Possible X-ray diagnostic for jet/disk dominance in Type 1 AGN
Possible X-ray diagnostic for jet/disk dominance in Type 1 AGN
Barbara J. Mattson⋆, Kimberly A. Weaver
NASA/Goddard Space Flight Center, Astrophysics Science Division, Greenbelt, MD, 20771
Christopher S. Reynolds
Department of Astronomy, University of Maryland, College Park, MD, 20742
ABSTRACT
Using Rossi X-ray Timing Explorer Seyfert 1 and 1.2 data spanning 9 years,
we study correlations between X-ray spectral features. The sample consists of
350 time-resolved spectra from 12 Seyfert 1 and 1.2 galaxies. Each spectrum
is fitted to a model with an intrinsic powerlaw X-ray spectrum produced close
to the central black hole that is reprocessed and absorbed by material around
the black hole. To test the robustness of our results, we performed Monte Carlo
simulations of the spectral sample. We find a complex relationship between the
iron line equivalent width (EW ) and the underlying power law index (Γ). The
data reveal a correlation between Γ and EW which turns over at Γ . 2, but
finds a weak anti-correlation for steeper photon indices. We propose that this
relationship is driven by dilution of a disk spectrum (which includes the narrow
iron line) by a beamed jet component and, hence, could be used as a diagnostic
of jet-dominance. In addition, our sample shows a strong correlation between
R and Γ, but we find that it is likely the result of modeling degeneracies. We
also see the X-ray Baldwin effect (an anti-correlation between the 2-10 keV X-
ray luminosity and EW ) for the sample as a whole, but not for the individual
galaxies and galaxy types.
Subject headings: galaxies: Seyfert, X-rays: galaxies
also Department of Astronomy, University of Maryland, College Park, MD and Adnet Systems, Inc.,
Rockville, MD
http://arxiv.org/abs/0704.1587v1
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1. Introduction
Time-resolved X-ray spectroscopy studies of active galactic nuclei (AGN) offer the op-
portunity to investigate emission regions near the central black hole. In fact, X-ray spec-
troscopy offers the clearest view of processes occurring very close to the black hole itself,
probing matter to its final plunge into the black hole. Armed with such information, we can
unlock the structure of the innermost regions of AGN.
Typical X-ray spectra of AGN show an underlying powerlaw produced near the central
black hole with signatures of reprocessed photons often present. These reprocessed photons
show up as an Fe Kα line at ∼6.4 keV and a “reflection hump” which starts to dominate near
10 keV and is produced by the combined effects of photoelectric absorption and Compton
downscattering in optically-thin cold matter irradiated by the hard X-ray continuum. The
Fe Kα line has been observed in both type 1 (unabsorbed) and type 2 (absorbed) Seyfert
galaxies. It has been attributed to either the broad line region, the accretion disk, the molec-
ular torus of unification models (Antonucci 1993), or some combination of these. Signatures
of reflection have also been observed in both Seyfert 1 and 2 galaxies.
If the unification models are correct, we should see similar spectral correlations be-
tween Seyfert 1 and 2 galaxies, with any differences easily attributable to our viewing
angle. Regardless of the accuracy of the reflection models, we expect changes in the un-
derlying continuum to drive changes in the reprocessing features. However, results from
X-ray spectral studies of AGN have so far produced puzzling results. Samples of Seyfert
1 observations from ASCA (Weaver, Gelbord & Yaqoob 2001) and Rossi X-ray Timing Ex-
plorer (Markowitz, Edelson, & Vaughan 2003) have shown no obvious relationship between
changes in the continuum and iron line. Several galaxies have shown an anticorrelation
between reflection and/or iron line equivalent width and the source flux; e.g. NGC 5548
(Chiang et al. 2000), MCG −6-30-15 (Papadakis et al. 2002), NGC 4051 (Papadakis et al.
2002; Wang et al. 1999), NGC 5506 (Papadakis et al. 2002; Lamer, Uttley & McHardy 2000).
Recent data from Suzaku on MCG −6-30-15, on the other hand, show that the iron line and
reflection remain relatively constant while the powerlaw is highly variable (Miniutti et al.
2006). Zdziarski, Lubiński & Smith (1999) found that Seyfert galaxies and X-ray binaries
show a correlation between the continuum slope and reflection fraction, so those with soft
intrinsic spectra show stronger reflection than those with hard spectra. However, other stud-
ies have found either a shallower relationship than Zdziarski et al. (Perola et al. 2002) or an
anticorrelation (Papadakis et al. 2002; Lamer, Uttley & McHardy 2000).
