A&A 365, L134-L139 (2001)
J. N. Reeves1 - M. J. L. Turner1 - K. A. Pounds1 - P. T. O'Brien1 - Th. Boller2 - P. Ferrando3 - E. Kendziorra4 - S. Vercellone5
Send offprint request: J. N. Reeves
1 - X-Ray Astronomy Group, Department of Physics and Astronomy, Leicester University, Leicester LE1 7RH, UK
2 - Max-Planck-Institut für extraterrestrische Physik, Postfach 1603,
85748 Garching, Germany
3 - Service d'Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France
4 - Institut für Astronomie und Astrophysik - Astronomie, University of Tübingen,
Waldhäuser Strasse 64,
72076 Tübingen, Germany
5 - Istituto di Fisica Cosmica ``G.
Occhialini'', C.N.R., Via Bassini 15, 20133 Milano, Italy
Received 28 September 2000 / Accepted 20 October 2000
Abstract
XMM-Newton observations of the low luminosity, radio-quiet quasar Markarian 205 have revealed a unique iron K emission line profile. In marked contrast to the broad and redshifted
iron K line commonly seen in ASCA observations of Seyfert 1 galaxies, we find that a substantial amount of the line flux in Mrk 205 occurs above the neutral line energy of 6.4 keV. Furthermore, we find that the iron line emission has two distinct components, a narrow, unresolved neutral
line at 6.4 keV and a broadened line centred at 6.7 keV. We suggest that the most likely origin of the 6.7 keV line is from X-ray reflection off the surface of a highly ionised accretion disk, whilst the 6.4 keV component may arise from neutral matter distant from the black hole, quite possibly in the putative molecular torus. Overall this observation underlines the potential of XMM-Newton for using the iron K line as a diagnostic of matter in the innermost regions of AGN.
Key words: galaxies: active - quasars: individual: Markarian 205 - X-rays: galaxies
Since iron K
emission was found to be a common feature in
the X-ray spectra of AGN, it has been recognised as potentially a very
important probe of matter in the central nucleus. Observations with
the Ginga X-ray satellite first established the
iron K
line (at 6.4 keV) as a dominant feature in the
hard X-ray spectra of many broad-line Seyfert 1 galaxies
(Pounds et al. 1990; Nandra & Pounds 1994).
The line was often accompanied by a flattening of the
X-ray continuum (the Compton scattering
hump) above the line energy. A widely favoured interpretation of
these spectral features, as primary hard X-rays
being ``reflected'' by Compton-thick matter - evidence for the
putative accretion disk in AGN - was strengthened when
ASCA, with its higher resolution solid-state detectors,
was able to resolve the iron K emission line. The line profile of the
bright Seyfert 1 galaxy MCG -6-30-15 (Tanaka et al. 1995)
was found to be broadened (
), with a strong asymmetry
red-wards of 6.4 keV; the
extreme width and redshift of the iron line being attributed to
the strong gravity and high velocities of the matter in
the innermost regions around the putative
massive black hole. Subsequent observations of a sample of
Seyfert 1s with ASCA (Nandra et al. 1997) revealed that such
line profiles were apparently common in many nearby Seyfert 1s.
In contrast, the information on the more luminous, but generally fainter, quasars has remained inconclusive. Ginga detected iron K emission from only a few quasars (Williams et al. 1992; Lawson et al. 1997), whilst although ASCA increased the number with detected iron lines (Reeves et al. 1997; Reeves & Turner 2000), the data were not sufficient to resolve any of the line profiles. Despite this, a divergence in iron K line properties between the low luminosity Seyfert 1 galaxies and higher luminosity quasars has become apparent, with the line energy being closer to 6.7 keV, rather than 6.4 keV, and the line strength falling with increasing luminosity (Iwasawa & Taniguichi 1993; Nandra et al. 1997b). The sensitivity of the XMM-Newton EPIC instruments now offers the exciting prospect of observing the iron K emission of quasars in detail and hence studying the flow and physical state of matter in the nuclei of these most powerful of objects.
This paper presents the discovery, by XMM-Newton, of an unusual
iron K
line profile in the quasar Markarian 205,
observed during the Cal-PV (Calibration and Payload
Verification) phase. Mrk 205 is a nearby (z=0.071), low
luminosity quasar (MV=-23,
erg/s,
erg/s; Williams et al. 1992;
Reeves & Turner 2000) and is radio-quiet (Rush et al. 1996).
It is located only 0.7
away from the nearby (z=0.00468)
spiral galaxy NGC 4319 and is viewed through the outer
disk of this galaxy (Bahcall et al. 1992; Bowen & Blades
1993).
