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Title: The Variable Hard X-Ray Emission of NGC 4945 as Observed by NUSTAR

Here, 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 τThomson ~ 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 luminositymore » and assuming a bolometric correction typical of type 2 AGN, is in the range ~0.1-0.3 λEdd depending on the flux state. As a result, 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.« less
ORCiD logo [1] ;  [2] ;  [3] ;  [4] ;  [5] ;  [6] ;  [7] ; ORCiD logo [8] ;  [8] ;  [9] ;  [10] ;  [11] ;  [12] ;  [9] ;  [13] ;  [14] ;  [9] ;  [7] ;  [15] ;  [16] more »;  [8] ;  [17] « less
  1. ASDC-ASI, Roma (Italy); INAF-Osservatorio Astronomico di Roma, Monte Porzio Catone (RM) (Italy)
  2. INAF-Osservatorio Astronomico di Bologna, Bologna (Italy)
  3. INAF-Osservatorio Astronomico di Roma, Monte Porzio Catone (RM) (Italy)
  4. Pontificia Univ. Catolica de Chile, Santiago (Chile)
  5. INAF-Osservatorio Astrofisico di Arcetri, Firenze (Italy); Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States)
  6. Pontificia Univ. Catolica de Chile, Santiago (Chile); Space Science Institute, Boulder, CO (United States)
  7. The Pennsylvania State Univ., University Park, PA (United States)
  8. California Inst. of Technology (CalTech), Pasadena, CA (United States)
  9. Durham Univ., Durham (United Kingdom)
  10. Univ. of California, Berkeley, CA (United States)
  11. Technical Univ. of Denmark, Lyngby (Denmark)
  12. Univ. of California, Berkeley, CA (United States); Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
  13. Columbia Univ., New York, NY (United States)
  14. ETH Zurich, Zurich (Switzerland)
  15. SLAC National Accelerator Lab., Menlo Park, CA (United States)
  16. Univ. 'a Roma Tre, Roma (Italy)
  17. NASA Goddard Space Flight Center (GSFC), Greenbelt, MD (United States)
Publication Date:
OSTI Identifier:
Report Number(s):
Journal ID: ISSN 1538-4357; arXiv:1407.3974
Grant/Contract Number:
Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Name: The Astrophysical Journal (Online); Journal Volume: 793; Journal Issue: 1; Journal ID: ISSN 1538-4357
Institute of Physics (IOP)
Research Org:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org:
USDOE Office of Science (SC); National Aeronautics and Space Administration (NASA)
Country of Publication:
United States
79 ASTRONOMY AND ASTROPHYSICS; Astrophysics; ASTRO; GRQC; galaxies: active; galaxies: individual (NGC 4945); X-rays: galaxies