X-raying the Intergalactic OVI Absorbers

TL;DR

This study uses stacked X-ray spectra to investigate the physical state of intergalactic OVI absorbers, setting upper limits on associated X-ray lines and constraining the temperature of the warm-hot intergalactic medium.

Contribution

It provides the first stringent upper limits on X-ray absorption lines associated with intergalactic OVI absorbers, constraining their temperature and ionization state.

Findings

No detectable X-ray absorption lines found.

Upper limits on ionic column densities established.

Average temperature of OVI-bearing gas constrained to log[T(K)]<=5.7.

Abstract

The observed intergalactic OVI absorbers at z>0 have been regarded as a significant reservoir of the ``missing baryons''. However, to fully understand how these absorbers contribute to the baryon inventory, it is crucial to determine whether the systems are collisionally ionized or photoionized (or both). Using the identified intergalactic OVI absorbers as tracers, we search for the corresponding X-ray absorption lines, which are useful for finding the missing baryons and for revealing the nature of the OVI absorbers. Stacking the Chandra grating spectra along six AGN sight lines, we obtain three spectra with signal-to-noise ratios of 32, 28, and 10 per 12.5 mA spectral bin around the expected OVII Kalpha wavelength. These spectra correspond to OVI absorbers with various dynamic properties. We find no detectable NeIX, OVII, OVIII, NVII, or CVI absorption lines in the spectra, but the high counting statistics allows us to obtain firm upper limits on the corresponding ionic column densities (in particular N(OVII)<=10 N(OVI) on average at the 95% confidence level). Jointly analyzing these non-detected X-ray lines with the averaged OVI column density, we further limit the average temperature of the OVI-bearing gas to be log[T(K)]<=5.7 in collisional ionization equilibrium. We discuss the implications of these results for physical properties of the putative warm-hot intergalactic medium and its detection in future X-ray observations.