PMN J0525-3343: soft X-ray spectral flattening in a blazar at z=4.4

Abstract

We report optical, radio and X-ray observations of a new distant blazar, PMN J0525-3343, at a redshift of 4.4. The X-ray spectrum measured from ASCA and BeppoSAX flattens below a few keV, in a manner similar to the spectra of two other z>4 blazars, GB 1428+4217 (z=4.72) reported by Boller et al and RXJ 1028.6-0844 (z=4.28) by Yuan et al. The spectrum is well fitted by a power-law continuum which is either absorbed or breaks at a few keV. An intrinsic column density corresponding to 2 x 10^23 H-atoms cm-2 at solar abundance is required by the absorption model. This is however a million times greater than the neutral hydrogen, or dust, column density implied by the optical spectrum, which covers the rest-frame UV emission of the blazar nucleus. We discuss the problems raised and suggest that, unless there is intrinsic flattening in the spectral distribution of the particles/seed photons producing X-rays via inverse Compton scattering, the most plausible solution is a warm absorber close to the active nucleus.