We present the results of a long ( \sim 93 { ksec } ) XMM-Newton observation of the bright BL-Lac object PKS 0548-322 ( z = 0.069 ) .
Our Reflection Grating Spectrometer ( RGS ) spectrum shows a single absorption feature at an observed wavelength \lambda = 23.33 \pm 0.01 Å which we interpret as OVI K \alpha absorption at z = 0.058 , i.e. , \sim 3000 { km } { s } ^ { -1 } from the background object .
The observed equivalent width of the absorption line \sim 30 { m \AA } , coupled with the lack of the corresponding absorption edge in the EPIC pn data , implies a column density N _ { OVI } \sim 2 \times 10 ^ { 16 } { cm } ^ { -2 } and turbulence with a Doppler velocity parameter b > 100 { km } { s } ^ { -1 } .
Within the limitations of our RGS spectrum , no OVII or OV K \alpha absorption are detected .
Under the assumption of ionisation equilibrium by both collisions and the extragalactic background , this is only marginally consistent if the gas temperature is \sim 2.5 \times 10 ^ { 5 } { K } , with significantly lower or higher values being excluded by our limits on OV or OVII .
If confirmed , this would be the first X-ray detection of a large amount of intervening warm absorbing gas through OVI absorption .
The existence of such a high column density absorber , much stronger than any previously detected one in OVI , would place stringent constraints on the large-scale distribution of baryonic gas in the Universe .