Soft X-Ray Excess from Shocked Accreting Plasma in Active Galactic Nuclei

Abstract

We propose a novel theoretical model to describe a physical identity of the soft X-ray excess, ubiquitously detected in many Seyfert galaxies, by considering a steady-state, axisymmetric plasma accretion within the innermost stable circular orbit (ISCO) around a black hole (BH) accretion disk. We extend our earlier theoretical investigations on general relativistic magnetohydrodynamic (GRMHD) accretion which has implied that the accreting plasma can develop into a standing shock for suitable physical conditions causing the downstream flow to be sufficiently hot due to shock compression. We numerically calculate to examine, for sets of fiducial plasma parameters, a physical nature of fast MHD shocks under strong gravity for different BH spins. We show that thermal seed photons from the standard accretion disk can be effectively Compton up-scattered by the energized sub-relativistic electrons in the hot downstream plasma to produce the soft excess feature in X-rays. As a case study, we construct a three-parameter Comptonization model of inclination angle ${\theta }_{\mathrm{obs}}$, disk photon temperature $k{T}_{\mathrm{in}}$ and downstream electron energy $k{T}_e$ to calculate the predicted spectra in comparison with a 60 ks {\it XMM-Newton}/EPIC-pn spectrum of a typical radio-quiet Seyfert 1 AGN, Ark~120. Our ${\chi }^2$-analyses demonstrate that the model is plausible in successfully describing data for both non-spinning and spinning BHs with the derived range of $61.3\mathrm{keV}\lesssim k{T}_e\lesssim 144.3\mathrm{keV}$, $21.6\mathrm{eV}\lesssim k{T}_{\mathrm{in}}\lesssim 34.0\mathrm{eV}$ and $17.5\degr \lesssim θ_{\rm obs} \lesssim 42.6\degr$ indicating a compact Comptonizing region of $3-4$ gravitational radii that resembles the putative X-ray coronae.

accepted to ApJ; 39 pages, 11 figures

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Cited by in corpus (13)

• Shocks in relativistic viscous accretion flows around Kerr black holes
• Low angular momentum relativistic hot accretion flow around Kerr black holes with variable adiabatic index
• Properties of two-temperature magnetized advective accretion flow around rotating black hole
• GRB Observational Properties
• Long term X-Ray Observations of Seyfert 1 Galaxy Ark 120: On the origin of soft-excess
• Probing Nuclear and Circumnuclear Properties of NGC 6300 using X-ray Observations
• Dimming of Continuum Captured in Mrk 841: New Clues on the Nature of the Soft X-ray Excess
• Investigating the nuclear properties of highly accreting active galactic nuclei with XMM-Newton
• Standing shocks in magnetized advection accretion flows onto a rotating black hole
• The contribution of bulk Comptonization to the soft X-ray excess in AGN
• A unified model of supernova driven by magnetic monopoles
• Hamiltonian Formulation of Relativistic Magnetohydrodynamic Accretion on a General Spherically Symmetric and Static Black Hole: Quantum effects on Shock States
• On the origin of core radio emissions from black hole sources in the realm of relativistic shocked accretion flow