Discovery of Galactic O iv and O v X-ray absorption due to transition temperature gas in the PKS 2155-304 spectrum

Far-ultraviolet (FUV) observations have revealed transition temperature gas (TTG; log T(K) 5), located in the lower Galactic halo and in 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 coverage of important wavelengths with at least two independent instruments allowed us to reliably detect 10 Galactic lines with better than 99.75% confidence. We discovered significant absorption from blended O iv transitions 1s-2p ²S (22.571 Å), 1s-2p ²P (22.741 Å), and 1s-2p ²D (22.777 Å), and from the O v transition 1s-2p (22.370 Å) from TTG at log T(K)=5.2 ± 0.1. 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(O iv)/N(O v) is not significantly different from that in collisional ionisation equilibrium (CIE). Under CIE we obtained N_OIV = 3.6 ± 2.0 ×10¹⁵ cm^-2, corresponding to N_H = 1.0 ± 0.5 ×10¹⁹(Z_⊙/Z_TTG) cm^-2.