C07. We also fix the FWHM of the broad
feature, which could not be resolved in the individual O VII
lines, to the previously measured value: 1.4 Å, corresponding
to about 19 000 km s-1
for the blend. We find that the intrinsic line luminosity decreased
with respect to the 2003 observation going from
to
in units of
1040 erg s-1.
The same line luminosity is measured also using the three RGS data sets
separately. The inclusion of the line improves the fit by
,
corresponding to only
significance. The broad line
is also a necessary emission component to correctly fit some underlying
absorption features (e.g. O V,
O VI and O VII),
important in the global fitting of the
ionized absorber parameters (Fig. 3). The inclusion of
this low-ionization absorber alone improves the fit of the RGS spectrum
by
.
In the RGS band we detect the narrow O VII
forbidden line with a flux
(
erg cm-2 s-1),
consistent with the Chandra measurement (C07). The
line is marginally detected (Table 3), probably
because of a neighboring bad pixel. For other relevant narrow lines,
such as O VIII LyLy
,
Ne IX and Ne X,
we only obtain upper limits on the luminosity (Table 3).
Table 3: List of line luminosities as measured by Epic-PN (inst. 1) and RGS (inst. 2).
![]() |
Figure 3: RGS spectrum in the O VII region in terms of ratio from a continuum fit. For display purpose, only the rebinned data from orbit 1087 and 1088 are shown. The measurements reported in the paper are based on the full RGS exposure. The labels indicate some of the main features of the warm absorber. |
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![]() |
Figure 4:
Upper panel: residuals, in terms of |
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Both the PN and MOS2 spectra show a complex structure between 6.4 and
7 keV: a prominent Fe K
line at
6.4 keV
(rest-frame) and a broad emission feature, consistent with a blend of
Fe XXVI and Fe K
at
keV and
keV
respectively (Fig. 4,
upper panel). Because the MOS2 spectral analysis deliver a slightly
different powerlaw slope at energies >2 keV
(Table 2),
and the data are affected by lower statistics, we restrict
the iron line analysis to the Epic-PN data only. At first we modeled
the Fe K
line with a single unresolved Gaussian. Although the general fit
improves (
),
this model does not provide a good fit for the line, as evident in
Fig. 4
(lower panel). Next, we considered the line as composed of an
unresolved Gaussian and a very broad component, leading to a
significant improvement of the fit (
,
Table 4).
The very broad component has a FWHM of
keV,
corresponding to
14 000 km s-1.
We tested this very broad component of the Fe K
line against a relativistically smeared profile (Laor
1991). With this model, the line should be viewed at an angle
of
,
with a radial emissivity law r-q,
with
(Fig. 5,
Table 4).
These parameters mimic a regular, almost symmetric, line
profile (e.g. Reynolds &
Nowak 2003). The statistical improvement with respect to a
simple very-broad Gaussian is:
,
corresponding to
.
For the Fe K
feature, we considered also a single Gaussian with the width free to
vary (Table 3).
The line centroid is
in the rest
frame of the source, and the
intrinsic luminosity is
erg s-1.
The line is resolved by Epic-PN, with a FWHM of
keV (corresponding
to
km s-1).
The statistical improvement with respect to a single
unresolved line is
.
Table 4:
Parameters for the very broad component of the Fe K
line in a two-components model.
The inclusion of other two narrow lines, at the energy of the
Fe XXVI and the Fe K
further improves the fit by
.
For consistency, we tested the Laor model, with the same line
parameters, to the Fe K
,
fixing the line ratio to 0.135 (Yaqoob et al. 2007; Palmeri et al. 2003).
As expected the flux of a broad Fe K
line has a negligible effect on the fit, therefore we ignore this
additional
component in subsequent fits. This negligible contribution of a broad
profile of the Fe K
also strengthens our assumption that the bump seen in the 6.9-7.05
region is a blend of
two narrow lines (Fe XXVI and
Fe K
)
rather than a broad Fe K
line arising from the accretion disk. This fit is shown in
Fig. 5.
The reduced
of a fit with all emission lines included, but no absorption, is
308/238. With this final component (see below), the
.
