Transcript Document

Active Galactic Nuclei 9
Black Holes and Revelations
A reflection origin
for the soft and hard
X-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
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)
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 (Vaughan+04)
Suzaku (2007/04/01 - 100 ks)
Soft
excess
Reflection
hump
Iron
K line
Power law
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.
The nature of soft excess - I
.995/471
kT = 0.14 keV
fdr = 0.07
E1 = 6.64 keV
E2 = 6.97 keV
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.
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
The nature of soft excess - II
.895/470
E1 = 6.64 keV
E2 = 6.97 keV
kTe = 0.37 keV
fdr = 0.05
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)
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.
v/c = 0.5
fdr = 0.05
E1 = 6.52 keV
E2 = 6.97 keV
Blurred reflection model
Reflection
component
Hard power-law
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.
Thermal
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.
.877/468
fbr = 0.38
fdr = 0.31
fbr = 0.25
fdr = 0.05
kdblur*reflionx
E1 = 6.46 keV
E2 = 6.97 keV
The blurred reflection model turns
out to be successful in reproducing
both the soft excess and the highenergy 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.
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
fbr = 0.31
fbr = 0.08
fdr < 0.01
E1 = 6.40 keV
E2 = 6.64 keV
E3 = 7.00 keV
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.
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