Testing a double AGN hypothesis for Mrk 273
  The ULIRG Mrk 273 contains two infrared nuclei, N and SW, separated by 1
arcsec. A Chandra observation has identified the SW nucleus as an absorbed
X-ray source with nH ~4e23 cm-2 but also hinted at the possible presence of a
Compton thick AGN in the N nucleus, where a black hole of 10^9 Msun is inferred
from the ionized gas kinematics. The intrinsic X-ray spectral slope recently
measured by NuSTAR is unusually hard (photon index of ~1.3) for a Seyfert
nucleus, for which we seek an alternative explanation. We hypothesise a
strongly absorbed X-ray source in N, of which X-ray emission rises steeply
above 10 keV, in addition to the known X-ray source in SW, and test it against
the NuSTAR data, assuming the standard spectral slope (photon index of 1.9).
This double X-ray source model gives a good explanation of the hard continuum
spectrum, the deep Fe K absorption edge, and the strong Fe K line observed in
this ULIRG, without invoking the unusual spectral slope required for a single
source interpretation. The putative X-ray source in N is found to be absorbed
by nH = 1.4(+0.7/-0.4)e24 cm-2. The estimated 2-10 keV luminosity of the N
source is 1.3e43 erg/s, about a factor of 2 larger than that of SW during the
NuSTAR observation. Uncorrelated variability above and below 10 keV between the
Suzaku and NuSTAR observations appears to support the double source
interpretation. Variability in spectral hardness and Fe K line flux between the
previous X-ray observations is also consistent with this picture.