The lines' parameters and their significance are listed in
Table 3.
In all the lines fitting, we also let the normalization free to vary
towards negative values. This allowed us to use the F-test as an
indication of the
significance of the lines (Protassov
et al. 2002).
![]() |
Figure 5:
Spectrum and best fit in the iron line region as observed by Epic-PN.
The solid lines display the broad and narrow
components added to fit the emission features: a prominent Fe K |
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2.3 The underlying absorption
Mrk 279 shows a complex absorption spectrum (C07), whose
characteristics are best studied using RGS. In analogy with C07, we
modeled the absorption with two Galactic components,
that is a neutral gas (
,
Elvis et al. 1989)
and a ionized gas, highlighted by the O VII
absorption line at 21.6 Å, with
cm-2
and temperature
eV
(C07, Williams et al. 2006).
Further, we detect other two, photoionized, gas components intrinsic to
the source. We report the average physical parameters of the warm
absorbers as measured by RGS and the Epic-PN in Table 5. The absorbers are
characterized by a low
.
Therefore absorption lines, well detected in the high-resolution
spectrum (Fig. 3),
rather than edges (which are in this case shallow features in the Epic
spectrum), are the signature of the absorbers. The inclusion of a lower
ionization gas alone already improves the Epic-PN fit by
.
The inclusion of a two-component gas, with column densities and
ionization parameters free to vary leads to an improvement of the fit
of
,
bringing the reduced
to
1.20. A
detailed analysis of the complex absorption and a comparison with
previous results will be presented in a forthcoming paper (Ebrero
et al., in prep.).
3 Modeling
of the Fe K
line region
In this section we evaluate the contribution of different region in the proximity of the AGN to the spectrum above 6 keV.
3.1 The LOC model for the broad emission lines
Table 5: Best fit parameters for the absorbed spectrum as measured by RGS and Epic-PN.
Here we investigate a possible physical link between the
Fe K
line and the optical BLR. In order to quantitatively evaluate the
contribution of the BLR to any X-ray broad lines, we used the ``locally
optimally
emitting clouds'' model (Baldwin
et al. 1995). This model considers the observed line
luminosity as a sum of lines emitted at different density n
and distances r, weighted by a powerlaw
distribution for n and r:
and
respectively (see e.g. C07, Korista et al. 1997, for
details). For the present observation we can benefit from the results
obtained in C07: they evaluated the ``structure'' of the BLR (i.e.
the distribution of r and the covering factor)
fitting 11 UV lines observed by HST-STIS and FUSE simultaneously to the
Chandra-LETGS data. They found a slope for r,
and a covering factor
%, keeping
the slope for nfixed to -1 (Korista
& Goad 2000). The general structure of the BLR is not
expected to have changed significantly in the 2.5 years that have
elapsed since the Chandra observation. Therefore we
apply those value also to the present spectral energy distribution
(SED).
However, the ionizing continuum, and as a consequence the line
luminosities, may have changed. In Fig. 6 the
SED used for the present observation (2005) is displayed (solid line).
The optical points are measured
by OM using the U (344 nm), UVW1
(291 nm), UVM2 (231 nm) and UVW2
(212 nm) filters. The unabsorbed X-ray continuum is evaluated
using the Epic-PN data. For energies higher than 10 keV, the
power law continuum was extended and artificially cut off at
150 keV.
On the very low energy end of the SED (far infrared to radio band), the
shape was taken from a standard AGN SED
template used in Cloudy (Ferland 2004).
We used Cloudy (version 07.02.02, Ferland
et al. 1998) to calculate a line luminosity grid
over a large range of r and nvalues.
As in C07, r ranged between
1014.7-18 cm. The value of n
ranged between 108-12.5 cm-3.
For both parameters the spacing of the values was 0.125 in log.
For comparison, we show also the SED used for the 2003 observation. The
overall optical flux is higher for the present data, while the X-ray
continuum remained almost unchanged in flux and soft X-ray spectrum.
