X-ray Polarization of Inverse Compton Scattering by Thermal and Nonthermal Electrons

Abstract

X-ray emission from accretion-powered astrophysical systems is widely interpreted as inverse Compton (IC) scattering between energetic electrons and soft photons. Besides the emitted intensity, the polarization of this radiation provides important information about the physical properties of the electrons involved. We investigate how different electron populations shape both the spectrum and polarization of IC emission in the X-ray band. We consider three electron populations: purely thermal, purely nonthermal power-law, and a hybrid population combining both components. We take both numerical simulations and semi-analytic calculations. We first attempt the cases for thermal and nonthermal electrons, respectively. We then focus on the hybrid population, which is expected to be realistic in high-energy object environments. For the case of hybrid electrons, the scattered emission separates into three energy regimes. At low energies ($\lesssim 0.1$ keV), it is dominated by thermal electrons; at high energies ($\gtrsim 4$ keV), it is governed by the nonthermal component. Between these limits, a transition band ($\sim 0.1$-$4$ keV) appears in which both components contribute. The degree of polarization varies smoothly across these regimes, and the behavior in the transition band directly traces the relative importance of thermal and nonthermal electrons. We further show that for partially polarized seed photons, the scattered polarization scales linearly with the incident polarization while its frequency dependence remains unchanged. These results show that X-ray polarimetry provides a powerful diagnostic of the electron energy distribution in accretion-powered systems.

14 pages, 12 figures, accepted for publication in MNRAS

References in corpus (13)

• Array Programming with NumPy
• Modelling the behaviour of accretion flows in X-ray binaries
• Polarized x-rays constrain the disk-jet geometry in the black hole x-ray binary Cygnus X-1
• Properties of AGN coronae in the NuSTAR era II: hybrid plasma
• The X-ray Polarization of the Accretion Disk Coronae of Active Galactic Nuclei
• Polarization constraints on the X-ray corona in Seyfert Galaxies: MCG-05-23-16
• Prospects for differentiating extended coronal geometries in AGNs with the IXPE mission
• X-ray Polarization of the Black Hole X-ray Binary 4U 1630-47 Challenges Standard Thin Accretion Disk Scenario
• IXPE and NICER view of Black hole X-ray binary 4U 1630-47: First significant detection of polarized emission in thermal state
• Detection of X-ray polarized emission and accretion-disk winds with IXPE and NICER in the black-hole X-ray binary 4U 1630-47
• A New Fast Monte Carlo Code for Solving Radiative Transfer Equations based on Neumann Solution
• Monte-Carlo Applications for Partially Polarised Inverse External-Compton Scattering (MAPPIES) -- I. Description of the code and First Results
• Polarization-sensitive Compton scattering by accelerated electrons