Mrk 205 is also unusual in that it has a very weak UV bump
(McDowell et al. 1989), suggesting that the thermal disc
emission is hidden in the EUV.
We now present the XMM-Newton observations of Mrk 205.
Values of H0=50 km s-1 Mpc-1 and q0=0.5are assumed and all fit parameters are given in the quasar rest-frame.
Mrk 205 was observed during orbit 75 of the Cal-PV phase of XMM-Newton.
The observations with the EPIC MOS (Turner et al. 2001) and PN
(Strüder et al. 2001) detectors were split
into 3 parts, each of 17 ksec duration,
to test a variety of sub-window modes. Three
observations each were made with the MOS 1 and 2 cameras, in Full Window,
Partial Window 2 and Partial Window 3 modes. For the PN,
two observations were made in Full Window mode and
one in Large Window mode.
The data were screened with the XMM SAS (Science Analysis
Software) and pre-processed using the latest CCD gain values known
at the time of the observation.
X-ray events corresponding to patterns 0-12 for the 2 MOS cameras
(similar to grades 0-4 in ASCA) were used; for the PN, only
pattern 0 events (single pixel events) were selected.
Additional electronic noise was also removed from the MOS detectors.
A low energy cut of 200 eV was applied
and known hot or bad pixels were removed during screening.
The non X-ray background remained low throughout the observations.
A screened MOS-1 image of Mrk 205 is shown in Fig. 1.
We then proceeded to extract spectra and light curves for both the source
and background in each of the three observations, separately for each EPIC
detector. A circular source region of 1' diameter was defined around the
centroid position of Mrk 205, with the background being taken from an
offset position close
to the source. This resulted in 3 source spectra, for each of
the 3 EPIC CCD cameras. There was little flux or spectral variability of
Mrk 205
during the observations. Indeed a spectral
fit of a power-law plus Gaussian emission line (near to 6.4 keV)
yielded consistent results for each of the three observations.
Therefore we proceeded to co-add the 3 observations, for each detector. Furthermore as there was little difference between the MOS 1 and MOS 2 spectra (both in the spectral
shape and the iron line profile described later), we combined these into a
single spectral file to maximise signal-to-noise.
The resultant process left one combined EPIC-MOS
spectrum, with a total exposure of 100 ksec (both MOS, co-added) and
one PN spectrum, with a total exposure of 44 ksec. The integrated
X-ray flux from Mrk 205, over the 0.5-10 keV range, was
8 10-12 ergs cm-2 s-1. At this flux
level, the effect of photon pile-up, even in MOS full-frame mode, is
negligible. The background subtracted spectra
were fitted, using XSPEC V11.0, with the latest response
matrices produced by the EPIC team; the systematic level of uncertainty
is
.
Finally spectra were binned to a minimum of 20 counts per
bin, to apply the
minimalisation technique.
![]() |
Figure 1: An EPIC MOS-1 image of the Mrk 205 observation. Mrk 205 is located in the centre, the extended source near the chip gap is the nearby elliptical galaxy NGC 4291 |
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We initially fitted the hard X-ray (2.5-10 keV)
spectrum of Mrk 205 with a power law and neutral absorption corresponding
to the line-of-sight
column through our Galaxy, of
(Elvis et al. 1989).
From our initial inspection of the EPIC spectra, it was
obvious that deviations were present between 6-7 keV suggestive of
iron K shell band emission.
We then proceeded to parameterise the profile of the observed feature in
terms of simple
Gaussian models for both the MOS and PN (see Table 1 for a summary of
fits). Note all errors are quoted at the 90% confidence level.
Fit | Instrument | Narrow-Line | Broad/Disk Line | ![]() | Ea | EWb |
![]() |
Ea | ![]() ![]() |
EWb |
![]() |
1. PL + 2 |
MOS only |
![]() |
![]() |
32.4 |
![]() |
300+280-180 | 125+95-75 | 10.1 | 363.5/341 |
2. PL + 2 |
PN only | 6.46+0.02-0.08 | ![]() |
23.7 | 6.78+0.12-0.16 | 200+300-120 | 140+110-55 | 21.7 | 786/789 |
3. PL + 2 |
PN + MOS |
![]() |
![]() |
53 |
![]() |
250+190-130 | 135+70-60 | 30 | 1095/1093 |
4. PL + DISK |
PN + MOS | - | - | - | 6.4f | 45![]() |
215+80-50 | 56 | 1122/1096 |
5. PL + DISK |
PN + MOS | - | - | - |
![]() |
![]() ![]() |
300+65-70 | 68 | 1110/1095 |
6. PL + GA + DISK |
PN + MOS |
![]() |
![]() |
16 |
![]() |
![]() ![]() |
225+75-105 | 68 | 1094/1093 |
7. PL + 3 |
PN + MOS |
![]() |
![]() |
50 |
![]() |
10f | ![]() |
14 | |
![]() |
10f | ![]() |
19 | 1095/1092 |
Initially we fitted the MOS and PN data separately, to check for
consistency
between the 2 datasets. For the MOS, the addition
of a narrow (
keV) iron K line at
6.4 keV, to the
underlying power-law,
improved the fit significantly (
for
2 parameters). Even after addition of a narrow iron line, there were still
residuals present at energies >6.4 keV.