![]() |
Figure 6: The SED of Mrk 279 in 2005 measured by XMM- Newton (solid line). The optical data are measured by OM, while the broad band X-ray continuum is measured by Epic-PN. The dotted line shows the SED of the 2003 data for comparison (C07). |
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In order to infer the BLR contribution to the Fe K
line, we have to rely on the O VII
triplet in the X-ray band, whose luminosity could be totally accounted
for by the LOC model (C07). Therefore we assume that all the emission
from the broad line of the O VII
triplet comes from the BLR. The O VII
line luminosity (
erg cm-2 s-1)
from the RGS data can be explained by the LOC model using
the new 2005 SED, with the values of
and CV
set by the previous analysis, within the errors. Given these
constraints, the
resulting value for
,
which is here the only free parameter, is found to be consistent with
previous results. Once this constraint is set, we can
predict also the contribution of the Fe K
and other X-ray lines.
In Sect. 2.2
we have provided two statistically acceptable descriptions of the
Fe K
line: a
narrow+very-broad component model and a single, resolved, Gaussian.
Only for the latter case the FWHM is similar to the
one directly measured from the UV data (
-9500 km s-1,
Gabel et al. 2005).
In Fig. 7
we show the BLR line
luminosities predicted by the LOC model in the X-ray band. Along with
the O VII observed luminosity we
also plot the observed value for the Fe K
line in the case of a single Gaussian model (Table 3). The predicted
O VII/Fe K
luminosity ratio is about 6.3 within the narrow
1.14-1.20 range for
.
Therefore, the BLR contribution to the Fe K
line is then
erg s-1,
about 30 times smaller than the broad Fe K
component measured from the data. For comparison we also made a
prediction on the iron line flux based on the 2003 SED and the BLR
parameters derived in C07. In that case the Fe K
luminosity of iron from the BLR is
erg s-1.
![]() |
Figure 7:
The solid thick line shows the predicted BLR line luminosities in the
X-ray band in units of 1040 erg s-1
using the LOC model, using as constraints
|
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3.2 Variability of the continuum and iron line complex
Lines emitted at a few gravitational radii from the black hole may show
significant short term variability. We examined the Fe K
line parameters in the three time segments of our observation using a
simple Gaussian model. We find that the flux, the FWHM,
centroid energy and the underlying
powerlaw slope are nearly the same in response to the central-source
variation. In order to further test a variability of the Fe K
flux we analyzed both an archival XMM-Newton-pn data
set with a net exposure time of
26 ks, collected in May 2002 and Chandra
HETGS data, collected about 10 days later in May 2002 (Yaqoob &
Padmanabhan 2004; Scott et al. 2004). In
Table 6
we show the comparison of the line parameters of
all observations since 2002, for a single-line model.
Prior to 1994, Mrk 279 showed a significant
2-10 keV flux variation, ranging from
erg cm-2 s-1
(Weaver et al. 2001).
The 2002 Chandra-HETGS observation, revealed a very
low-flux state of the source
(
erg cm-2 s-1,
Scott et al. 2004),
a factor of two lower than what measured by XMM-Newton,
shortly before. In the last XMM-Newton observation
the source shows a 2-10 keV flux of
erg cm-2 s-1.
We see that, despite the change in the continuum flux (a factor two),
the Fe K
line parameters did not dramatically change in three years time, within
the errors.
In the Chandra data the decline of the
HETGS effective area and the low signal-to-noise ratio hampered the
detection of the Fe XXVI and Fe
K
lines. On the contrary, in the 2002 XMM-Newton
spectrum, both lines, although blended, are
detected at about 95 and 90% confidence level for Fe XXVI
and Fe K
,
respectively. The line luminosities are the same we obtain in 2005,
within the errors.
Table 6:
Parameters of the Fe K
at different epochs.
