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We present the results from the Suzaku observation of the powerful radio-loudquasar RBS315 (z=2.69), for which a previous XMM-Newton observation showed anextremely flat X-ray continuum up to 10 keV (photon index Gamma=1.26) andindications of strong intrinsic absorption (N_H~10^22 cm^{-2} assuming neutralgas). The instrument for hard X-rays HXD/PIN allows us a detection of thesource up to 50 keV. The broad-band continuum (0.5-50 keV) can be well modeledwith a power-law with slope Gamma=1.5 (definitively softer than the continuummeasured by XMM-Newton) above 1 keV with strong deficit of soft photons. Thelow-energy cut-off can be well fitted either with intrinsic absorption (withcolumn density N_H~10^22 cm^{-2} in the quasar rest frame) or with a break inthe continuum, with an extremely hard (Gamma =0.7) power-law below 1 keV. Weconstruct the Spectral Energy Distribution of the source, using also optical-UVmeasurements obtained through a quasi-simultaneous UVOT/SWIFT observation. Theshape of the SED is similar to that of other Flat Spectrum Radio Quasars(FSRQs) with similar power, making this source an excellent candidate for thedetection in gamma-rays by GLAST. We model the SED with the synchrotron-InverseCompton model usually applied to FSRQs, showing that the deficit of softphotons can be naturally interpreted as due to an intrinsic curvature of thespectrum near the low energy end of the IC component rather than to intrinsicabsorption, although the latter possibility cannot be ruled out. We proposethat in at least a fraction of the radio-loud QSOs at high redshift the cut-offin the soft X-ray band can be explained in a similar way. Further studies arerequired to distinguish between the two alternatives.

Low energy cut-offs and hard X-ray spectra in high-z radio-loud quasars: the Suzaku view of RBS315