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tokens: UV O vi measurements. It is thus highly unlikely that the poorer data quality of X-ray spectra contribute much to the O vi problem. XMM-Newton detection of the z = 0 O vi absorption line toward Mrk 421 is robust, with a sigma detection ( mÅ, error; Rasmussen et al. 2007 ). Moreover, the line was also independently detected by Chandra with similar strength ( mÅ; Williams et al. 2005 ), a detection, increasing the combined significance to . Thus, we cannot just dismiss this robust detection as a statistical accident. 2. Saturation. If UV O vi lines are saturated, the column density might be underestimated (Arav et al. 2003 ). For the unresolved lines, the observed equivalent width (EW) is related both to the column density and the velocity dispersion parameter (or the Doppler parameter b ). The column density and the b -parameter can be disentangled by using different transitions of the same ion, as we did in Williams et al. ( 2005 ) for the z = 0 absorption along the Mrk 421 sightline. Using the UV O vi absorption line doublet, we constrained the b- parameter tightly and found that for this value of b , the X-ray O vi line has a column density about 0.5 dex higher than the UV line. If the X-ray O vi line is saturated, the column density would be even higher. Thus the O vi discrepancy cannot be attributed to simple phenomena such as saturation in the z = 0 systems, so the solution has to lie elsewhere, and may be different for different systems. 3. Variability. Another possibility is variability, discussed by Arav et al. ( 2003 ). AGN absorption lines are known to vary, so if UV and X-ray measurements are not obtained at the same time, the measured column densities may be different. However, variability cannot be the cause of the O vi discrepancy in CGM/IGM systems that do not vary. 4. Size of the emission region. In AGNs, the size of the X-ray emission region is known to be smaller than the UV continuum size. Therefore, if part of the O vi X-ray absorption happens inside the UV continuum region, and both X-ray and UV O vi absorption takes place outside the UV emitting region, there might be a discrepancy, with X-ray O vi absorption probing a larger column density. Once again, this cannot be the cause of the O vi discrepancy in CGM/ISM systems that are far from the background quasars. 5. Excitation. In a high-temperature plasma, electron impact excitation and recombination may suppress absorption from the ground level and contribute to emission. However, the Einstein A 21 coefficient for the 2 p –2 s decay is extremely large (), which means any excited O vi atom decays immediately to the ground state. Even in a plasma with an unrealistically high collision/photoexcitation rate (), the standard Boltzmann equation only allows for the ground state to be depleted by a factor of one-half, and the observed discrepancy is much larger than this. 6. Atomic physics. The wavelengths, oscillator strengths, photoionization cross-sections of the UV O vi λλ