3.3 The Fe XXVI line at 6.9 keV
There are no detectable changes in the Fe XXVI
line in a 3-years time scale. The absence of any of the lines (or a
blend of lines) of the Fe XXV
triplet is also peculiar (Table 4, Fig. 5). In order to
understand the properties of the gas emitting Fe XXVI,
we created a grid of
column density, log
-24.5, and ionization
parameter, log
-7,
using Cloudy. For lower-column-density, lower-ionization
parameter gas, the predicted emission line would be too weak, or
absent. For an easy scaling of the luminosity, we initially considered
the covering factor (CV)
equal to unity. This is the fraction of light that is occulted by the
absorber
. A covering factor of one
is of course unrealistic, because in this case absorption lines of the
same ions should be observed. Therefore CV
must be less than one. We also simplistically assume that a single gas
component is responsible for all Fe XXVI
emission. Therefore, we took the measured intrinsic luminosity of
Fe XXVI as the reference line to
estimate the covering factor.
We only have upper limits on other highly ionized lines (namely
N VII, O VIII,
Ne X and Fe XXV,
Table 4),
therefore a formal
fit cannot be performed.
However, those limits contain information that can be used to
constraint the parameters. In Fig. 8 the set of
parameters which are consistent with the measurements are displayed.
![]() |
Figure 8: Emission line contribution to the high ionization lines (Fe XXVI, Fe XXV, Ne X, O VIII). Triangles: data. Light shaded line: range of models viable to fit the data. |
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In Fig. 9,
we show the parameter space of the possible solutions.
We see that the ionization parameter sufficient to produce
Fe XXVI, but not Fe XXV
has to be larger that 5.3.
The ionization parameter linearly correlates with the column density,
as for the highest values of log
the emission lines are visible only if there is sufficient emitting
material. For the same reason, for the same column density, log
and CV
correlate linearly. On the other hand, the covering factor
anti-correlates with
(Fig. 9).
Indeed, for a high column density less material on the line of sight is
needed to produce the observed emission.
4 Discussion
4.1 A complex continuum
![]() |
Figure 9: Parameter space of parameters viable to produce the observed high-ionization emission spectrum. Upper panel: Log of the ionization parameter as a function of the log of the column density. Lower panel: covering factor of the Fe XXVI line as a function of the log of the column density. |
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The continuum spectrum of Mrk 279 needs at least three
component to be correctly interpreted: a modified black body and a
broken powerlaw (Table 2).
In principle, reflection from both the accretion disk and from
distant matter should be present (Krolik & Kriss 1995; Matt
et al. 1991; George & Fabian 1991; Nandra
et al. 2007). The only difference would be that if
arising from the accretion disk, the reflection and the associated iron
line should be modified by relativistic effects. The harder tail with
and the double component of the iron line profile (Sect. 2.2) may be
reminiscent of a reflection emission component. We therefore tested
this scenario, in comparison with our phenomenological model for the
continuum, including both types of reflection
in the model (model REFL in SPEX). This model simultaneously considers
the Compton reflected continuum (Magdziarz
& Zdziarski 1995)
and the fluorescent emission from Fe K
(Zycki
et al. 1999; Zycki & Czerny 1994) from a
Schwarzschild black hole. General relativity effects and convolution
with an accretion
disk effects can be easily switched off in this model. Free parameters
are the normalization of the reflected power law and
its spectral index
,
the reflection scale
s, the emissivity scale
(Zycki et al. 1999), the
inclination angle i of the disk of the iron line
emission. In the unblurred reflection, we fixed the inclination angle
to 60
,
based on the average found for a large sample of Seyferts (Nandra et al. 2007).
We find that the energy range E>3 keV
can be well fitted by a combination of a reflector with relativistic
properties and by a distant reflector. The former accounts for the
broad
component of the Fe K
line, while the latter models the narrow component of the line
(Table 7)
in a similar way as obtained in the two-component fit (Fig. 5).
The basic parameters of the disk (e.g. the inclination angle,
Table 7)
are in agreement with what was found from the Fe K
line fit using the Laor (1991)
model. We note
however that here a comparison is not straightforward as in Zycki & Czerny (1994) a
diskline from a Schwarzschild (rather than Kerr) black hole is
included.
The two reflection component cannot fit simultaneously the
broad band continuum, as the soft energy portion of the spectrum is
significantly underestimated. To reach an acceptable fit, this region
has to be modeled again by a modified black body and a single powerlaw
(Table 7).
