N. Schartel 1 - P. M. Rodríguez-Pascual 1 - M. Santos-Lleó 1 - J. Clavel 2 - M. Guainazzi 1 - E. Jiménez-Bailón 1 - E. Piconcelli 1
1 - XMM-Newton Science Operations Centre, ESA, RSSD, ESAC, Apartado 50727,
28020 Madrid, Spain
2 -
Astrophysics Mission Division, ESA, SCT-SA, ESTEC, Postbus 299,
2200 AG - Noordwijk, The Netherlands
Received 5 August 2004 / Accepted 17 November 2004
Abstract
We present the analysis of XMM-Newton observations of
three X-ray weak quasars: PG 1001+054, PG 1535+547 and PG 2112+059.
All objects are absorbed by ionized material showing high column
densities,
to
,
and ionization parameters,
to
.
The spectra of PG 1535+547 require an additional
partial covering by neutral material with a column density
of
at a covering factor of
0.96.
The spectra of PG 1535+547 show systematic residuals in
the energy range from 4 keV to
6 keV,
which are inconsistent with K
-fluorescence-emission
of neutral or ionized iron under the assumption
of a Gaussian line profile.
They can be described with a relativistic
disk line (Laor) and establish therefore the second
X-ray weak quasar with such a spectral characteristic.
Our results together with the findings of Brinkmann et al. (2004, A&A, 414, 107)
and Piconcelli et al. (2004a, MNRAS, 351, 161),
indicate that warm absorbers characterized by high column
densities and ionization parameters are
typical of X-ray weak quasars.
The occurrence of a variable relativistic broad
Fe K
fluorescence
line in two out of the five well studied X-ray weak quasars
might indicate a second general characteristic of the entire
object class.
Based on observations obtained with XMM-Newton, an ESA science
mission with instruments and contributions directly funded by
ESA Member States and NASA.
Key words: X-ray: galaxies - quasars: individual: PG 1001+054 - quasars: individual: PG 1535+547 - quasars: individual: PG 2112+059
The average dependency of the ratio of X-ray luminosity to optical luminosity as a function of redshift and optical luminosity for quasars was established by Avni & Tananbaum (1982). About two years later it was becoming evident that the ratio of X-ray to optical luminosity can be very low, e.g. PKS 1004+13 (Elvis & Fabbiano 1984). In 1997 the population of such quasars could be better constrained as it was discovered that the X-ray emission in about 10% of quasars is fainter, by a factor 10-30, than expected on the basis of their luminosity at other wavelengths. These quasars form a distinct class, and are called "X-ray weak'' or "soft X-ray weak'' quasars.
The population of X-ray weak quasars was constrained
by Laor et al. (1997) based on
ROSAT observations of a complete sub-sample
of the Bright Quasar Survey (Green 1976; Schmidt & Green 1983),
characterized by low redshift, ,
and low
Galactic equivalent column density,
.
Three out of the 23 studied quasars showed an X-ray
flux which was significantly lower, by a factor 10-30, than expected
on the basis of their emission at other wavelengths.
A bimodal distribution of the X-ray luminosity with respect
to optical measurements was also found in a sample of AGNs
studied by Wang et al. (1996), where
6 out of the 86 analyzed AGN show an
(Laor et al. 1997).
As the ROSAT spectra of X-ray weak quasars
do not show a distinctive spectral slope,
Laor et al. (1997)
discussed two possible scenarios for the origin
of their X-ray weakness:
either that for unknown reasons the X-ray emission
mechanism of quasars is intrinsically bimodal
or that the direct X-ray flux is obscured
and only scattered X-ray flux can be observed.
In this context, it is interesting to note that
X-ray absorption by partially ionized gas ("warm absorber'')
was detected in only
of the whole
population in the Laor sample in sharp contrast
with lower luminosity AGNs, where it is much more
frequent (
Reynolds 1997).
Recent studies (Porquet et al. 2004;
Piconcelli et al. 2004b) indicate that the
fraction of quasars showing warm absorber
is comparable with the fraction found for
lower luminosity AGNs.
