High-Density Effects in X-ray Reflection Models from Accretion Disks

TL;DR

This paper investigates how increasing the density in X-ray reflection models of accretion disks affects the spectra, revealing significant modifications especially at energies below 2 keV, with implications for astrophysical observations.

Contribution

It introduces high-density effects into X-ray reflection models, demonstrating their impact on spectra and improving interpretation of observational data.

Findings

High densities (>10^{17} cm^{-3}) significantly alter reflection spectra.

Thermal continuum is boosted at energies below 2 keV.

Models are limited by current atomic data quality.

Abstract

Current models of the spectrum of X-rays reflected from accretion disks around black holes and other compact objects are commonly calculated assuming that the density of the disk atmosphere is constant within several Thomson depths from the irradiated surface. An important simplifying assumption of these models is that the ionization structure of the gas is completely specified by a single, fixed value of the ionization parameter $\xi$, which is the ratio of the incident flux to the gas density. The density is typically fixed at $n_e=10^{15}$ cm$^{-3}$. Motivated by observations, we consider higher densities in the calculation of the reflected spectrum. We show by computing model spectra for $n_e \gtrsim 10^{17}$ cm$^{-3}$ that high-density effects significantly modify reflection spectra. The main effect is to boost the thermal continuum at energies $\lesssim 2$ keV. We discuss the implications of these results for interpreting observations of both AGN and black hole binaries. We also discuss the limitations of our models imposed by the quality of the atomic data currently available.