Joint NuSTAR and Chandra analysis of the obscured quasar in IC 2497 -
  Hanny's Voorwerp system
  We present new Nuclear Spectroscopic Telescope Array (NuSTAR) observations of
the core of IC 2497, the galaxy associated with Hanny's Voorwerp. The combined
fits of the Chandra (0.5-8 keV) and NuSTAR (3-24 keV) X-ray spectra, together
with WISE mid-IR photometry, optical longslit spectroscopy and optical
narrow-band imaging, suggest that the galaxy hosts a Compton-thick AGN ($N_{\rm
H} \sim 2 \times 10^{24}$ cm$^{-2}$, current intrinsic luminosity $L_{\rm bol}
\sim 2-5 \times 10^{44}$ erg s$^{-1}$) whose luminosity dropped by a factor of
$\sim$50 within the last $\sim 100$ kyr. This corresponds to a change in
Eddington ratio from $\rm \lambda_{Edd} \sim$ 0.35 to $\rm \lambda_{Edd} \sim$
0.007. We argue that the AGN in IC 2497 should not be classified as a
changing-look AGN, but rather we favour the interpretation where the AGN is
undergoing a change in accretion state (from radiatively efficient to
radiatively inefficient). In this scenario the observed drop in luminosity and
Eddington ratio corresponds to the final stage of an AGN accretion phase. Our
results are consistent with previous studies in the optical, X-ray and radio
although the magnitude of the drop is lower than previously suggested. In
addition, we discuss a possible analogy between X-ray binaries and an AGN.