Table 1: PG-name, redshift and Galactic column density. The redshifts are taken from Véron-Cetty & Véron (2000).
Based on archival ROSAT, ASCA and Einstein
observations of the 87 quasars in the
Boroson & Green (1992)
sample, Brandt et al. (2000)
searched
systematically for X-ray weak quasars.
They found that the distribution of
provides a
clear separation between X-ray weak and
"normal'' quasars, where the former ones fulfill
,
which holds for
11%
of the entire sample.
X-ray weak quasars show a strong correlation between
and the C IV absorption equivalent width,
which suggests that absorption is the primary cause of
their soft X-ray-weakness.
This correlation reveals a continuum of
absorption properties connecting unabsorbed quasars,
X-ray warm absorber quasars, soft X-ray weak
quasars and Broad Absorption Line (BAL) quasars.
However, soft X-ray weak quasars exhibit distinctive
properties.
They are characterized by
low [O III] luminosity and equivalent widths.
Most of them are located toward the weak
[O III] end of the Boroson & Green eigenvector 1,
likewise many BAL quasars.
As it has been suggested for AGN with similar eigenvectors,
it is possible that X-ray weak quasars
are characterized by extreme physical parameters
in the nuclear region, like a high mass accretion
rate relative to the Eddington limit (
).
We report here on XMM-Newton observations of three X-ray weak quasars performed in 2002 and 2003 (Table 1). These observations together with our approved observing program for 2004, observations performed in the guaranteed program of XMM-Newton and an observing program of Brinkmann et al. (2004), will provide XMM-Newton observations of the entire sample of X-ray weak quasars established by Brandt et al. (2000).
The paper is organized as follows. In Sect. 2 we describe optical characteristics and previous X-ray studies of the three quasars. The XMM-Newton observations and their data reduction are described in Sect. 3. In Sect. 4 the X-ray spectral analysis of each quasar is provided. The spectral energy distribution is discussed in Sect. 5. Finally, in Sect. 6 we compare our findings with the previous results and infer some constraints on the regime of soft X-ray weak quasars from our results.
Table 2: Exposure details: (1) observation ID; (2) exposure number used in Table 3.
The quasar has the lowest
value in the
sample studied by Wang et al. (1996) and in the
sample studied by Laor et al. (1997).
Based on a comparison between the ROSAT X-ray observations
and the UV absorption and emission lines,
Wang et al. (2000) concluded that the observed UV line
depth is much lower than expected from the
X-ray absorbing column density.
PG 2112+059 was detected with ROSAT which is generally
unusual for BAL quasars (Green & Mathur 1996).
The number of counts accumulated during the ROSAT observations
was enough to allow an estimate of the
absorbing column
density of
,
for
(Wang et al. 1996).
An ASCA observation from October 1999 measured
a photon index of
if absorption was
fixed at the Galactic equivalent column density
(Gallagher et al. 2001), which is rather flat
for a typical quasar.
In addition the fit was rather poor, i.e.
it could be rejected at a 96% confidence level.
From the statistical point of view the data can
equally well be described with a power law absorbed
by neutral material, partial covering or with an
ionization edge (Gallagher et al. 2001).
A comparison between the low energy ASCA data (<1 keV)
with an extrapolated power-law model that was fitted
on the spectrum above 2 keV
shows that the spectrum recovers toward the lowest
energy bins.
As neutral cold absorption would depress all
flux below
3 keV, the comparison speaks
against a cold neutral absorber favoring either
partial covering or ionized absorption.
Comparing the ASCA observation with ROSAT
data Gallagher et al. (2004)
found flux variability of almost a factor of four.
An even larger flux and spectral variability was found
with respect to the Chandra observation of PG 2112+059 from
September 2002 (Gallagher et al. 2004), where
the continuum was a factor 3.5
lower than in the ASCA observation.
The Chandra spectrum can be modeled by a power law
folded by a warm absorber as well as by a power
law folded by partially covering neutral absorption
with equal statistical significance.
The spectral variability was interpreted as evidence
for an increase of the absorbing column density by Gallagher et al. (2004),
as the two observations cannot be modeled with an
absorber of constant column density.
