We present the results from the Suzaku observation of the powerful radio-loud quasar RBS 315 ( z = 2.69 ) , for which a previous XMM-Newton observation showed an extremely flat X-ray continuum up to 10 keV ( photon index \Gamma = 1.26 ) and indications of strong intrinsic absorption ( N _ { H } \sim 10 ^ { 22 } cm ^ { -2 } assuming neutral gas ) .
The instrument for hard X-rays HXD/PIN allows us a detection of the source up to 50 keV .
The broad-band continuum ( 0.5-50 keV ) can be well modeled with a power-law with slope \Gamma = 1.5 ( definitively softer than the continuum measured by XMM-Newton ) above 1 keV with strong deficit of soft photons .
The low-energy cut-off can be well fitted either with intrinsic absorption ( with column density N _ { H } \sim 10 ^ { 22 } cm ^ { -2 } in the quasar rest frame ) or with a break in the continuum , with an extremely hard ( \Gamma = 0.7 ) power-law below 1 keV .
We construct the Spectral Energy Distribution of the source , using also optical-UV measurements obtained through a quasi-simultaneous UVOT/SWIFT observation .
The shape of the SED is similar to that of other Flat Spectrum Radio Quasars ( FSRQs ) with similar power , making this source an excellent candidate for the detection in \gamma -rays by GLAST .
We model the SED with the synchrotron-Inverse Compton model usually applied to FSRQs , showing that the deficit of soft photons can be naturally interpreted as due to an intrinsic curvature of the spectrum near the low energy end of the IC component rather than to intrinsic absorption , although the latter possibility can not be ruled out .
We propose that in at least a fraction of the radio-loud QSOs at high redshift the cut-off in the soft X-ray band can be explained in a similar way .
Further studies are required to distinguish between the two alternatives .