The 2-79 keV X-ray Spectrum of the Circinus Galaxy with NuSTAR,
  XMM-Newton and Chandra: a Fully Compton-Thick AGN
  The Circinus galaxy is one of the nearest obscured AGN, making it an ideal
target for detailed study. Combining archival Chandra and XMM-Newton data with
new NuSTAR observations, we model the 2-79 keV spectrum to constrain the
primary AGN continuum and to derive physical parameters for the obscuring
material. Chandra's high angular resolution allows a separation of nuclear and
off-nuclear galactic emission. In the off-nuclear diffuse emission we find
signatures of strong cold reflection, including high equivalent-width neutral
Fe lines. This Compton-scattered off-nuclear emission amounts to 18% of the
nuclear flux in the Fe line region, but becomes comparable to the nuclear
emission above 30 keV. The new analysis no longer supports a prominent
transmitted AGN component in the observed band. We find that the nuclear
spectrum is consistent with Compton-scattering by an optically-thick torus,
where the intrinsic spectrum is a powerlaw of photon index $\Gamma = 2.2-2.4$,
the torus has an equatorial column density of $N_{\rm H} =
(6-10)\times10^{24}$cm$^{-2}$ and the intrinsic AGN $2-10$ keV luminosity is
$(2.3-5.1)\times 10^{42}$ erg/s. These values place Circinus along the same
relations as unobscured AGN in accretion rate-vs-$\Gamma$ and $L_X$-vs-$L_{IR}$
phase space. NuSTAR's high sensitivity and low background allow us to study the
short time-scale variability of Circinus at X-ray energies above 10 keV for the
first time. The lack of detected variability favors a Compton-thick absorber,
in line with the the spectral fitting results.
