The hot circumgalactic medium of the Milky Way: evidence for
  super-virial, virial, and sub-virial temperature, non-solar chemical
  composition, and non-thermal line broadening
  For the first time, we present the simultaneous detection and
characterization of three distinct phases at $>10^5$ K in $z=0$ absorption,
using deep $\it{Chandra}$ observations toward Mrk 421. The extraordinarily high
signal-to-noise ratio ($\geqslant60$) of the spectra has allowed us to detect a
$\it{hot}$ phase of the Milky Way circumgalactic medium (CGM) at
3.2$^{+1.5}_{-0.5}\times$ 10$^7$ K, coexisting with a $\textit{warm-hot}$ phase
at 1.5$\pm$0.1$\times$10$^6$ K and a $\textit{warm}$ phase at
3.0$\pm$0.4$\times$10$^5$ K. The $\textit{warm-hot}$ phase is at the virial
temperature of the Galaxy, and the $\textit{warm}$ phase may have cooled from
the $\textit{warm-hot}$ phase, but the super-virial $\textit{hot}$ phase
remains a mystery. We find that [C/O] in the $\textit{warm}$ and
$\textit{warm-hot}$ phases, [Mg/O] in the $\textit{warm-hot}$ phase and [Ne/O]
in the $\textit{hot}$ phase are super-solar, and the $\textit{hot}$ and the
$\textit{warm-hot}$ phases are $\alpha-$enhanced. Non-thermal line broadening
is evident in the $\textit{warm-hot}$ and the $\textit{hot}$ phases and it
dominates the total line broadening. Our results indicate that the $>10^5$ K
CGM is a complex ecosystem. It provides insights on the thermal and chemical
history of the Milky Way CGM, and theories of galaxy evolution.
