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

Abstract

We report on observations of NGC1068 with NuSTAR, which provide the best constraints to date on its > 10 keV spectral shape. The NuSTAR data are consistent with past instruments, with no strong continuum or line variability over the past two decades, consistent with its classification as a Compton-thick AGN. The combined NuSTAR, Chandra, XMM-Newton, and Swift BAT spectral dataset offers new insights into the complex secondary emission seen instead of the completely obscured transmitted nuclear continuum. The critical combination of the high signal-to-noise NuSTAR data and the decomposition of the nuclear and extranuclear emission 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 without requiring unrealistic physical parameters (e.g., large Fe overabundances, inconsistent viewing angles, poor fits to the spatially resolved spectra). A multi-component reflector with three distinct column densities (e.g., with best-fit values of N_H = 1.5×10^(23), 5×10^(24), and 10^(25) 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 component provides the bulk of the flux to the Compton hump while the lower N_H component produces much of the line emission, effectively decoupling two key features of Compton reflection. We also find that ≈ 30% of the neutral Fe Kɑ line flux arises from >2" (≈140 pc) and is clearly extended, 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 Compton-thick spectra from observations with poorer signal-to-noise and/or more distant objects.

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