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) similar to 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 (similar to 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 S-2 (22.571 angstrom), 1s-2p P-2 (22.741 angstrom), and 1s-2p 2D (22.777 angstrom), and from the O v transition 1s-2p (22.370 angstrom) 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(OIV)/N(O v) is not significantly different from that in collisional ionisation equilibrium (CIE). Under CIE we obtained N-OIV = 3.6 +/- 2.0 x 1015 cm 2, corresponding to N-H = 1.0 +/- 0.5 x 10(19) Z(circle dot)/Z(TTG) cm(-2).