Soft X-Ray Excess from Shocked Black Hole Accretion

Abstract

A certain class of luminous galaxies hosting supermassive black holes (BHs) is known to exhibit an excess of “soft X-rays” in their X-ray spectra. However, its physical identity and origin are yet to be understood. In this work, we systematically study a sample of 9 well-documented narrow-line Seyfert 1 active galactic nuclei by utilizing the archival XMM-Newton/EPIC spectra in the context of general relativistic magnetohydrodynamic model. In this scenario, thermal accretion disk photons (in UV) are Compton up-scattered by nonthermal energetic electrons in the hot downstream accretion due to shock compression, producing the observed soft excess. Our spectral model consists primarily of the shock Comptonization component and the underlying continuum including reflection from the accretion disk. Based on χ 2-statistics, we successfully constrain the model parameters, most notably electron energy of the downstream flow kT e , effective disk blackbody temperature kT bb, and inclination angle θ for a given BH spin. The disk temperature is commonly found to be kT bb 10 eV and the electron energy ranges from kT e 75–160 keV (for Schwarzschild BHs) to 126–232 keV (for Kerr BHs) depending on inclination. Our analyses imply that the characteristics of the observed soft excess are strongly dependent on the properties of the downstream accretion flow and BH spin.