期刊
ASTROPHYSICAL JOURNAL
卷 793, 期 1, 页码 -出版社
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/793/1/26
关键词
galaxies: active; galaxies: individual (NGC 4945); X-rays: galaxies
资金
- NASA [NNG08FD60C, GO3-14109X]
- National Aeronautics and Space Administration
- ASI/INAF [I/037/12/0-011/13]
- STFC [ST/J003697/1, ST/K501979/1, ST/I001573/1]
- Leverhulme Trust
- Fondecyt [11100449]
- Anillo [ACT1101]
- Swiss National Science Foundation [PP00P2_138979/1]
- California Institute of Technology NuSTAR [44A-1092750]
- NASA ADP [NNX10AC99G]
- Basal-CATA [PFB-06/2007]
- CONICYT-Chile [ACT1101, FONDECYT 1101024]
- Millennium Institute of Astrophysics (MAS) of Iniciativa Cientifica Milenio del Ministerio de Economia, Fomento y Turismo [IC120009]
- STFC [ST/J003697/1, ST/K501979/1, ST/J003697/2, ST/I001573/1, ST/L00075X/1] Funding Source: UKRI
- Science and Technology Facilities Council [ST/J003697/2, ST/J003697/1, ST/L00075X/1, ST/I001573/1, ST/K501979/1] Funding Source: researchfish
We present a broadband (similar to 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 tau(Thomson) similar to 2.9, and a global covering factor for the circumnuclear gas of similar to 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 similar to 0.1-0.3 lambda(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.
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