The variable hard X-ray emission of NGC 4945 as observed by NUSTAR

Simonetta Puccetti, Andrea Comastri, Fabrizio Fiore, Patricia Arévalo, Guido Risaliti, Franz E. Bauer, William N. Brandt, Daniel Stern, Fiona A. Harrison, David M. Alexander, Steve E. Boggs, Finn E. Christensen, William W. Craig, Poshak Gandhi, Charles J. Hailey, Michael J. Koss, George B. Lansbury, Bin Luo, Greg M. Madejski, Giorgio MattDominic J. Walton, Will Zhang

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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 luminosity and assuming a bolometric correction typical of type 2 AGN, is in the range ∼0.1-0.3 λEdddepending 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/LEddvalues for obscured AGNs.

Original languageEnglish (US)
Article number26
JournalAstrophysical Journal
Issue number1
StatePublished - Sep 20 2014

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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