The Chandra data statistically require a broad iron K
fluorescence emission line, in contrast to the
1999 ASCA data where the line could be excluded at
a very high confidence level.
PG 1535+547 shows a narrow Balmer line with
FWHM
and
therefore satisfies the criterion to be a NLQ
(Véron-Cetty et al. 2001; Boroson & Green 1992).
The optical Fe II emission is relatively strong
(Phillips 1978), which
supports the quasar's classification
as the optical Fe II emission in general
anti-correlates with the Balmer line FWHM.
PG 1535+547 is the X-ray weakest quasar detected in the Boroson & Green (1992) AGN sample by Brandt et al. (2000).
A power law fit to the ASCA spectrum with
absorption fixed at the Galactic value plus
and additional intrinsic absorption component
resulted in a photon index of
,
which is abnormally flat for a typical type 1 AGN
(Gallagher et al. 2001).
A statistically better fit was obtained by the
same authors with a partial
covering model.
The best fit model parameters were: an intrinsic
neutral absorption column density of
with covering factor of
0.91+0.07-0.26 and
.
For comparison, a fit with an ionized absorber left
clear systematical residuals and was characterized
by
for the same number of
degrees of freedom.
We used the standard Science Analysis System (SAS) v.5.4.1 (linux version, Loiseau 2004), and recent calibration files (November/December 2003) for the processing and extraction of the data.
The OM observations were processed with the OMICHAIN routine of SAS which applied the required astrometric and photometric corrections. The raw pn and MOS data were processed with the EPCHAIN and EMCHAIN tasks provided by SAS in order to generate the relative linearized event files. For pn only events with patterns 0-4, which characterize single and double events, were selected. For MOS events with patterns 0-12 were chosen, which correspond to single, double, triple and quadruple events. The applied pattern selection ensures a good energy calibration, and allows the usage of standard detector response and effective area calculation.
The screening for time ranges with low radiation background level was done following the method described by Piconcelli et al. (2004a). The source extraction area, the energy range used for the screening and the corresponding rejection thresholds, the background extraction area, the net count rate for the source and the finally utilized accumulated exposure times for each exposure are shown in Table 3.
All source events were extracted within a circular region centered at the peak of the emission, which was determined by eye. The details are provided in Table 3. The background was determined from source-free regions nearby the source. For all MOS exposures the background was determined from an annulus around the extraction area of the source counts, whereas an offset circular region was chosen for the pn background extraction. The background for each exposure was determined from counts collected at the same CCD in order to minimize calibration uncertainties. In addition the center of the circular background extraction region of the pn data was selected to have similar CCD y-coordinates which minimizes the charge transfer efficiency dependency on the detector coordinates. The locations are specified in the Table 3.
The detector response and the effective area were generated with the SAS tasks RMFGEN and ARFGEN, respectively.
Table 3: Source extraction and low background screening process: (1) an annulus with an inner radius of 1' and an outer radius 5' centered on the source position was used as reference area for the screening. The data were binned with 10 s; (2) coordinates in J2000; (3) extraction radius for the source; (4) energy range considered for the screening; (5) rejection threshold for screening; (6) for annular background areas the inner and the outer radius are provided. (7) background corrected count rate in the energy range of 0.3-12.0 keV for pn and 0.5-10.0 keV for MOS, respectively; (8) accumulated exposure time.
Following the current calibration recommendation (Kirsch 2003), we restricted the analysis of the MOS spectra to the energy range from 0.5 keV to 10.0 keV and of pn spectra to the 0.3 to 12.0 keV energy range, respectively. Due to the low source count rates found in the MOS observations, we added the MOS1 and MOS2 spectra of each source and determined the corresponding mean detector response and auxiliary files.
The source spectra were binned to ensure a constant signal-to-noise
ratio for each bin,
restricting to a maximum of three bins per energy
resolution element.
With this choice for the binning over-sampling is avoided
and the Gaussian distribution is approximately reached
which authorizes the use of statistics.
The spectral analysis was performed with XSPEC 11.2.0
(linux version, Arnaud 1996).
