Variable Hard X-Ray Emission of NGC 4945 as Observed by NuSTAR.

摘要

We present a broadband (approximately 0.5 – 79 keV) spectral and temporal analysis of multiple NuSTAR observations combined with archival Suzaku and Chandra data of NGC4945, the brightest extragalactic source at 100 keV. We observe hard X–ray (> 10 keV) flux and spectral variability, with flux variations of a factor 2 on timescales of 20 ksec. A variable primary continuum dominates the high energy spectrum (> 10 keV) in all the states, while the reflected/scattered flux which dominates at E< 10 keV stays approximately constant. From modelling the complex reflection/transmission spectrum we derive a Compton depth along the line of sight of Thomson approximately 2.9, and a global covering factor for the circumnuclear gas of approximately 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 that Compton–scattered. This demonstrates the effectiveness of spectral analysis in constraining the geometric properties of the circumnuclear gas, and validates similar methods used for analyzing the spectra of other bright, Compton–thick AGN. The lower limits on the e–folding energy are between 200 ? 300 keV, consistent with previous BeppoSAX, Suzaku and Swift BAT observations. The accretion rate, estimated from the X–ray luminosity and assuming a bolometric correction typical of type 2 AGN, is in the range approximately 0.1 ? 0.3 Edd depending on the flux state. The substantial observed X–ray luminosity variability of NGC4945 implies that large errors can arise from using single–epoch X–ray data to derive L/LEdd values for obscured AGNs.