Residuals mainly at the crossing points of the continuum components
determine the value of the reduced
(Table 7).
Therefore a broad band modeling only in terms of reflection is not
straightforward. In addition, hard X-rays coverage is not available and
this fit is based on the 0.3-10 keV
continuum shape and an iron line with two blended components. This
limited knowledge on the broad band behavior makes the fit parameters
very uncertain.
Table 7: Alternative fit of the continuum, including reflection.
4.2 The
contribution of the BLR to the Fe K
line
We have quantitatively evaluated the BLR contribution to the emission
of the prominent Fe K
line at 6.4 keV. The highly ionized specie of Fe XXVI
has a negligible role in the BLR, therefore the emission we see at
those
energies in the X-ray spectrum must come from some other location
(Sect. 4.4).
We based this analysis on previous results for the BLR structure and on
the present detection of the O VII
broad line in the
RGS spectrum. From the extensive experience of broad lines studies in
optical spectra of AGN, it is known that measurements of broad and
shallow emission features can be affected by large uncertainties and
should be cautiously treated. The limitation is due for instance to the
signal-to-noise ratio of the continuum level. Resolution, effective
area and calibration of the instrument are
also relevant effects. However, known, possibly not completely
calibrated, instrumental narrow features (23.05,
23.35 Å, de Vries
et al. 2003) fall at the border of the O VII
emission region. Observation-specific bad pixels (flagged as bad
channels in the spectrum) are automatically taken into account when
computing the instrument response, but they may certainly cause
additional uncertainty when fitting narrow features. The O VII
broad line is 1.4 Å wide and the determination of its flux is
not
significantly influenced by bad pixels. Considering also the very
significant, independent, detection in the Chandra-LETGS
data, we treat this feature as an intrinsically-low-flux O VII
line. The line flux measured in 2005 is indeed weaker than in 2003
(Sect. 2.2),
even though the SED of 2005 shows an increased availability of optical
and UV photons (Fig. 6).
Such a behavior of the line intensity is however not
unexpected, as the single line luminosities are also sensitive to the
long term flux history and the shape of the SED both in the UV and
X-ray band (at least of the previous two
months, given the typical size of the BLR). The X-ray continuum of
Mrk 279 has been observed to change in a month time-scale
(Sect. 3.2),
however the information on the light curve of Mrk 279 is
sparse, as the most
recent observation prior to the XMM-Newton pointing
has been done in 2003 (C07). Therefore a reconstruction of the SED
prior to the
present XMM-Newton observation cannot be performed.
In this study we find that only a small fraction of the broad
Fe K
line can be produced in the BLR. From the LOC modeling we find that the
Fe K
line luminosity, coming from the BLR, should be about six times smaller
than the O VII triplet. This
translates in a contribution to the Fe K
line which is about 30 times smaller than that observed, implying a 3%
contribution. However, the elemental abundance close to a
black hole, especially of iron,
might be different from Solar. Abundances up to 7 times Solar have been
suggested (e.g. Fabian
et al. 2002). The same BLR model,
but with the Fe abundance enhanced by a factor seven only reduces the
O VII/Fe K
luminosity ratio to about 4.5. However, such an overabundance implies
much stronger X-ray iron lines from the L-M shells, which are not
observed. If we artificially consider the luminosity of the
Fe K
as entirely produced in the BLR, we should observe an O VII
line with a luminosity as large as
erg s-1,
which is clearly inconsistent with both the present and
archival measurements (C07). Moreover the Ne IX
and O VIII broad lines (with
predicted luminosity of about
erg s-1)
would be clearly visible in the spectrum, against the observational
evidence.
The same discrepancy applies if we consider the very broad profile of
the Fe K
line as arising from the BLR
(Sect. 2.2).
In that case, the BLR contribution would be 23 times smaller than
observed. The unresolved component of the Fe K
would instead be
about 6 times smaller. If this were the case, the contribution of the
BLR to the Fe K
line would not be negligible
(
16%).
