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

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 Å), 1s-2p ${}^2$P (22.741 Å) and 1s-2p ${}^2$D (22.777 Å), and from the OV transition 1s-2p (22.370 Å) from TTG at $\log T(\mathrm{K})=5.2\pm 0.1$. 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(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${}^{-2}$.

Accepted for publication in Astronomy and Astrophysics

References in corpus (8)

• Time-Dependent Ionization in Radiatively Cooling Gas
• On the Putative Detection of z>0 X-ray Absorption Features in the Spectrum of Markarian 421
• Thermal radiation processes
• XMM-Newton Survey of Local O VII Absorption Lines in the Spectra of Active Galactic Nuclei
• Tracing the Cosmic Metal Evolution in the Low-Redshift Intergalactic Medium
• Confirming the Detection of an Intergalactic X-ray Absorber Toward PKS 2155-304
• Chandra and Far Ultraviolet Spectroscopic Explorer Observations of z~0 Warm-Hot Gas Toward PKS 2155-304
• Structual Study of Galactic Hot Gas toward Markarian 421 from X-Ray Absorption and Emission Lines

Cited by in corpus (9)

• The Dispersion of Fast Radio Bursts from a Structured Intergalactic Medium at Redshifts z < 1.5
• 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
• The Properties of the Galactic Hot Gaseous Halo from X-ray Emission
• Hot WHIM counterparts of FUV OVI absorbers: Evidence in the line-of-sight towards quasar 3C 273
• Cosmic metal invaders: Intergalactic OVII as a tracer of the warm-hot intergalactic medium within cosmic filaments in the EAGLE simulation
• The OVI mystery: mismatch between X-ray and UV column densities
• A search for the missing baryons with X--ray absorption lines towards the blazar 1ES 1553+113
• The ionic composition of the local absorber towards 3C 273
• Parts-per-million-accurate determination of the K$\alpha$ photoionization resonance of Be-like oxygen with resolution of its ${}^{16}$O-${}^{18}$O isotopic shift