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

Free access article

Issue		A&A Volume 442, Number 3, November II 2005


Page(s)		L53 - L56
Section		Letters
DOI		http://dx.doi.org/10.1051/0004-6361:200500189

A&A 442, L53-L56 (2005)
DOI: 10.1051/0004-6361:200500189

Letter

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

E. Piconcelli and M. Guainazzi

European Space Astronomy Center (ESA), Apartado 50727, 28080 Madrid, Spain
e-mail: Enrico.Piconcelli@sciops.esa.int

(Received 18 May 2005 / Accepted 14 September 2005)

Abstract
We present the analysis and the results of a 20 ks XMM-Newton observation of the extremely X-ray loud ( $L_{\rm X} \approx 5 \times 10^{47}$ 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 ( $\Gamma = 1.23\pm0.01$ ) as well as to discover the presence of a sharp drop below $\approx$ 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 ( $N_{\rm H}^{z}$ = 1.62 ^+0.09_-0.09 $\times$ 10²² cm^-2) and lukewarm ( $N_{\rm H}^{z}$ = 2.2 ^+0.9_-0.3 $\times$ 10²² cm^-2; $\xi$ = 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.

Key words: galaxies: active -- quasar: general -- X-ray: individual: RBS 315

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