Post on 16-Dec-2015
A reflection originA reflection originfor the soft and hardfor the soft and hard
X-ray excess of Ark 120X-ray excess of Ark 120
Ferrara, 2010 May 24-27
in collaboration with:
Andy Fabian, Rubens Reis, Dom Walton(Institute of Astronomy, Cambridge)
Emanuele NardiniDipartimento di Fisica e Astronomia
Università di Firenze
Active Galactic Nuclei 9Black Holes and Revelations
Absorption-free AGN broadband X-ray emission
Soft excessThermal emission, cold Comptonization,
smeared absorption or blurred reflection?
Reflection humpPhotoelectric absorption
plus Compton back-scattering
Power lawHot Comptonization of soft photons in a coronal region above the disc
Iron K lineFluorescent line emission
(broad and/or narrow profile)
Ark 120: a bare Seyfert galaxy
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.
Suzaku (2007/04/01 - 100 ks)
Softexcess
Power law
Reflectionhump
IronK line
XMM-Newton RGS (Vaughan+04)
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.
T ~ M-1/4
0.01 LEdd - 6 Rg
0.1 LEdd - 2 Rg
0.3 LEdd - 1.3 Rg
The nature of soft excess - I
kT = 0.14 keVfdr = 0.07
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.
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.
E1 = 6.64 keVE2 = 6.97 keV
.995/471
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)
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 = 0.5fdr = 0.05
E1 = 6.64 keVE2 = 6.97 keV
E1 = 6.52 keVE2 = 6.97 keV
kTe = 0.37 keVfdr = 0.05
.895/470
1.02/470
Blurred reflection model
Hard power-law component
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.
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.
kdblur*reflionx
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.
E1 = 6.46 keVE2 = 6.97 keV
fbr = 0.25fdr = 0.05
fbr = 0.38fdr = 0.31
.877/468
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.
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.
XMM-Newton
E1 = 6.40 keVE2 = 6.64 keVE3 = 7.00 keVfbr = 0.31
fbr = 0.08fdr < 0.01
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 MNRASA light bending model for the X-ray temporal and spectral properties of accreting BHs, Miniutti & Fabian 2004 MNRASA comprehensive range of X-ray ionized-reflection models, Ross & Fabian 2005 MNRASAn explanation for the soft X-ray excess in active galactic nuclei, Crummy et al. 2006 MNRASThe impact of accretion disk winds on the X-ray spectra of AGN (II.), Schurch, Done & Proga 2009 ApJ
Summary