The Hot, Accreted Halo of NGC 891
  Galaxies are surrounded by halos of hot gas whose mass and origin remain
unknown. One of the most challenging properties to measure is the metallicity,
which constrains both of these. We present a measurement of the metallicity
around NGC 891, a nearby, edge-on, Milky Way analog. We find that the hot gas
is dominated by low metallicity gas near the virial temperature at
$kT=0.20\pm0.01$ keV and $Z/Z_{\odot} =
0.14\pm0.03$(stat)$^{+0.08}_{-0.02}$(sys), and that this gas co-exists with
hotter ($kT=0.71\pm0.04$ keV) gas that is concentrated near the star-forming
regions in the disk. Model choices lead to differences of $\Delta Z/Z_{\odot}
\sim 0.05$, and higher $S/N$ observations would be limited by systematic error
and plasma emission model or abundance ratio choices. The low metallicity gas
is consistent with the inner part of an extended halo accreted from the
intergalactic medium, which has been modulated by star formation. However,
there is much more cold gas than hot gas around NGC 891, which is difficult to
explain in either the accretion or supernova-driven outflow scenarios. We also
find a diffuse nonthermal excess centered on the galactic center and extending
to 5 kpc above the disk with a 0.3-10 keV $L_X = 3.1\times 10^{39}$ erg
s$^{-1}$. This emission is inconsistent with inverse Compton scattering or
single-population synchrotron emission, and its origin remains unclear.