For the estimates of the spectral parameters the modified
minimum
method is used (Kendall & Stuart 1973),
which requires the applied binning with constant
signal-to-noise in order to avoid biasing effects
due to the intrinsic correlation between the accumulated
counts and their error.
The errors of estimated parameters are given
for the 90% confidence region for a single interesting
parameter (
,
Avni 1976).
The various absorption components were modeled assuming cosmic abundances according to Anders & Grevesse (1989), and photoelectric absorption cross sections as provided by Balucinska-Church & McCammon (1992), with the exception for helium, for which the cross sections were taken from Yan et al. (1998).
All fits were performed assuming Galactic foreground absorption. The Galactic column densities were fixed at the values inferred from 21 cm observations. They are provided in Table 1 together with the reference to the corresponding publications. The intrinsic absorption was always fitted with the redshift fixed at the value of the source as provided in Table 1.
Table 4:
Spectral fits of the EPIC spectra of the quasars.
The errors for the 90% confidence level are provided if
.
Fits labeled with
have
and consequently
no errors were determined.
Fits labeled with
were performed over the 2-10 keV energy range for MOS and the
2-12 keV energy range of pn, respectively.
Explanation of labels: (1) explanation of the used models:
pl: power-law (XSPEC: pow),
nab: neutral absorber at the redshift of the source (XSPEC: zphabs),
pab: partial covering with neutral absorber at the redshift of the source (XSPEC: zpcfabs),
iab: ionized absorption at the redshift of the source (XSPEC: absori),
ga: Gaussian line at the redshift of the source (XSPEC: zgauss),
la: relativistic disk line after Laor (XSPEC: laor, Laor et al. 1997);
Parameters: (a) equivalent column density; (b) covering fraction;
(c) ionization level of absorbing material
,
as defined in XSPEC;
(d) photon flux at
;
(e) degrees of freedom.
For a neutral absorber, an absorbed power-law model and a
partially covered source model can both be
excluded from the resulting poor
statistics of 156.7 (
)
and 71.1 (
), respectively.
We fitted the absorption through ionized material as
represented by the XSPEC model absori (Done et al. 1992).
For this fit we forced the absori photon index
to be equal to the (free) continuum
power law index.
The temperature of the absorbing material was fixed at
and its redshift was fixed at
the redshift of the source.
In addition the iron abundance was pegged to the
value in Anders & Grevesse (1989), as described above.
This fit of a power law absorbed by ionized material, i.e. a warm
absorber, lead to an improvement of
with respect to the partial covering model and
provides an acceptable
parameterization of the spectra.
Despite the relative large
(
), the low signal
to noise ratio of the data does not warrant a more
complex model.
The estimated parameters and the statistical results
are provided in Table 4.
Contour plots of column density versus ionization
parameter are given in Fig. 1.
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Figure 1: PG 1001+054: the confidence levels to 68.3%, 90% and 99% for two interesting parameters: column density and ionization parameter are shown. |
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The pn and the co-added MOS spectra were binned with a signal-to-noise ratio of 6, which provides 45 channels in total.
The values of
and the degrees of freedom
for all fits are provided in Table 4.
A power law model absorbed by intrinsic neutral material
can be ruled out
(
and
).
Although a partial covering model decreases
the value of
,
it can still be ruled out at the 99% confidence.
The existence of systematic residuals also
argue strongly against both models.
The X-ray spectra can be well described by a power law absorbed by ionized material. The choice of fixed and free parameters was the same as for PG 1001+054. The best fit parameters are provided in Table 4. The significance contours for the absorbing column density versus the ionization parameter are shown in Fig. 2.
In order to allow an investigation of the
variability with respect to previous Chandra and ASCA
observations (Gallagher et al. 2004) we determined the
flux for the energy bands 0.5-2.0 keV and
2.0-7.0 keV to be
and
,
respectively.
The spectra of the XMM-Newton observation do
not show residuals around the energy range
of the Fe K
fluorescence emission line.
But it has to be considered that the individual
energy bins cover a wide energy range due to the
low statistics at higher energies.
