Hard X-ray View of HCG 16 (Arp 318)
  We report the hard X-ray (3-50 keV) view of the compact group HCG 16 (Arp
318) observed with Nuclear Spectroscopic Telescope Array (NuSTAR). NGC 838 and
NGC 839 are undetected at energies above 8 keV, showing no evidence of heavily
obscured active galactic nuclei (AGNs). This confirms that these are
starburst-dominant galaxies as previously suggested. We perform a comprehensive
broadband (0.3-50 keV) X-ray spectral analysis of the interacting galaxies NGC
833 and NGC 835, using data of NuSTAR, Chandra, and XMM-Newton observed on
multiple epochs from 2000 to 2015. NuSTAR detects the transmitted continua of
low-luminosity active galactic nuclei (LLAGNs) in NGC 833 and NGC 835 with
line-of-sight column densities of $\approx 3 \times10^{23}$ cm$^{-2}$ and
intrinsic 2-10 keV luminosities of $\approx 3\times10^{41}$ erg s$^{-1}$. The
iron-K$\alpha$ to hard X-ray luminosity ratios of NGC 833 and NGC 835 suggest
that their tori are moderately developed, which may have been triggered by the
galaxy interactions. We find that NGC 835 underwent long-term variability in
both intrinsic luminosity (by a factor of 5) and absorption (by $\Delta N_{\rm
H} \approx 2\times10^{23}$ cm$^{-2}$). We discuss the relation between the
X-ray and total infrared luminosities in local LLAGNs hosted by spiral
galaxies. The large diversity in their ratios is consistent with the the
general idea that the mass accretion process in the nucleus and the star
forming activity in the disk are not strongly coupled, regardless of the galaxy
environment.
