XMM-Newton confirmation of a warm absorber in the z=4.4 blazar PMN J0525-3343

We present the analysis and results from a series of XMM-Newton observations of the distant blazar PMN J0525-3343, at a redshift of 4.4. The X-ray spectrum shows a spectral flattening below ~1 keV confirming earlier results from ASCA and BeppoSAX. The spectrum is well fitted by a power-law continuum with either a spectral break or absorption; no sharp features are apparent in the spectrum. No variability is seen in the individual lightcurves although there is evidence of small longer term variations (~ months in the blazar frame) in both flux and (0.1-2.4)/(2-10) keV flux ratio. Very low levels of optical-UV extinction and the lack of any evidence of a Lyman-limit system at the quasar redshift rule out neutral absorption and we argue that the most plausible explanation is the presence of a warm, ionized absorber. Very strong C IV absorption in the optical spectrum already implies the presence of highly ionized material along the line of sight. Warm absorber models using the photoionization code CLOUDY are consistent with both the X-ray and C IV data, yielding a column density \~10^(22.3-22.5) cm^-2 and ionization parameter ~10^(0.8-1.2) erg cm s^-1.