However, the FWHM of the core of the Fe K
line, as measured by Chandra-HETGS is
4200+3350-2950 km s-1
(Nandra 2006), which is
inconsistent with the FWHM of the UV lines (Gabel et al. 2005).
Moreover, BLR line are subjected to
significant variability both in the UV (e.g., Goad
& Koratkar 1998) and in the X-rays (Steenbrugge et al. 2009;
and present work), while we could not detect any significant change in
the iron line flux, measured in a few occasions by different
instruments (Sect. 3.2).
This also supports the idea that the BLR contribution should be
minimal. An important note is that the BLR parameters of
Mrk 279 were determined first using UV data only (see C07 and
Sect. 3.1)
and then applied to the X-ray data. With this approach, any X-ray line
only provides useful
upper limits, within the range imposed already by the CV
value, on the normalization of the LOC model
(Fig. 7).
Therefore, in principle, given a BLR model and an SED, the contribution
of the BLR to the Fe K
line can be calculated without the support of other X-ray lines. As a
test, we also predicted the iron line
luminosity considering the SED and BLR parameters derived for the 2003
data set. In that case,
reminding that we do not have any simultaneous measurement of the
Fe K
line in 2003, the contribution of the BLR to the
line would be roughly 17%.
Here for the first time we have tested the connection between
the optical BLR and the Fe K
line, using the intrinsic luminosity of the lines (rather than the FWHM)
and relying on a possible physical model for the BLR (the LOC model, Baldwin et al. 1995).
The idea of a non-relation between the BLR and
the Fe K
line has been already tested comparing the width of the optical broad
lines with the width of Fe K
line (e.g. Nandra
2006; Sulentic
et al. 1998). In at least one case, (the LINER
galaxy NGC 7213, Bianchi
et al. 2008) a simultaneous optical-X-ray
observation revealed the same width for the H
and Fe K
lines,
suggesting a common origin in the BLR. However the UV and X-ray lines
might come in regions closer to the black
hole with respect to the lower ionization lines in the optical,
implying a larger FWHM. Moreover NGC 7213
can be considered an outsider object, as has been proven to totally
lack the reflection component
(Bianchi et al. 2003).
Therefore a
direct comparison with classical Seyfert 1 galaxies (as is
Mrk 279) may not be possible.
4.3 The origin of the Fe K
line
Any variability of the iron line would easily suggest a close
interaction between the central source and the region producing the
iron line, e.g. the accretion disk. The line does not need to be
relativistically smeared, as also Gaussian-shaped lines may arise from
the accretion disk (Yaqoob
et al. 2003). Using ASCA data Weaver
et al. (2001) found variability in the line centroid
at the 1.6
level (90% confidence).
The variability was observed on a time scale of about 20 ks
following a 2-10 keV central-flux variation of
15%. The
average EW and FWHM as measured by ASCA were
however consistent with the present XMM-Newton data.
Over a time scale of at least 3 years, the Fe K
line are, in first approximation, stable in luminosity, width and
centroid energy within the errors (Table 6). To which
extent the line is stable is difficult to assess, as some measurements
are
affected by large uncertainties. Formally, the line could have changed
of as much as a factor of two or more in luminosity. In a scenario
where a dominant component of the line does not respond to the central
source variation, an origin of the iron line in the accretion disk can
be justified by light-bending (e.g. Gallo et al. 2007; Fabian &
Vaughan 2003). In that case the apparent flux is allowed to
change even of a large factor (up to four) while the line-flux change
is marginal (Miniutti
et al. 2003). A number of factors may influence the
iron line behavior. The limited knowledge of the exact nature of the
emission components (both Fe K
line and continuum) in this source does not allow us to make
predictions on the variation
of the line in response to the continuum changes on either short or
long time scales.
A symmetric line profile, possibly constant over a long time
scale, may also suggest a
relatively unperturbed environment, like for example the molecular
torus, 1 pc
away from the central source. In the
framework of the unification model (e.g. Antonucci
1993), the iron line is a natural consequence of the
obscuring torus.
