NuSTAR Spectroscopy of Multi-Component X-ray Reflection from NGC 1068

TL;DR

This study uses NuSTAR and other X-ray observatories to analyze the complex multi-component reflection in NGC 1068, revealing a multi-layered structure with distinct column densities and spatial origins of reflected emission.

Contribution

It introduces a multi-component reflector model with three distinct column densities that better explains the observed X-ray spectral features of NGC 1068.

Findings

A multi-component reflector fits the spectral data better than a monolithic model.

Approximately 30% of the Fe Kalpha line flux originates from regions outside the parsec-scale torus.

Different N_H components dominate the Compton hump and line emission, decoupling key reflection features.

Abstract

We report on observations of NGC1068 with NuSTAR, which provide the best constraints to date on its $>10$~keV spectral shape. We find no strong variability over the past two decades, consistent with its Compton-thick AGN classification. The combined NuSTAR, Chandra, XMM-Newton, and Swift-BAT spectral dataset offers new insights into the complex reflected emission. The critical combination of the high signal-to-noise NuSTAR data and a spatial decomposition with Chandra allow us to break several model degeneracies and greatly aid physical interpretation. When modeled as a monolithic (i.e., a single N_H) reflector, none of the common Compton-reflection models are able to match the neutral fluorescence lines and broad spectral shape of the Compton reflection. A multi-component reflector with three distinct column densities (e.g., N_H~1.5e23, 5e24, and 1e25 cm^{-2}) provides a more reasonable fit to the spectral lines and Compton hump, with near-solar Fe abundances. In this model, the higher N_H components provide the bulk of the Compton hump flux while the lower N_H component produces much of the line emission, effectively decoupling two key features of Compton reflection. We note that ~30% of the neutral Fe Kalpha line flux arises from >2" (~140 pc), implying that a significant fraction of the <10 keV reflected component arises from regions well outside of a parsec-scale torus. These results likely have ramifications for the interpretation of poorer signal-to-noise observations and/or more distant objects [Abridged].