A reflection origin for the soft and hard X-ray excess of Ark 120

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Active Galactic Nuclei 9 Black Holes and
Revelations
A reflection originfor the soft and hardX-ray
excess of Ark 120
Emanuele Nardini Dipartimento di Fisica e
Astronomia Università di Firenze
in collaboration with Andy Fabian, Rubens Reis,
Dom Walton (Institute of Astronomy, Cambridge)
Ferrara, 2010 May 24-27
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Absorption-free AGN broadband X-ray emission
Soft excess Thermal emission, cold
Comptonization, smeared absorption or blurred
reflection?
Power law Hot Comptonization of soft photons in
a coronal region above the disc
Reflection hump Photoelectric absorption plus
Compton back-scattering
Iron K line Fluorescent line emission (broad
and/or narrow profile)
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Ark 120 a bare Seyfert galaxy
A significant contribution to the X-ray
luminosity of Seyfert 1 galaxies arises from the
soft excess component. The presence of complex
and/or variable absorption can mask or mimic this
critical feature.
XMM-Newton RGS (Vaughan04)
Suzaku (2007/04/01 - 100 ks)
Ark 120 is a Broad Line Seyfert with no evidence
of obscuration in the IR/optical/UV. Also,
stringent upper limits can be placed on the ionic
column densities of any possible warm X-ray
absorber. The Suzaku observation shows prominent
iron emission and substantial spectral curvature
at both low and high energies.
Reflection hump
Soft excess
Iron K line
Power law
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The nature of soft excess - I
Due to its smoothness and the lack of strong
spectral features the soft excess is consistent
with different models. In analogy with BH binary
systems it can be accounted for by thermal
emission from the disc, but this interpretation
is rather controversial.
.995/471
kT 0.14 keV fdr 0.07
E1 6.64 keV E2 6.97 keV
T M-1/4
The observed quasi-blackbody temperature is much
higher than predicted for a standard accretion
disc and almost independent of BH mass over
several orders of magnitude. Also, it does not
seem to follow the Stefan-Boltzmann law.
0.3 LEdd - 1.3 Rg
0.1 LEdd - 2 Rg
0.01 LEdd - 6 Rg
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The nature of soft excess - II
A more physical explanation invokes cold
Comptonization of EUV disc photons, but this
implies the existence of either a single plasma
with hybrid electron distribution or two
scattering regions with different temperatures
and optical depths. (Compactness problem)
.895/470
E1 6.64 keV E2 6.97 keV
kTe 0.37 keV fdr 0.05
1.02/470
Smeared absorption can take place in partially
ionized and highly turbulent material above the
disc, but the latest simulations of the velocity
and density structure of any realistic
accretion disc wind rule out this origin for the
soft excess.
E1 6.52 keV E2 6.97 keV
v/c 0.5 fdr 0.05
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Blurred reflection model
Hard power-law component
The intense X-ray illumination of the disc outer
layers is also expected to produce a wealth of
emission lines dominating the reflected spectrum
below 2 keV.
Reflection component
Thermal component
The relativistic motion of the inner accretion
flow blurs the sharp atomic features into the
smooth shape of the soft excess. The necessity
of taking into account strong relativistic
effects is confirmed by the detection of a broad
component in the iron K line profile.
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.877/468
The blurred reflection model turns out to be
successful in reproducing both the soft excess
and the high-energy Compton hump of Ark 120
without requiring extreme parameters. This
interpretation is therefore the most convincing
solution at present, also because of the minimal
set of geometrical and physical assumptions
involved.
fbr 0.38 fdr 0.31
E1 6.46 keV E2 6.97 keV
fbr 0.25 fdr 0.05
kdblurreflionx
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An independent test to discriminate between
blurred reflection and cold Comptonization is
provided by spectral variability and timing
analysis. Anyway, four different energy bands in
Ark 120 show the same variability pattern.
XMM-Newton
E1 6.40 keV E2 6.64 keV E3 7.00 keV
fbr 0.31 fbr 0.08 fdr lt 0.01
A reflection scenario for Ark 120 is also
supported by the high-quality XMM-Newton
spectrum, which however suggests a larger
complexity involving the ionization and/or
blurring parameters.
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Summary

• The problem of soft excess
• Extra emission exceeding the hard power law
  extrapolation below 2 keV
• Its smooth spectral shape is well-explained by
  different physical models
• Necessity to avoid possible contamination from
  absorption effects
• Resort to high-energy data to distinguish among
  the interpretations
• Blurred reflection in Ark 120
• A Broad Line Seyfert 1 galaxy free from complex
  intrinsic absorption
• Photoionization of the disc gives rise to many
  broad emission lines
• Blurred and cold reflection account for both the
  soft and hard excess
• Open issues and future work
• Ionization and blurring mismatch among the
  reflection components?
• Spectral analysis of a large sample of AGN with
  little obscuration
• Test the light bending model and the origin of
  the illuminating source
• Variability, timing analysis, frequency-dependent
  lags, reverberation

Selected References Is the soft excess in active
galactic nuclei real?, Gierlinski Done 2004
MNRAS A light bending model for the X-ray
temporal and spectral properties of accreting
BHs, Miniutti Fabian 2004 MNRAS A comprehensive
range of X-ray ionized-reflection models, Ross
Fabian 2005 MNRAS An explanation for the soft
X-ray excess in active galactic nuclei, Crummy et
al. 2006 MNRAS The impact of accretion disk winds
on the X-ray spectra of AGN (II.), Schurch, Done
Proga 2009 ApJ