Here we present the first results of a larger study of the X-ray spectral properties of
Seyfert galaxies observed by the Rossi X-ray Timing Explorer (RXTE ). Our full study
consists of observations of 30 galaxies. In this letter, we focus on the spectral results from
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the subset of 12 Seyfert 1 and 1.2 galaxies. In § 2 we present our method of data analysis,
including our sample selection criteria (§ 2.1), a description of our data pipeline (§ 2.2), and
results of our spectral analysis (§ 2.3). We discuss the implications of our results in § 3 and
detail our conclusions in § 4.
2. Data Analysis
2.1. The Sample
The RXTE public archive1 represents one of the largest collections of X-ray data for
AGN, with pointed observations of over 100 AGN spanning 10 years. The RXTE bandpass
allows the study of absorption and iron line properties of AGN spectra, as well as a glimpse
at the Compton reflection hump. We use data from the RXTE proportional counter array
(PCA), which is sensitive to energies from 2 to 60 keV and consists of five Proportional
Counter Units (PCUs). Most of the sources in our sample do not show significant counts in
the RXTE Hard Energy X-ray Timing Experiment (HEXTE), so we do not include HEXTE
data in this study.
To focus this study, we choose only Seyfert galaxies for which the RXTE public archive
contained a minimum of two pointings separated by at least two weeks. We further required
the total observed time be > 40 ks. These selection criteria led to a sample of 40 Seyfert
galaxies. For the analysis presented here, we examine the 18 Seyfert 1 and 1.2 galaxies. Six
galaxies were eliminated after they were put through our data pipeline (see § 2.2 for more),
so the final sample presented here consists of 12 galaxies, listed in Table 1. Because the data
come from the public archive, the sample is not uniform from galaxy to galaxy or even from
observation to observation; however, we use the Standard 2 data, which provides a standard
data mode for these diverse observations.
2.2. Data Pipeline
To ensure consistent data reduction of the large volume of data, we developed a data
pipeline. The Standard 2 data for each observation was reduced using a combination of
FTOOLs and the Pythonr scripting language. The pipeline produces time-resolved spectra,
1Hosted by the High Energy Astrophysics Science Archive and Research Center (HEASARC;
http://heasarc.gsfc.nasa.gov/)
http://heasarc.gsfc.nasa.gov/
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each with a minimum of 125,000 net photons, which are extracted using standard PCA
selection criteria and background models (Jahoda et al. 2006). Sources which did not have
sufficient net photons for even one spectrum were eliminated from the final sample (Table 1
shows the final sample with the 6 eliminated sources listed in the table notes). Each spectrum
includes 1% systematic errors. We are confident in the instrument response and background
models up to energies of ∼25 keV, so we ignore channels with higher energies.
2.3. Spectral Fitting and Results
The data pipeline produced 350 spectra for the 12 galaxies in our sample. Each spectrum
was fitted from 3 to 25 keV with an absorbed Compton reflection model plus a Gaussian iron
line. In xspec, the PEXRAV (Magdziarz & Zdziarski 1995) model simulates the effects of an
exponentially cut-off powerlaw reflected by neutral matter and has seven model parameters:
photon index of the intrinsic underlying power-law (Γ), the cutoff energy of the power law
in keV (Ec), the relative amount of reflection (R), the redshift (z), the abundance of heavy
elements in solar units (Z), the disk inclination angle (i), and the photon flux of the power
law at 1 keV in the observer’s frame (A). The relative amount of reflection is normalized to
1 for the case of an isotropic source above a disk of neutral material (Ω = 2π). Adding a
Gaussian line (energy in keV (EFe), physical width (σ) in keV, and normalization in units of
photons cm−2 s−1) and an absorbing column (NH , in cm
−2) yields a total of 12 parameters.
We fixed the following values in PEXRAV: Ec = 500 keV, Z = 1.0, and cos i = 0.95.
This inclination represents an almost face-on disk; however, since we are seeking trends in
the spectral parameters, rather than absolute values, the precise value is not important to
this study. In addition, z is fixed at the appropriate value from the NASA Extragalactic
Database for each galaxy2. After fitting all spectra to this model, we derived the mean
Gaussian width for each source (Table 1), then held σ fixed for a second fit to the model.