In type 1 objects, the expected equivalent width would be of
the order of 100 eV. However geometrical effects would easily
reduce it of a factor of two (Krolik & Kallman 1987; Nandra
et al. 2007). In the case of Mrk 279, the
EW of the narrow iron line is
eV, in agreement
with the
theoretical prediction. The presence of Fe K
,
insures that the ionization state of iron is less than Fe XVII
(Yaqoob et al. 2007).
A further constraint on the ionization state of iron would be based on
the Fe K
/Fe K
branching
ratio, which ranges from 0.12 for Fe I
up to 0.17 for Fe IX (Palmeri et al. 2003).
Unfortunately, considering the associated errors on the observed line
fluxes, we only obtain a lower limit of 0.1 to that ratio.
4.4 The nature of the Fe XXVI line
He-like, sometimes associated with H-like, iron lines have been
detected in a number of AGN spectra. These lines can be associated to
high column density, photoionized gas (Bianchi et al. 2005; Longinotti
et al. 2007, and references therein). The spectrum
of Mrk 279 does exhibit a narrow emission line at 6.9 keV,
consistent with emission from Fe XXVI,
but interestingly Fe XXV is not
detected, showing the presence of extremely highly ionized gas.
Interpreting this emission as a product of photoionization, we find
that a high column-density (
cm-2)
with a very-high ionization parameter (log
)
is necessary. A range of
and log
and covering factors is allowed to explain the Fe XXVI
emission and at the same time be
consistent with the measured upper limits on the other high ionization
lines: Fe XXV, Ne X
and O VIII (see
Sect. 3.3
and Fig. 9).
The covering factor is on average 0.65. As we do not see any associated
absorption, this gas must be
out of our line of sight. The EW of the Fe XXVI
line (
eV)
is consistent with an origin in photoionized circumnuclear gas at the
distance of the obscuring torus (Bianchi
& Matt 2002). The torus
itself would produce the bulk of the neutral-weakly ionized iron
emission, while a highly
ionized outer layer of photoionized gas would be responsible for H-like
and He-like iron. In our case the gas is
so ionized to
suppress even Fe XXV.
Another possibility is that the Fe XXVI
line is formed in a hot galactic wind, driven by starburst activity
(Chevalier & Clegg 1985).
However, a very hot (log -10 keV) plasma would
be needed to produce the observed Fe XXVI
line
luminosity via collisional ionization. Such high temperatures have been
proposed for example for the diffuse
emission of the Galactic ridge (e.g. Tanaka
2002). However, this interpretation is problematic with
respect to the confinement of the plasma within the host galaxy, the
source of such heat (e.g. Masai
et al. 2002) and the
contamination by point-like, hard sources in the diffuse emission
spectrum (e.g. in NGC 253, Strickland et al. 2002).
Another way to model the high ionization spectrum in terms of a
galactic wind, is to invoke quasi-thermal
acceleration of electrons which then would produce X-rays with thermal
photons. Iron lines would be produced by
recombination of background iron ions recapturing electrons, as
proposed by Masai et al.
(2002) for the Galactic ridge
emission.
5 Conclusions
We have performed a detailed modeling of the spectrum of
Mrk 279, observed for 160 ks by XMM-Newton.
Thanks to the broad band coverage we had the opportunity to study in
detail
both the low-energy emission lines (observed by RGS) and the iron K
complex at around 6.4 keV (observed by Epic).
We have extended the ``locally optimally emitting cloud''
model, which has been conceived to model the UV broad lines coming
from the BLR (Baldwin et al.
1995), first
to the soft X-ray band (C07) and, in the present paper, to the
Fe K
line. The BLR can easily account for
the O VII broad emission (C07),
while it contributes marginally to
the luminosity of the broad iron line at 6.4 keV (about 3%,
this work). This is the first attempt to evaluate the BLR contribution
to the luminosity of the iron line using a global modeling (rather than
relying on the line FWHM only). Further
investigation on high quality data of similar sources are of course
necessary to test the robustness of our results.
The bulk of the Fe K
narrow emission line shows no remarkable variability, in flux and shape
over at least three years (Sect. 3.2) and it may be
produced by reflection from distant and cold matter. The EW of the line
(
70 eV)
is also consistent with this picture.
