Discovery of Galactic OIV and OV X-ray absorption due to transition
  temperature gas in the PKS 2155-304 spectrum
  FUV observations have revealed the transition temperature gas (TTG;
$\log{T({\mathrm{K}})}$ ~ 5), located in the lower Galactic halo and in the
High Velocity Clouds. However, the corresponding X-ray absorption has so far
remained mostly undetected. In order to make an improvement in this respect in
Galactic X-ray absorption studies, we accumulated very deep (~3 Ms) spectra of
the blazar PKS 2155-304 obtained with the spectrometers RGS1, RGS2, LETG/HRC
and LETG/ACIS-S and studied the absorption lines due to the intervening
Galactic components. The very high quality of the data and the coverage of
important wavelengths with at least two independent instruments allowed us to
reliably detect ten Galactic lines with better than 99.95% confidence. We
discovered significant absorption from blended OIV transitions 1s-2p $^2$S
(22.571 \AA ), 1s-2p $^2$P (22.741 \AA ) and 1s-2p $^2$D (22.777 \AA ), and
from the OV transition 1s-2p (22.370 \AA ) from TTG at $\log{T({\mathrm{K}})}
\thinspace = \thinspace 5.2\pm0.1$. A joint X-ray and FUV analysis indicated
that photoionisation is negligible for this component and that the gas is in a
cooling transition phase. However, the temperature is high enough that the
column density ratio N(OIV)/N(OV) is not significantly different from that in
collisional ionisation equilibrium (CIE). Under CIE we obtained
$N_{\mathrm{OIV}}$= 3.6$\pm$2.0 $\times 10^{15}$ cm$^{-2}$, corresponding to
$N_{\mathrm{H}}$ = 1.0$\pm$0.5 $\times 10^{19}
\frac{Z_{\odot}}{Z_{\mathrm{TTG}}}$ cm$^{-2}$.