Our final model has free parameters: Γ, R, A, EFe, iron line normalization and NH . To
prevent xspec from pursuing unphysical values of the parameters, we set the following hard
limits: 0 ≤ Γ ≤ 5, 0 ≤ R ≤ 5, 5.5 ≤ EFe ≤ 7.5 keV, and 0 ≤ σ ≤ 1.5 keV (for the free-σ
fits).
Looking at the iron line equivalent width (EW ) and Γ, we find a complex relationship
with a “hump” peaking near Γ ∼ 2.0 (Figure 1a). The EW -Γ plot shows a correlation for
Γ . 2.0 and an anti-correlation for Γ & 2.0, with a peak near Γ ∼ 2.0 with EW ∼ 250
eV. We also find a strong correlation between R and Γ (Figure 2a), with a best-fit line of
2http://nedwww.ipac.caltech.edu/
http://nedwww.ipac.caltech.edu/
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R = −0.87 + 0.54 Γ (χ2 = 506/349 = 1.46).
We performed a Monte Carlo simulation to determine if our results were an artifact
of modeling degeneracies. Each spectrum in the Monte Carlo sample was simulated with
NH=10
22 cm−2, Γ=2.0, R=1.0, EFe=6.4 keV, and σFe=0.23 keV. The flux and exposure
times were randomly varied for each spectrum. The flux was varied by randomly choosing A
from a uniform distribution between 0.004 and 0.06 photons keV−1 cm−2 s−1. The exposure
time was randomly generated from a uniform distribution between 300 and 11000s. The
ranges for A and the exposure time represent the range of A and exposure for the spectra
in the full sample.
We generated 200 spectra: 100 simulated using an RXTE Epoch 3 response, 50 using an
Epoch 4 response, and 50 using an Epoch 5 response, roughly corresponding to our RXTE
sample. Each spectrum was then fitted to the same model as our full sample. The R over Γ
plot (Figure 2b) clearly shows a strong correlation with a best-fit line of R = −7.3 + 4.1 Γ
(χ2 = 28.96/159 = 0.182), which strongly suggests that the observed R-Γ correlation is a
result of modeling degeneracies. The correlation shows a much steeper relationship than the
Seyfert 1 data, due to the large number of Seyfert 1 spectra showing R ∼ 0.
EW and Γ, however, do not suffer the same degeneracies, which is clear from the Monte
Carlo results (Figure 1b). Based on the lack of correlation in our Monte Carlo results, we
are confident that the shape of the EW -Γ plot for the data sample is real.
To further examine the EW -Γ relationship, we reproduced the EW -Γ plot to show the
contribution from each galaxy (Figure 3). The radio-loud galaxies form the rising leg, with
the quasar, 3C 273, anchoring the low Γ-low EW portion of the plot. The Seyfert 1 (radio
quiet) and 1.2 galaxies tend to congregate at the peak and the falling leg of the plot. The
one narrow-line Seyfert 1 diverges from the main cluster of points.
Finally, we examined EW as a function of the intrinsic 2-10 keV X-ray luminosity (Lx),
using H0 = 70 km s
−1 Mpc−1. We fitted the data for each galaxy, each type, and the sample
as a whole to linear and powerlaw models. The data were well-fit for either model. For
consistency with other publications, we report here the powerlaw results. For the sample
as a whole, we see an anticorrelation, i.e. the X-ray Baldwin effect (Iwasawa & Taniguchi
1993), with EW ∝ L−0.14±0.01x . When examining galaxy types, however, the anticorrelation
does not always hold up (Table 1). We find an anticorrelation in the radio loud galaxies and
the Seyfert 1.2s, but a marginal correlation for the quasar and radio quiet Seyfert 1s.
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3. Discussion
3.1. EW -Γ Relationship
The simulations of George & Fabian (1991) for the observed spectrum from an X-ray
source illuminating a half-slab showed that the spectra should include a “Compton hump”
and an iron line. They found that the iron line EW should decrease as the spectrum softens.
