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