The very broad component of the Fe K
line may be modeled by a line arising from an accretion disk,
associated with relativistically smeared reflection. However, a formal
fit using broad-band reflection models is affected by large
uncertainties and degeneracy of the
parameters, as we cannot access
the region above 10 keV and we only have to rely on the
Fe K
profile, which is in turn formed by two line-components.
We also detected emission from Fe XXVI,
but not from Fe XXV. This implies
an extremely highly ionized medium. Using the
available constraints on other highly ionized ions (e.g. Ne X,
O VIII and N VII)
we could limit the column density of such gas
cm-2
with an increasingly higher ionization parameter
.
The
covering factor of the gas is 0.65 on average. Such a
gas is predicted by the AGN unification model, and it could be
associated with a highly ionized outer layer of the obscuring torus
(e.g. Krolik & Kallman 1987).
The Space Research Organization of The Netherlands is supported financially by NWO, The Netherlands Organization for Scientific Research.
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Footnotes
- ...
SPEX
- http://www.sron.nl/divisions/hea/spex/version2.0/release/
- ... 2 keV
- Several examples are provided at http://xmm2.esac.esa.int/cgi-bin/ept/preview.pl
- ... absorber
- This is different from the global covering factor, the fraction of emission intercepted by the absorber averaged over all lines of sight, which is has been estimated to be about 0.5 (e.g. Crenshaw et al. 2003).
- ... scale
- The parameter s is a scaling factor for the reflected
luminosity:
. For an isotropic source above the disk s=1, corresponding to an equal contribution from direct and reflected spectrum.
All Tables
Table 1: XMM- Newton observation log.
Table 2: Best-fit continuum parameters for a phenomenological model.
Table 3: List of line luminosities as measured by Epic-PN (inst. 1) and RGS (inst. 2).
Table 4:
Parameters for the very broad component of the Fe K
line in a two-components model.
Table 5: Best fit parameters for the absorbed spectrum as measured by RGS and Epic-PN.
Table 6:
Parameters of the Fe K
at different epochs.
Table 7: Alternative fit of the continuum, including reflection.
All Figures
![]() |
Figure 1: Epic-PN light curves in the 0.3-2 and 2-10 keV band. The maximal variation is about 36%. |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Best fit for the Epic-PN data. The model includes a modified black body
(dotted line), a broken powerlaw (dashed
line), both absorbed by ionized gas, a
broad Fe K |
Open with DEXTER | |
In the text |
![]() |
Figure 3: RGS spectrum in the O VII region in terms of ratio from a continuum fit. For display purpose, only the rebinned data from orbit 1087 and 1088 are shown. The measurements reported in the paper are based on the full RGS exposure. The labels indicate some of the main features of the warm absorber. |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Upper panel: residuals, in terms of |
Open with DEXTER | |
In the text |
![]() |
Figure 5:
Spectrum and best fit in the iron line region as observed by Epic-PN.
The solid lines display the broad and narrow
components added to fit the emission features: a prominent Fe K |
Open with DEXTER | |
In the text |
![]() |
Figure 6: The SED of Mrk 279 in 2005 measured by XMM- Newton (solid line). The optical data are measured by OM, while the broad band X-ray continuum is measured by Epic-PN. The dotted line shows the SED of the 2003 data for comparison (C07). |
Open with DEXTER | |
In the text |
![]() |
Figure 7:
The solid thick line shows the predicted BLR line luminosities in the
X-ray band in units of 1040 erg s-1
using the LOC model, using as constraints
|
Open with DEXTER | |
In the text |
![]() |
Figure 8: Emission line contribution to the high ionization lines (Fe XXVI, Fe XXV, Ne X, O VIII). Triangles: data. Light shaded line: range of models viable to fit the data. |
Open with DEXTER | |
In the text |
![]() |
Figure 9: Parameter space of parameters viable to produce the observed high-ionization emission spectrum. Upper panel: Log of the ionization parameter as a function of the log of the column density. Lower panel: covering factor of the Fe XXVI line as a function of the log of the column density. |
Open with DEXTER | |
In the text |
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