Therefore we added a second line component, with the energy and
line width both free to vary. The fit improved still further
(
for 3 extra parameters), with the overall
fit-statistic of
(fit 1 in Table 1).
The narrow line energy is consistent with neutral iron
(
keV),
whilst the ``broad'' Fe line has
a higher energy of
keV and an
rms width of
eV (Fig. 2).
Next we fitted the PN data, which provided still tighter constraints on
the Fe K emission. A significant improvement in the simple power law fit
(
for 2 parameters) was again obtained on adding a
narrow line near 6.4 keV.
As for the MOS, the fit was further improved
(
for 3 extra parameters) by adding a second broad
Fe line component (fit 2 in Table 1).
Again the energy of the narrow line is consistent with neutral iron
(
E=6.46+0.02-0.08 keV), whereas the energy of the ``broad'' line
is near to 6.7 keV (
E=6.78+0.12-0.16 keV).
The PN line profile, for the double Gaussian model, is also plotted in
Fig. 2.
As the line spectral fits for the MOS and PN are fully consistent, we
proceeded to fit the 2 spectra simultaneously, allowing for a slight
normalisation difference between the 2 detectors. The addition of both the
narrow 6.4 keV line and the broader line at 6.7 keV were both highly
significant (for the narrow line,
for 2 parameters;
broad line,
for 3 parameters) with the overall
fit-statistic being very good (
,
fit 3).
The narrow line is consistent with emission from neutral iron
(
keV, equivalent width
eV), whilst the broader
line corresponds to ionised matter (
keV,
eV and
EW=135+65-60 eV).
We do not detect an iron K edge, the upper-limit on the
edge, constrained between 7 and 9 keV, is
.
However, we
cannot rule out the presence of a continuum reflection component, from
either neutral or moderately ionised material, with a covering
fraction of 0.5 (corresponding to
).
Note the best-fit power-law index of
is
consistent with other radio-quiet quasars (Reeves et al. 1997).
Finally we plot confidence contours between line energy and normalisation
for both lines in Fig. 3, illustrating the
separate detection (at >99.9% confidence) of both the
narrow line at 6.4 keV and the broad, blue-shifted component
at 6.7 keV.
![]() |
Figure 2: The EPIC Fe line profile of Mrk 205, from the MOS and PN. A broad line component at 6.7 keV is present, as well as a narrow emission line at 6.4 keV. The line profile differs considerably from those observed in Seyfert 1 galaxies (MCG -6-30-15, Tanaka et al. 1995; NGC 3516, Nandra et al. 1999), where most of the line flux is redshifted below 6.4 keV |
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![]() |
Figure 3: Confidence contour plot for the double Gaussian Fe line profile. Contours correspond to the 68%, 90%, 99% and 99.9% confidence levels respectively. Clearly both the narrow line at 6.4 keV and the broad component at 6.7 keV are detected at a high level of significance |
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Given the complexity of the Fe line in Mrk 205, we checked
whether this could be reconciled with a relativistic disk-line
profile (Fabian et al. 1989). In the first trial, the
rest-frame energy of the disk-line was fixed at 6.4 keV, appropriate
for neutral iron (i.e. < Fe XX).
This yielded a poor fit to the
observed profile (fit 4, Table 1), for a disk inclination of
.
This is not surprising as the diskline model requires that most of the line
flux is located below 6.4 keV, which is clearly not observed.
Next we freed the rest-frame
energy in the disk-line model (fit 5). This yielded a better fit
(
), with an increased line energy of
keV,
consistent with a higher ionisation of iron. However inspection of the
line residuals indicated significant emission still remaining
at 6.4 keV. Adding a second narrow, neutral iron line to the model
(fit 6) did produce an acceptable fit, with the diskline rest energy
at
keV. Thus the observations are only
consistent with a diskline origin if the disk ionisation is
high enough to produce He-like iron at
6.7 keV and
if the narrow, neutral Fe line component is also present.
