The dust scattering component of X-ray extinction: Effects on continuum fitting and high-resolution absorption edge structure

TL;DR

Dust scattering significantly impacts X-ray extinction measurements, affecting continuum fitting and absorption edge analysis, especially in high-resolution spectra, and must be included for accurate astrophysical modeling.

Contribution

This paper introduces a comprehensive simulation study showing the importance of including dust scattering in X-ray extinction models, which was previously neglected.

Findings

Dust scattering causes a 25% overestimation of NH when ignored.

High-resolution spectra reveal enhanced extinction and altered absorption edge shapes due to dust scattering.

Inclusion of dust scattering improves spectral fits and physical interpretation of X-ray data.

Abstract

Small angle scattering by dust grains causes a significant contribution to the total interstellar extinction for any X-ray instrument with sub-arcminute resolution (Chandra, Swift, XMM-Newton). However, the dust scattering component is not included in the current absorption models: phabs, tbabs, and tbnew. We simulate a large number of Chandra spectra to explore the bias in the spectral fit and NH measurements obtained without including extinction from dust scattering. We find that without incorporating dust scattering, the measured NH will be too large by a baseline level of 25%. This effect is modulated by the imaging resolution of the telescope, because some amount of unresolved scattered light will be captured within the aperture used to extract point source information. In high resolution spectroscopy, dust scattering significantly enhances the total extinction optical depth and the shape of the photoelectric absorption edges. We focus in particular on the Fe-L edge at 0.7 keV, showing that the total extinction template fits well to the high resolution spectrum of three X-ray binaries from the Chandra archive: GX 9+9, XTE J1817-330, and Cyg X-1. In cases where dust is intrinsic to the source, a covering factor based on the angular extent of the dusty material must be applied to the extinction curve, regardless of angular imaging resolution. This approach will be particularly relevant for dust in quasar absorption line systems and might constrain clump sizes in active galactic nuclei.