On Estimating the High-Energy Cutoff in the X-ray Spectra of Black Holes via Reflection Spectroscopy

TL;DR

This paper demonstrates that reflection spectroscopy using NuSTAR and Suzaku data can accurately estimate the high-energy cutoff in black hole X-ray spectra up to 1 MeV, even for faint sources.

Contribution

The study introduces a method to constrain the high-energy cutoff in black hole spectra using reflection modeling with existing X-ray telescopes.

Findings

Reflection spectra are highly sensitive to the coronal spectral shape.

E_cut can be constrained up to 1 MeV with current instruments.

Faint sources as low as 1 mCrab can be analyzed effectively.

Abstract

The fundamental parameters describing the coronal spectrum of an accreting black hole are the slope $\Gamma$ of the power-law continuum and the energy $E_{cut}$ at which it rolls over. Remarkably, this parameter can be accurately measured for values as high as 1 MeV by modeling the spectrum of X-rays reflected from a black hole accretion disk at energies below 100 keV. This is possible because the details in the reflection spectrum, rich in fluorescent lines and other atomic features, are very sensitive to the spectral shape of the hardest coronal radiation illuminating the disk. We show that fitting simultaneous NuSTAR (3-79 keV) and low-energy (e.g., Suzaku) data with the most recent version of our reflection model RELXILL, one can obtain reasonable constraints on $E_{cut}$ at energies from tens of keV up to 1 MeV, for a source as faint as 1 mCrab in a 100 ks observation.