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Published September 20, 2014 | Submitted + Published
Journal Article Open

The Variable Hard X-Ray Emission of NGC 4945 as Observed by NuSTAR

Abstract

We present a broadband (~0.5-79 keV) spectral and temporal analysis of multiple NuSTAR observations combined with archival Suzaku and Chandra data of NGC 4945, the brightest extragalactic source at 100 keV. We observe hard X-ray (>10 keV) flux and spectral variability, with flux variations of a factor of two on timescales of 20 ks. A variable primary continuum dominates the high-energy spectrum (>10 keV) in all states, while the reflected/scattered flux that dominates at E <10 keV stays approximately constant. From modeling the complex reflection/transmission spectrum, we derive a Compton depth along the line of sight of τ_(Thomson) ~ 2.9, and a global covering factor for the circumnuclear gas of ~0.15. This agrees with the constraints derived from the high-energy variability, which implies that most of the high-energy flux is transmitted rather than Compton-scattered. This demonstrates the effectiveness of spectral analysis at constraining the geometric properties of the circumnuclear gas, and validates similar methods used for analyzing the spectra of other bright, Compton-thick active galactic nuclei (AGNs). The lower limits on the e-folding energy are between 200 and 300 keV, consistent with previous BeppoSAX, Suzaku, and Swift Burst Alert Telescope observations. The accretion rate, estimated from the X-ray luminosity and assuming a bolometric correction typical of type 2 AGN, is in the range ~0.1-0.3 λ_(Edd) depending on the flux state. The substantial observed X-ray luminosity variability of NGC 4945 implies that large errors can arise from using single-epoch X-ray data to derive L/L_(Edd) values for obscured AGNs.

Additional Information

© 2014 American Astronomical Society. Received 2014 April 15; accepted 2014 July 14; published 2014 September 2. This work was supported under NASA Contract NNG08FD60C and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. We thank the NuSTAR Operations, Software, and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). S.P., A.C., F.F., and G.M. acknowledge support from the ASI/INAF grant I/037/12/0–011/13. A.C. acknowledges the Caltech Kingsley visitor program. P.G. acknowledges support from STFC (grant reference ST/J003697/ 1). G.B.L. acknowledges support from STFC (grant reference ST/K501979/1). D.M.A. acknowledges support from STFC (grant reference ST/I001573/1) and from the Leverhulme Trust. P.A. acknowledges financial support from Fondecyt grant 11100449 and Anillo ACT1101. G.R. acknowledges financial support from grant NASA GO3–14109X. M.K. gratefully acknowledges support from Swiss National Science Foundation Grant PP00P2_138979/1. W.N.B. and B.L. acknowledge support from California Institute of Technology NuSTAR subcontract 44A–1092750 and NASA ADP Grant NNX10AC99G. F.E.B. acknowledges support from Basal–CATA PFB–06/2007, CONICYT–Chile (grants FONDECYT 1101024 and "EMBIGGEN" Anillo ACT1101), and Project IC120009 "Millennium Institute of Astrophysics (MAS)" of Iniciativa Científica Milenio del Ministerio de Economía, Fomento y Turismo. S.P. is grateful to Tahir Yaqoob for useful discussions on the mytorus model.

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Published - 0004-637X_793_1_26.pdf

Submitted - 1407.3974v1.pdf

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