Simulation-Based Prediction of Black Hole Fe K$α$ Line Profiles

Abstract

One of the most useful spectral diagnostics of accreting black hole systems is the Fe K$α$ fluorescence line. Detected in many systems, it is often used to estimate the black hole spin, as its breadth is attributed to relativistic kinematics near the spin-dependent innermost stable circular orbit (ISCO). In a companion paper, we showed how continuum spectra emitted by accreting black holes can be derived from snapshots of general relativistic magnetohydrodynamics simulations by combining radiation transfer solutions for the disk body and the corona. In this paper, we focus on the Fe K$α$ line, solving its transfer problem on the basis of local ionization and thermal balance. Its equivalent width is $\sim 25-225$ eV, depending mainly on viewing angle, for an accretion rate of 1$\%$ Eddington. Contrary to common assumptions, the illuminating X-ray spectrum and ionization parameter $ξ$ can be strong functions of radius; e.g. $ξ\propto r^{-1.5}$ in this simulation. Consequently, the region of the disk near the ISCO is completely ionized and contributes almost no Fe K$α$ photons; most of the flux is made at radii $\gtrsim 10 r_g$. The lines are broadened by a combination of relativistic Doppler shifts, Compton broadening in the disk atmosphere, and the differing line energies emitted by different Fe ions. These new mechanisms expand the parameter space of acceptable models, including the possibility of broad line profiles without large black hole spin; physical trends revealed by the simulations can refocus fitting efforts on the most relevant sections of the parameter space.