XMM-Newton discovery of soft X-ray absorption in the high-z superluminous Blazar RBS 315

Abstract

We present the analysis and the results of a 20 ks XMM-Newton observation of the extremely X-ray loud (Lx ≈ 5 × 1047 erg s-1) flat-spectrum radio quasar RBS 315 at a redshift of 2.69. This EPIC observation has allowed us to strongly constrain the slope of the continuum (F = 1.23 ± 0.01) as well as to discover the presence of a sharp drop below ≈ 2 keV in its spectrum. Such a flat photon index and the huge luminosity suggest that the X-ray emission is due to the low energy tail of the Comptonized spectrum, produced from plasma in a relativistic jet oriented close to our line of sight. Even though the hypothesis of a break in the continuum cannot be completely discarded as an explanation of the soft X-ray cutoff, the presence of intrinsic absorption appears more plausible. Spectral fits with cold (NH2 = 1.62-0.09+0.09 × 1022 cm-2) and lukewarm (NHz = 2.2-0.3+0.9 × 1022 cm-2; ζ = 15-12+38 erg cm-2 s-1) absorbers are statistically indistinguishable. Remarkably, our results are very similar to those reported so far for other absorbed high-z Blazars observed by XMM-Newton. The existence of this "homogeneous" class of jet-dominated superluminous obscured QSOs at high z therefore could be important in the context of the formation and cosmological evolution of radio-loud objects. © ESO 2005.