Modeling the Non-Thermal X-ray Tail Emission of Anomalous X-ray Pulsars

TL;DR

This paper presents a magnetospheric model for the non-thermal X-ray tail emission in AXPs, explaining the flat, polarized spectra via resonant magnetic Compton upscattering of surface thermal X-rays.

Contribution

It introduces a kinetic equation formalism with QED magnetic scattering cross sections to model non-thermal X-ray emission in AXPs, highlighting the role of magnetospheric geometry.

Findings

Produced flat, polarized spectra consistent with observations

Spectral features depend on observer orientation and scattering location

Constraints from Comptel upper bounds are discussed

Abstract

The paradigm for Anomalous X-ray Pulsars (AXPs) has evolved recently with the discovery by INTEGRAL and RXTE of flat, hard X-ray components in three AXPs. These non-thermal spectral components differ dramatically from the steeper quasi-power-law tails seen in the classic X-ray band in these sources, and can naturally be attributed to activity in the magnetosphere. Resonant, magnetic Compton upscattering is a candidate mechanism for generating this new component, since it is very efficient in the strong fields present near AXP surfaces. In this paper, results from an inner magnetospheric model for upscattering of surface thermal X-rays in AXPs are presented, using a kinetic equation formalism and employing a QED magnetic scattering cross section. Characteristically flat and strongly-polarized emission spectra are produced by non-thermal electrons injected in the emission region. Spectral results depend strongly on the observer's orientation and the magnetospheric locale of the scattering, which couple directly to the angular distributions of photons sampled. Constraints imposed by the Comptel upper bounds for these AXPs are mentioned.