Therefore, we added a redshifted Gaussian emission
line with centroid energy and width fixed at the
values found by Gallagher et al. (2004):
and
.
The resulting fit leads to an improvement of
corresponding to an
F-statistics probability of 97.91% and as such
does not allow to claim a detection of the line.
The best fitted line emissivity,
is
corresponding to an equivalent width,
.
Table 5:
Spectral fits of the emission line of PG 1535+547.
The corresponding continuum fits are provided in Table 4.
Fits labeled with
were performed over the 2-10 keV energy range for MOS and the
2-12 keV energy range of pn, respectively.
(1) The following two models are used:
ga: Gaussian line at redshift of source (XSPEC: zgauss),
la: relativistic disk line after Laor (XSPEC: laor, Laor 1991);
Parameters:
(a) energy of line in rest frame of quasar;
(b) line width (
);
(c) total photons in line;
(d) inclination angle
![]() |
Figure 2: PG 2112+059: the confidence levels to 68.3%, 90% and 99% for two interesting parameters: column density and ionization parameter are shown. |
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A power law continuum absorbed by neutral material
at the redshift of the source,
a continuum absorbed by neutral material partially
covering the source, as well as a continuum
absorbed by ionized material
can be excluded according to the
obtained
values with respect to the degrees
of freedom.
The choice of fixed and free parameters was the
same as for PG 1001+054.
The best fit parameters, the
corresponding
-values and degrees
of freedom are provided in Table 4.
A model assuming a continuum absorbed by
ionized material and partially covered by a neutral
absorber at the same time leads to a fit
acceptable at the 95% confidence level
(fit 10 in Table 4).
The best-fit model, the data points and the
resulting residuals are
shown in Fig. 3.
The large excess between 4 keV and 6 keV
indicates the presence of a fluorescence iron emission
line.
Adding a red-shifted Gaussian emission line
(XSPEC model zgauss) to the applied model improves the
obtained value by
.
In Fig. 4 the 68.3%, 90% and 99%
confidence level for two interesting parameters,
line energy in the rest-frame of PG 1535+547 and
line width, are shown.
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Figure 3:
PG 1535+547: the data compared with
the best fitted model for a power law continuum absorbed
by ionized material and partially covered by neutral
material (pn: filled circles and MOS filled triangles).
The best fit parameters are given in Table 4.
Systematic residuals are appearing between 4 and
6 keV, i.e. the energy range of the fluorescence
Fe K![]() |
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A satisfactory description of the data is provided if the
fluorescence iron emission is described with a relativistically
broadened line profile emitted in an accretion disk
around a Kerr black hole (model laor in XSPEC, Laor 1991).
The line energy was fixed at the energy of the
neutral Fe K
transition, i.e. 6.4 keV
in the rest frame of the quasar, which is 6.166 keV
in the observed frame.
The emissivity index
(
)
was fixed at
,
whereas the inner and the outer radius,
the inclination angle and the normalization were allowed to
vary.
The best fit model is plotted in Fig. 5,
together with the data and the residuals.
The model parameters are listed in Table 4.
The resulting
improves by
with respect to the model without
emission line and by
relative
to the model with
Gaussian profile, significant at the 99.99%
and 99.89% level, respectively.
As shown in Fig. 6 the column density of
the warm absorber is correlated with the column density
of the partially covering neutral material.
For the ionized absorber the confidence levels of 68.3%, 90% and 99% for two interesting parameters,
column density and ionization parameter, are shown
in Fig. 7.
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Figure 4:
PG 1535+547: the confidence levels to 68.3%, 90% and 99% for the two interesting parameters of a Gaussian line,
line energy and and line width, are shown. Both
are in the rest-frame of PG 1535+547.
