Presentation on theme: "A reflection origin for the soft and hard X-ray excess of Ark 120 Ferrara, 2010 May 24-27 in collaboration with: Andy Fabian, Rubens Reis, Dom Walton (Institute."— Presentation transcript:
A reflection origin for the soft and hard X-ray excess of Ark 120 Ferrara, 2010 May in collaboration with: Andy Fabian, Rubens Reis, Dom Walton (Institute of Astronomy, Cambridge) Emanuele Nardini Dipartimento di Fisica e Astronomia Università di Firenze Active Galactic Nuclei 9 Black Holes and Revelations
Absorption-free AGN broadband X-ray emission Soft excess Thermal emission, cold Comptonization, smeared absorption or blurred reflection? Reflection hump Photoelectric absorption plus Compton back-scattering Power law Hot Comptonization of soft photons in a coronal region above the disc Iron K line Fluorescent 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/ ks) Soft excess Power law Reflection hump Iron K 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/ L Edd - 6 R g 0.1 L Edd - 2 R g 0.3 L Edd R g The nature of soft excess - I kT = 0.14 keV f dr = 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. E 1 = 6.64 keV E 2 = 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 = 0.5 f dr = 0.05 E 1 = 6.64 keV E 2 = 6.97 keV E 1 = 6.52 keV E 2 = 6.97 keV kT e = 0.37 keV f dr = / /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. E 1 = 6.46 keV E 2 = 6.97 keV f br = 0.25 f dr = 0.05 f br = 0.38 f dr = /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 E 1 = 6.40 keV E 2 = 6.64 keV E 3 = 7.00 keV f br = 0.31 f br = 0.08 f dr < 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 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 MNRAS The impact of accretion disk winds on the X-ray spectra of AGN (II.), Schurch, Done & Proga 2009 ApJ Summary