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 (L_X ≈ 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 (Γ = 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 (N_H^z = 1.62^+0.09_-0.09 × 10²² cm^-2) and lukewarm (N_H^z = 2.2^+0.9_-0.3 × 10²² cm^-2; ξ = 15⁺³⁸_-12 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.