The line energy is not compatible with an emission
of neutral iron at
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Figure 5:
PG 1535+547: the data compared with
the best fitted model for a power-law continuum absorbed
by ionized material and partially covered by neutral
material. The fluorescence Fe K![]() |
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Figure 6: PG 1535+547: the confidence levels to 68.3%, 90% and 99% for the two interesting parameters: column density of ionized absorber and column density of neutral absorber are shown. |
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Figure 7: PG 1535+547: the confidence levels to 68.3%, 90% and 99% for the two interesting parameters, column density of the ionized absorber and ionization parameter, are shown. |
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To investigate the nature of the central black hole we calculated confidence contours for the inner and the outer radius of the iron line emission area of the accretion disk. The 68.3%, 90% and 99% confidence level contours are shown in Fig. 8. Unfortunately, these results do not allow to distinguish between a non-rotating (Schwarzschild) and a rotating (Kerr) black hole.
Contrary to what is often found in quasars, the
PG 1535+547 data do not require soft X-ray excess
emission above the power law.
The count rate is low below 2 keV and does not
warrant such a complex model.
It is important however to check that the
relativistic iron K line is not an
artifact resulting from an improper placement
of the power law at low energies combined with the
introduction of an ionized instead of a
neutral absorber.
To test this hypothesis we repeated the fits described
above but restricting the spectral analysis to the
energy range above 2.0 keV.
The best fit parameters are listed in Table 4
and fits are labeled with .
As expected, the statistical significance is lower than
the significance obtained for the spectra covering the
whole energy range and even a description without
Fe K
fluorescence line emission becomes
acceptable.
However, the Gaussian line leads again to a line energy
which is not in agreement with the redshift of
PG 1535+547 and therefore must be excluded
on the basis of physical arguments.
Compared with the case without a line,
the addition of a
relativistic emission profile improves the fit
by
,
which is significant
at the 98.8% confidence level.
PG 1001+054 was detected in the first visible grism exposure
at an average flux of
Å-1 in the
3500-4500 Å band and in the second exposure at an
average flux of
Å-1,
respectively.
In both UV grism observations the source could not be found.
PG 2112+059 was observed with an average flux of
Å-1 in the
3500-4500 Å band visible grism observation.
Again, the source was not detected in the UV grism observations.
Both PG 1001+054 and PG 2112+059 had been observed with HST in
the UV-optical domain.
In order to have a snapshot of the optical-UV-X rays spectral
energy distribution of these objects during their XMM-Newton
observations we use the fluxes obtained with OM to
scale the high resolution high S/N UV-optical spectra.
The scaling factors are 1.4 for PG 1001+054 and 1.25 for PG 2112+059;
the latter is more uncertain given the small wavelength overlap
between HST and OM spectrum.
The scaling factor used for PG 1001+054 corresponds to its
brightness level in 1987 and 2002 and the scaling factor used
for PG 2112+059 corresponds to its brightness level in 1993 and 1995,
respectively.
It is remarkable that OM reveals a relatively strong
feature at 4060 Å, consistent with MgII
2800 emission.
As the quality of the HST spectra of PG 1535+547 is rather low,
we have used its IUE spectrum instead.
Figure 9 shows the observed UV spectra of the three quasars. The similarity of the Galactic absorption features is remarkable in spite of the largely different HI column density in PG 1001+054 and PG 2112+059. The low resolution, low signal-to-noise ratio of the IUE spectrum of PG 1535+547 does not show clearly identifiable absorption features, except for galactic MgII.
In order to compare the intrinsic spectra of the three sources,
the UV spectra were brought in the rest-frame of the
quasars.
In addition, the PG 2112+059 spectrum has been corrected for
galactic reddening using the Cardelli et al. (1989)
parametrization and assuming
E(B-V)=0.15.
For the galactic reddening we have used the NH/E(B-V) relation
given by Dickey & Lockman (1990).
After this correction, the overall UV spectral shapes of
PG 1001+054 and PG 2112+059 are very similar, but PG 1535+547
shows an opposite trend: flux increasing with wavelength.
A possibility to reverse the spectral shape of PG 1535+547 is to
apply some reddening correction.
However, the required color excess is too large for the galactic
HI column density and for the absence of a significant
2200 Å feature (Smith et al. 1997).
Therefore, if reddening is causing the atypical UV spectral shape
in PG 1535+547, a galactic origin is very unlikely.
In Fig. 10 and for the following analysis and modeling,
we have applied the correction for the rest wavelength using
the parametrization from Calzetti et al. (2000)
with
E(B-V)=0.35.
