The Variable Hard X-Ray Emission of NGC 4945 as Observed by NuSTAR
Hailey, Charles J.; Brandt, William N.; Comastri, Andrea; Gandhi, Poshak; Risaliti, Guido; Bauer, Franz E.; Luo, Bin; Walton, Dominic J.; Fiore, Fabrizio; Matt, Giorgio; Stern, Daniel; Harrison, Fiona A.; Boggs, Steve E.; Christensen, Finn E.; Craig, William W.; Madejski, Greg M.; Alexander, David M.; Puccetti, Simonetta; Koss, Michael J.; Lansbury, George B.; Arévalo, Patricia; Zhang, Will
Italy, Chile, United States, United Kingdom, Denmark, Switzerland
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/L Edd values for obscured AGNs.