Note that if the 6.7 keV line component does originate from an
ionised accretion disk, then one might still expect to see some line flux
redshifted below 6.4 keV. We therefore tried to add a third, broad emission
line component, with the energy constrained between 5 and 6
keV. However we were only able to place an upper-limit on this
redshifted component (EW<45 eV), and although the flux is
substantially lower it is consistent with the amount
of flux expected from our diskline fit to the ionised line.
Finally we checked whether a blend of several narrow lines, from various
ionisation species of iron, can model the data (fit 7). This did produce
an acceptable fit
for three narrow lines, with the
best-fit line energies at 6.4 keV, 6.6 keV and 6.85 keV, compatible
with a line blend from neutral, He-like and H-like iron, respectively.
We now defer further discussion of the iron emission until Sect. 5.
![]() |
Figure 4: The EPIC spectrum of Mrk 205, extrapolated to lower energies (0.3 keV). An excess of soft X-ray counts, above that of the hard power-law, is clearly present in the data |
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We now consider the broad-band EPIC spectrum of Mrk 205.
Extrapolating the best-fit hard X-ray spectrum,
with a power-law index of
,
to lower energies
gives a poor fit; there is a clear excess of soft X-ray counts
above that of the hard X-ray power-law slope (Fig. 4).
Refitting the continuum with a single power-law (with
Galactic absorption) still gives a poor fit (
,
1630 channels), albeit with a steeper photon index
(
).
An acceptable reduced chi-squared (
)
is obtained on adding a black-body component to model the soft
excess, with temperature
eV and
.
The
soft excess is relatively weak, representing only
15% of the
emission over the range 0.5-2 keV.
Given the apparent weakness of the soft X-ray excess in Mrk 205, and its
relatively high temperature (
T=1.5 106 K), it is unlikely that we
are observing the direct thermal emission from the disk in this bandpass
(Malkan & Sargent 1982).
A more likely origin of the soft excess in Mrk 205 is by enhanced
reflection of the primary power law continuum in the surface layers of an
ionised accretion disk. Fitting the broad-band
X-ray continuum of Mrk 205 with a hard power-law plus the reflected
contribution from an ionised disk
(using the PEXRIV model in XSPEC; Magdziarz & Zdziarski
1995) provides
a very good fit, including all of the soft X-ray excess.
The parameters of the fit are also reasonable;
,
ionisation parameter
erg cm s-1 and
.
Furthermore this emission could naturally
explain the ionised part (at 6.7 keV) of the iron line profile; He-like
Fe K emission from the disc is predicted in this range of ionisation
(Ross et al. 1999; Nayakshin et al. 2000).
Note that the detection
of both the narrow and broad Fe line components is still robust (at
>99.9% confidence), even after the addition of reflection to the
continuum.
Aside from the Fe K line emission, the EPIC X-ray spectrum of Mrk 205
appears to be devoid of other discrete emission or absorption
features. To check this we
constructed a high resolution RGS spectrum (from RGS1 and RGS2),
using the RGSPROC script in the SAS software.
The RGS spectra are in agreement with the EPIC data, the soft
X-ray (0.4-2 keV) spectrum of Mrk 205 is
essentially featureless. We place limits on the strength of the OVII
and OVIII edges, expected from a warm absorber, of
and
respectively. Furthermore we find
no evidence for neutral photoelectric absorption, other than Galactic; the
limit on
is
<1.5 1020 cm-2. Thus we are apparently
observing the bare quasar nucleus in Mrk 205.
There is also no measurable absorption
from the intervening galaxy NGC 4319,
consistent with the observation of a very highly ionised ISM
in the spiral arms of that galaxy (Bowen & Blades 1993).
We have found two separate components in the Fe K line profile of the quasar Mrk 205, observed with XMM-Newton. A narrow line at 6.4 keV is seen, presumably arising from neutral material in the quasar rest-frame, as well as a broad component at 6.7 keV. This observation provides the clearest example to date, from CCD-resolution data, of a distinct narrow Fe-K line component in a broad-lined, type I AGN. We now consider the likely physical origins of the iron line components.
The narrow line at 6.4 keV.
Detection of a strong, narrow iron emission line at
6.4 keV implies that a substantial quantity of cool
reprocessing material is present. This matter
has to lie outside of the line-of-sight - the X-ray spectrum is
completely unabsorbed - and therefore is presumably seen by reflection.
The line strength (
eV), implies
that the cool matter subtends a substantial solid angle,
of at least 1
steradian, assuming it is Thomson thick
and has a solar abundance of iron (e.g. George & Fabian 1991).
The most likely location of such material, within the framework of current
AGN models, would appear to be the molecular torus
(Antonucci 1993), with
hard X-rays from the central engine being reflected off the
inner surface of the torus and into the line-of-sight of the observer
(Krolik et al. 1994).
