# Discovery of Galactic OIV and OV X-ray absorption due to transition temperature gas in the PKS 2155-304 spectrum

## TL;DR

This study detects and analyzes X-ray absorption lines from transition temperature gas in the Galactic halo, revealing the physical state and ionization conditions of the gas through deep spectroscopic observations of PKS 2155-304.

## Contribution

It presents the first reliable detection of OIV and OV X-ray absorption lines from transition temperature gas using deep multi-instrument spectra, advancing understanding of Galactic halo gas.

## Key findings

- Detected ten Galactic absorption lines with high confidence.
- Identified OIV and OV lines from transition temperature gas at log T(K) ≈ 5.2.
- Estimated gas column densities consistent with collisional ionization equilibrium.

## Abstract

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$^{-2}$.