NuSTAR spectroscopy of multi-component X-Ray reflection from NGC 1068

Franz E. Bauer, Patricia Ar�valo, Dominic J. Walton, Michael J. Koss, Simonetta Puccetti, Poshak Gandhi, Daniel Stern, David M. Alexander, Mislav Baloković, Steve E. Boggs, William N. Brandt, Murray Brightman, Finn E. Christensen, Andrea Comastri, William W. Craig, Agnese Del Moro, Charles J. Hailey, Fiona A. Harrison, Ryan Hickox, Bin LuoCraig B. Markwardt, Andrea Marinucci, Giorgio Matt, Jane R. Rigby, Elizabeth Rivers, Cristian Saez, Ezequiel Treister, C. Megan Urry, William W. Zhang

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Abstract

We report on high-energy X-ray observations of the Compton-thick Seyfert 2 galaxy NGC 1068 with NuSTAR, which provide the best constraints to date on its >10 keV spectral shape. The NuSTAR data are consistent with those from past and current instruments to within cross-calibration uncertainties, and we find no strong continuum or line variability over the past two decades, which is in line with its X-ray classification as a reflection-dominated Compton-thick active galactic nucleus. The combined NuSTAR, Chandra, XMM-Newton, and Swift BAT spectral data set 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 NH) reflector, none of the common Compton reflection models are able to match the neutral fluorescence lines and broad spectral shape of the Compton reflection hump without requiring unrealistic physical parameters (e.g., large Fe overabundances, inconsistent viewing angles, or poor fits to the spatially resolved spectra). A multi-component reflector with three distinct column densities (e.g., with best-fit values of NH of 1.4 � 1023, 5.0 � 1024, and 1025 cm-2) provides a more reasonable fit to the spectral lines and Compton hump, with near-solar Fe abundances. In this model, the higher NH component provides the bulk of the flux to the Compton hump, while the lower NH component produces much of the line emission, effectively decoupling two key features of Compton reflection. We find that ≈30% of the neutral Fe K-α line flux arises from >2″ (≈140 pc) and is clearly extended, implying that a significant fraction (and perhaps most) of the <10 keV reflected component arises from regions well outside 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.

Original languageEnglish (US)
Article number116
JournalAstrophysical Journal
Volume812
Issue number2
DOIs
StatePublished - Oct 20 2015

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Bauer, F. E., Ar�valo, P., Walton, D. J., Koss, M. J., Puccetti, S., Gandhi, P., Stern, D., Alexander, D. M., Baloković, M., Boggs, S. E., Brandt, W. N., Brightman, M., Christensen, F. E., Comastri, A., Craig, W. W., Moro, A. D., Hailey, C. J., Harrison, F. A., Hickox, R., ... Zhang, W. W. (2015). NuSTAR spectroscopy of multi-component X-Ray reflection from NGC 1068. Astrophysical Journal, 812(2), [116]. https://doi.org/10.1088/0004-637X/812/2/116