Strong iron K
lines are believed to be formed in this way
in Seyfert 2 galaxies
(Turner et al. 1997), where the X-ray continuum
is at least partly obscured by the putative torus.
Clear evidence for this torus emission has hitherto not been found in a
quasar, where the (un-obscured) continuum level is much higher.
Our observation of a narrow 6.4 keV line from Mrk 205 could provide the
first such evidence.
Although reflection off the putative torus is attractive, we cannot rule out other alternatives. Emission from BLR clouds could account for some of the narrow line flux, although the predicted line strength is small (EW<50 eV; Leahy & Creighton 1993). Interestingly, a high resolution Chandra-HETG observation has also resolved a ``narrow'' Fe K line in the Seyfert 1 NGC 5548 (Yaqoob et al. 2000), here the authors attribute the line to BLR gas, but they cannot exclude the possibility of a torus component. Another possibility is that the line originates by X-ray reflection from the outer regions of the accretion disc. Normally the angle subtended by the outer disk, to the X-ray source, would be small. However a warped, concave disk could increase the amount of reflection at large radii, and account for a measurable narrow line component at 6.4 keV (Blackman 1999).
The broad line at 6.7 keV.
The most obvious explanation for the broad line in Mrk 205 is that
it originates from the inner accretion disc, where the material
velocities are high, as well as a strong gravity gradient.
However, the energy of the line emission is not consistent
with the diskline profiles observed in many Seyfert 1s
(Nandra et al. 1997), where the line flux is redshifted below
6.4 keV. Invoking a large inclination angle (
)
can circumvent this, but requires that the disc is orientated
near edge-on, which
appears to violate current AGN unification ideas (Antonucii 1993).
An alternative is that the disc matter is in a high
state of ionisation (with He or even H-like Fe).
This is a plausible explanation and we note high ionisation Fe lines near
6.7 keV have been claimed from ASCA data in several radio-quiet quasars
(Reeves & Turner 2000),
perhaps indicating a level of inner disc ionisation
increasing with accretion rate (Matt et al. 1999). Indeed
the red-wing of the Fe line profile may disappear if the very
innermost disc material becomes completely ionised (see Nandra et al.
1997b), consistent with our observation of Mrk 205, which we note
is 20
more luminous than the Seyfert 1 MCG -6-30-15.
It remains conceivable that the ``broad'' line does not originate from an accretion disc. A spherical rather than planar geometry could produce the observed profile, with the blue peak being produced naturally through transverse Doppler motion. Indeed the first formulation of X-ray reflection (Guilbert & Rees 1988) assumed a spherical distribution of clouds. Again we could be observing a blend of several Fe lines, from different ionised species and recall this model gave an equally good fit to the data (see fit 7). One possibility is the ionised lines originate from the warm electron scattering region associated with Seyfert 2 galaxies, which can produce substantial emission from He and H-like iron (Krolik & Kallman 1987). We note the spectrum of the Compton-thick Seyfert 2, NGC 1068, shows strong line emission from neutral, He-like and H-like iron (Marshall et al. 1993; Iwasawa et al. 1997). The difference in Mrk 205, where we observe the direct nuclear X-ray continuum, is that the equivalent widths of the lines are considerably smaller.
Observations with XMM-Newton have revealed a remarkable iron K line profile in the low luminosity quasar Mrk 205. The broad iron line component at 6.7 keV is inconsistent with the relativistic profile expected from the inner accretion disc (Fabian et al. 1989), unless the disc material is highly ionised. The narrow line at 6.4 keV probably arises from Compton scattering off distant cool material; we may infact be observing the reflected X-ray emission from the putative molecular torus in Mrk 205.
Acknowledgements
This work is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and the USA (NASA). EPIC was developed by the EPIC Consortium led by the Principal Investigator, Dr. M. J. L. Turner. The consortium comprises the following Institutes: University of Leicester, University of Birmingham (UK); CEA/Saclay, IAS Orsay, CESR Toulouse (France); IAAP Tuebingen, MPE Garching (Germany); IFC Milan, ITESRE Bologna, OAPA Palermo, Italy. EPIC is funded by: PPARC, CEA, CNES, DLR and ASI. The authors would also like to thank the EPIC instrument team, for their hard work during the calibration phase and the SOC and SSC teams for making the observation and subsequent analysis possible. It is a pleasure to thank Gareth Griffiths, Steve Sembay and Richard West for their help and advice since the launch. We also thank the referee (Paul Nandra) for providing a rapid reply to this paper and for some useful comments and suggestions.