This is easy to understand, since as the spectrum softens (Γ increases), there are fewer
photons with energies above the iron photoionization threshold. Our results show that the
relationship between EW and Γ is not quite so simple. We find a correlation between EW
and Γ when Γ . 2 and an anticorrelation when Γ & 2. Other researchers have found a
correlation for Seyfert 1 samples (Perola et al. 2002; Lubiński & Zdziarski 2001), but the
galaxies in their samples primarily fell in the Γ . 2 region. Page et al. (2004) also find that
their data suggest a slight correlation for a sample of radio loud and radio quiet Type 1
A close examination of our EW -Γ plot shows that the data for different galaxy types
progresses across the plot. The plot is anchored at the low-Γ, low-EW end by the quasar,
3C 273, in our sample. The rising arm of the plot, Γ ∼ 1.5− 2.0 and EW ∼ 0 − 300 eV, is
primarily formed by radio loud Seyfert 1 galaxies. The radio-quiet Seyfert 1 galaxies cluster
near the Γ ∼ 2.0, EW ∼ 300 eV peak of the hump, and the radio-quiet Seyfert 1.2 galaxies
form the falling arm of the plot for Γ > 2.0.
Physically, the most obvious difference between these sources is the presence or absence
of a strong jet. We propose that this relationship is driven by the degree of jet-dominance
of the source. The iron line features are associated with the X-ray emission from the disk.
Since the disk is essentially isotropic, it will excite an observable iron line from matter out of
our line-of-sight. On the other hand, the jet is beamed away from the obvious configurations
of matter in the system and, more importantly, is beamed toward us in the quasar and radio-
loud sources. Both of these jet-related phenomena reduce the observed equivalent width of
any iron line emission associated with the jet continuum.
In order for the Γ to increase as the jet-dominance decreases, the jet in these sources must
have a hard X-ray component, which implies that the radio-loud Seyferts in our sample are to
be associated with low-peaked BL Lac objects (LBLs). BL Lac objects show two broad peaks
in their spectral energy density plots (Giommi & Padovani 1994), with the lower-energy peak
due to synchrotron emission and the higher-energy peak due to inverse Compton emission.
BL Lacs are divided into two classes, depending on where the peaks occur: high-peaked BL
Lacs (HBLs) and LBLs. The X-ray continuum in the HBLs is rather soft, since we are seeing
the synchrotron spectrum cutting off in these sources. LBLs, on the other hand, tend to
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have a harder X-ray continua, since we are observing well into the inverse Compton part of
the spectrum (Donato, Sambruna, & Gliozzi 2005).
We also note that much of the falling arm of the EW -Γ relationship is formed by MCG
−6-30-15. Recent observations of MCG −6-30-15 by Suzaku have shown that the reflection
component, including the iron line, remains relatively constant Miniutti et al. (2006). We
would expect, then, that as Γ increases, the EW should decrease, which is exactly what we
see in our data.
3.2. R-Γ Relationship
Significant degeneracies between the photon index, absorbing column, and reflection
fraction can easily lead to false conclusions about spectral correlations. These degeneracies
occur as these three parameters trade off against each other in the modeling process, an effect
that is especially strong in the RXTE bandpass. Our R-Γ plot shows a strong correlation
which is mimicked in our Monte Carlo results. The few points that lie under the main
concentration are likely to be outliers, and not indicative of a subclass of galaxy. These
points all come from spectra that have been fitted to have NH = 0, and are primarily
radio-loud galaxies. We conclude that the observed R-Γ correlation in our sample cannot be
trusted as a real correlation.
3.3. EW -Lx Relationship
Looking at the EW -Lx relationship, we do see the X-ray Baldwin effect for our sample
as a whole, with a slighly shallower anticorrelation than reported elsewhere. We find EW ∝
L−0.14x , whereas Iwasawa & Taniguchi (1993) and Jiang, Wang & Wang (2006) find EW ∝
L−0.20x and Page et al. (2004) find EW ∝ L
−0.17
x . However, when Jiang, Wang & Wang
(2006) exclude the radio loud galaxies from their sample, they find EW ∝ L−0.10x .
We find, though, that when we examine our data on a galaxy-by-galaxy or type-by-type
basis, the effect is not consistent from source to source. At this point, we cannot determine
if these variations are real or are simply due to the small number of spectra for some of our
galaxies and types.
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4. Conclusions
We have examined time-resolved spectra of 12 Seyfert 1 and 1.2 galaxies observed by
RXTE over seven years. We find a complex relationship between the iron line equivalent
width and the continuum slope, with a correlation for Γ . 2 that turns over to an anticor-
relation for Γ & 2. We propose that this relationship is a possible diagnostic for jet- versus
disk-dominated sources, where jet-dominated sources show a correlation between EW and
Γ, and disk-dominated sources show an anticorrelation. We also see a strong correlation
between Γ and R which is likely an artifact of modeling degeneracies caused by the interplay
of Γ, R, and nH in the RXTE bandpass. Finally, we observe the X-ray Baldwin effect for
the sample as a whole, but not for each galaxy and galaxy type individually.