The applied intrinsic redding was determined
by iteration of the E(B-V) value until the UV spectrum was
becoming similar to the UV spectra of the other two quasars.
This intrinsic reddening might be associated with the neutral absorbing
material suggested by the X-rays data fits.
If this is the case, then
ratio and,
hence, the gas-to-dust ratio would be
10 times larger than typical galactic values, as already
found in other quasars (Alonso-Herrero et al. 2001).
In their analysis of the polarization spectrum, Smith et al. (1997)
conclude that dust is present in the BLR of this source,
scattering/reddening the continuum and emission lines, but only
partially; this result is in qualitative agreement with the partially
covering neutral absorber found in the X-ray spectra.
The optical-UV-X-ray spectral energy distribution, SED, of the
three sources is plotted in Fig. 11.
Taking these SEDs into account, we have run
CLOUDY models in order to match both the SED
and the warm absorber parameters obtained from the X-ray spectral
analysis.
The incident continuum on the gas has been defined as
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The model parameters are listed in Table 6 and the
results shown in Fig. 11.
We remark that the model parameters are not the
result of a fit to the SED; they have been chosen to match
the results of the X-ray spectral analysis and tuned to
give a reasonable representation of the SED.
As can be seen in Table 6, the parameter
of the intrinsic, incident continuum is
well above the value -2.0.
Our main purpose for running CLOUDY has been to investigate
if the highly ionized gas should be expected to imprint line
emission/absorption features into the spectrum.
In the plot of the outward emission predicted by CLOUDY
(Fig. 11) we have assumed rather narrow line
profiles (
)
which result in strongly peaked spectra.
The purpose is to show that some emission features are indeed
expected in the X-ray range, but no significant emission line
is predicted in the UV.
According to the results of the CLOUDY models described here, it
is rather difficult to get significant absorption features in
the UV since the column densities of the ionic species producing
UV absorptions are rather low (with the possible exceptions of
NV1240 Å and OVI
1033 Å): only for
Ar and heavier elements a significant fraction of their atoms can
retain more than 1-2 electrons.
The discussion is divided in four parts.
In the first one we discuss the X-ray
absorption characteristics of the three X-ray weak quasars.
The SED is discussed in the second part, the variability of
PG 2112-059 in the third one and
the Fe K
fluorescence emission line of PG 1535+547
is the subject of the fourth part.
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Figure 8:
PG 1535+547: confidence levels
to 68.3% (green), 90% (red) and 99% (black) for the two interesting
parameters: inner and outer radius, which characterize
the area of the accretion disk emitting fluorescence
Fe K![]() |
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Figure 9: The observed UV spectra of PG 1001+054, PG 2112+059 and PG 1535+547 show some common absorption features due to mildly ionized material in our Galaxy, but it is clear that the overall UV spectral shapes are widely different. |
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All three studied quasars are characterized by absorbers with high column densities and high ionization parameters. A neutral absorber in addition to the warm absorber is found only in the spectra of PG 1535+547. These findings are in agreement with the results obtained by Brinkmann et al. (2004) and Piconcelli et al. (2004a), who found strong absorption by ionized material in two X-ray weak quasars, namely PG 1411+442 and Mrk 304. The large effective area and improved spectral resolution of XMM-Newton for the first time allow to discriminate between competing absorption scenarios:
An ASCA observation from June 1999 could be described by either a warm absorber or a partial covering model; the latter allowing a slightly better statistical description of the data. Given the very large effective area of XMM-Newton in the soft X-ray region in comparison with ASCA and the fact that our description requires a partial covering of a neutral absorber in addition to a warm absorber, our results are in agreement with the findings of Gallagher et al. (2001).
