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 λEdd 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/LEdd values for obscured AGNs.

The variable hard X-ray emission of NGC 4945 as observed by NUSTAR / Puccetti, Simonetta; Comastri, Andrea; Fiore, Fabrizio; Arévalo, Patricia; Risaliti, Guido; Bauer, Franz E.; Brandt, William N.; Stern, Daniel; Harrison, Fiona A.; Alexander, David M.; Boggs, Steve E.; Christensen, Finn E.; Craig, William W.; Gandhi, Poshak; Hailey, Charles J.; Koss, Michael J.; Lansbury, George B.; Luo, Bin; Madejski, Greg M.; Matt, Giorgio; Walton, Dominic J.; Zhang, Will. - In: THE ASTROPHYSICAL JOURNAL. - ISSN 1538-4357. - STAMPA. - 793:(2014), pp. 26-42. [10.1088/0004-637X/793/1/26]

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

RISALITI, GUIDO;MATT, GIORGIO;
2014

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 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 λEdd 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/LEdd values for obscured AGNs.
2014
793
26
42
Puccetti, Simonetta; Comastri, Andrea; Fiore, Fabrizio; Arévalo, Patricia; Risaliti, Guido; Bauer, Franz E.; Brandt, William N.; Stern, Daniel; Harrison, Fiona A.; Alexander, David M.; Boggs, Steve E.; Christensen, Finn E.; Craig, William W.; Gandhi, Poshak; Hailey, Charles J.; Koss, Michael J.; Lansbury, George B.; Luo, Bin; Madejski, Greg M.; Matt, Giorgio; Walton, Dominic J.; Zhang, Will
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Utilizza questo identificatore per citare o creare un link a questa risorsa: https://hdl.handle.net/2158/1078228
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