This research has made use of data obtained from the High Energy Astrophysics Science
Archive Research Center (HEASARC), provided by NASA’s Goddard Space Flight Center.
This research has also 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.
CSR gratefully acknowledges support from the National Science Foundation under
grants AST0205990 and AST0607428.
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http://arxiv.org/abs/astro-ph/0609521
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Table 1. Sample of RXTE -observed Seyfert 1 and 1.2 galaxiesa
Galaxy Seyfert Fitted Average EW/Lx correlation
Typeb Spectrac σFeKα
d α WV/Num.
All -0.14+0.01
−0.01 700/350
Quasars +0.09+0.20
−0.25 105/81
3C 273 1 81 0.329 +0.09+0.20
−0.25 105/81
Broadline Seyfert 1s -0.24+0.14
−0.15 48.0/66
3C 111 1 4 0.239 +0.70+2.60
−1.52 0.654/4
3C 120f 1 40 0.261 -0.70+0.63
−0.61 20.9/39
3C 382 1 5 0.328 -0.80+1.69
−1.70 2.54/5
3C 390.3 1 17 0.203 -0.51+0.44
−0.41 2.70/17
Seyfert 1s (Radio quiet) 0.01+0.300.30 23.6/31
Ark 120 1 15 0.197 -0.66+0.58
−0.57 6.62/15
Fairall 9 1 16 0.155 +0.41+0.44
−0.44 11.1/16
Seyfert 1.2s -0.08+0.03
−0.03 192/169
IC 4329A 1.2 41 0.214 -0.55+0.36
−0.37 27.5/41
MCG -6-30-15 1.2 75 0.292 -0.65+0.34
−0.33 89.2/75
Mkn 509 1.2 16 0.102 -0.52+0.91
−0.99 7.57/16
NGC 7469 1.2 37 0.145 -0.58+0.30
−0.31 17.7/37
Narrow Line Seyfert 1 8.80+20.80
−6.08 0.196/3
TON S180 1.2 3 0.379 8.80+20.80
−6.08 0.196/3
aThe following sources were eliminated after running the data pipeline described
in the text, due to having no spectra with at least 125,000 net photons: Mkn 110,
PG 0804+761, PG 1211+143, Mkn 79, Mkn 335, and PG 0052+251.
bSeyfert type based on the NASA Extragalactic Database
cTotal number of spectra extracted using our data pipeline (§ 2.2).
dThe average physical width of the Fe Kα line for all spectra from a source when
fitted to the absorbed powerlaw model with Compton reflection and Gaussian iron
line (§ 2.3).
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eResults of fitting the X-ray luminosity over EW plot to a powerlaw model; e.g.
EW ∝ Lαx , where Lx is the 2-10 keV X-ray luminosity in ergs s
−1 and EW is the
iron line equivalent width in eV.
fOne 3C 120 spectrum shows a flare, where Lx jumps by ∼ 6×. The number
quoted above excludes this point from the sample. If we include the flare, we find
EW ∝ L
0.07(+0.18/−0.25)
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(a) (b)
Fig. 1.— Iron line equivalent width in eV (EW ) versus powerlaw photon index (Γ) for the
Seyfert 1/1.2 sample (a) and for the Monte Carlo simulations (b).
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(a) (b)
Fig. 2.— Reflection fraction (R) versus powerlaw photon index (Γ) for the Seyfert 1/1.2
sample (a) and for the Monte Carlo simulations (b). In both plots, the line shows the
best-fit linear model for the Monte Carlo simulations.
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Fig. 3.— The iron line equivalent width in eV versus the powerlaw photon index. This plot
is similar to the left panel in Figure 1, but with each galaxy plotted with a separate symbol.
The open circles are 3C 111, open squares are 3C120, pluses (+) are 3C273, open triangles
are 3C 382, open diamonds 3C 390.3, open stars Akn 120, open crosses Fairall 9, filled circles
IC 4329A, filled squares MCG −6-30-15, filled triangles Mkn 509, filled stars NGC 7469, and
asterisks (*) TON S180.
	Introduction
	Data Analysis
	The Sample
	Data Pipeline
	Spectral Fitting and Results
	Discussion
	EW- Relationship
	R- Relationship
	EW-Lx Relationship
	Conclusions