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Figure 10: Only when PG 2112+059 is corrected for reddening in our Galaxy and PG 1535+547 is corrected for intrinsic reddening the spectra at rest frame of the three objects become very similar. |
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Figure 11: Spectral energy distribution for PG 1001+054 , PG 2112+059 and PG 1535+547. The OM observations (red error bars) have been used to re-scale UV spectra (IUE and HST) for PG 1001+054 and PG 2112+059. Unfortunately, for PG 1535+547 the are no OM data available to assure the proper scaling of the optical-UV-X-ray ranges. Optical-UV have been corrected for galactic reddening and X-ray corrected for galactic absorption. Input ionizing spectra in Cloudy models are drawn as dashed lines and output (transmitted and diffuse emission) as solid lines, both in the same color as object data. |
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Table 6:
CLOUDY models for the three quasars: (1)
where Q(H) is the number of
hydrogen ionizing photons, r is the distance of the gas to the
continuum source, n is the gas number density and c the speed of light.
After correcting for intrinsic absorption in PG 1535+547,
it is remarkable that roughly the same incident spectral shape works
for the three objects,
and that only the column density and ionization
parameters make the difference in the X-ray spectra.
Given the observed
flux of the sources and the estimates of the ionization parameters and
spectral shape from the CLOUDY models, the only undefined quantity is nr2.
Unfortunately, the location and density of the absorber are weakly
constrained by photoionization models and we cannot discriminate
between a low density (
)
absorber in the
interstellar medium of the host galaxy (
)
or a high
density material (
)
inside the nuclear region with
.
The latter is a typical distance-density combination for the
BLR clouds in nearby Seyfert 1 galaxies.
However, the ionization parameters in Fig. 6 are
significantly larger than the typical BLR values
(
).
This means that the ionized absorber is
different from the material responsible for the broad UV
emission lines,
consistent with our CLOUDY results that
no significant UV emission is expected from the absorbing
material (Sect. 5 and Fig. 11).
The former solution is reminiscent of the molecular torus
postulated by unification schemes, both in terms of distances
(
1 < d < 10 pc) and densities
(
).
The ionization parameter is however very different.
We speculate that the warm absorber in X-Ray weak quasars could
originate in material boiled off the torus inner wall by the
intense nuclear radiation field.
In such case, X-ray weak quasars would be normal quasars seen
at a special angle such that our line of sight to the nucleus grazes the
upper tip of the torus.
An alternative possibility is that the torus is lacking in
X-ray weak quasars, being replaced by a high ionization screen
transparent to UV and optical photons.
This would explain the deficit of "type 2'' quasars in
existing surveys.
PG 2112+059 shows significant X-ray variability (Gallagher et al. 2004). In comparison with the September 2002 Chandra observation, our data from May 2003 show a factor 6.7 increase of the 0.5-2.0 keV flux and a factor 2.1 increase in the 2-7 keV band. Our fluxes remain nevertheless lower than those measured with ASCA in November 1999 by factors 0.66 and 0.47, respectively. The larger variability amplitude in the soft X-ray band is consistent with the ionized absorber model: as the flux increases, the absorbing material gets more ionized and its opacity in the soft X-ray decreases.
The spectrum of PG 2112+059 obtained by Chandra in
September 2002, required a broad iron K
fluorescence emission line (Gallagher et al. 2004).
A line as observed by Chandra is detected in the XMM-Newton
data at the 98% confidence level only.
Interestingly, the equivalent width is about half
the equivalent width obtained for the Chandra
observation which has to be compared with a
factor of
2.1 increase of the continuum
flux in the 2.0-7.0 keV energy band.
Therefore, the two observations are consistent with
continuum variation while the
flux of the iron line remains constant.
The XMM-Newton spectra of PG 1535+547 statistically require a broad iron line, whereas the line was absent from 1999 ASCA data. This points toward a variable emission line.
Given that broad Fe K
fluorescence emission lines
are only rarely seen in Chandra or XMM-Newton observations
of AGNs,
it is quite remarkable that two of the three quasars studied
here show a variable broad Fe K
fluorescence emission
line.
Although the number of studied objects is far too small to
draw definitive conclusions, we should consider that
variable broad Fe K
fluorescence emission
lines might be a second characteristic of X-ray weak
quasars in general.
This would then suggest exceptional physical parameters
or viewing angles as proposed by
Brandt et al. (2000).
Acknowledgements
This research has made use of the NASA/ IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.