Blazars are often characterized by a spectral break at soft X-rays , whose origin is still debated .
While most sources show a flattening , some exhibit a blackbody-like soft excess with temperatures of the order of \sim 0.1 keV , similar to low-luminosity , non-jetted Seyferts .
Here we present the analysis of the simultaneous XMM-Newton and NuSTAR observation of the luminous FSRQ 4C+25.05 ( z = 2.368 ) .
The observed 0.3–30 keV spectrum is best described by the sum of a hard X-ray power law ( \Gamma = 1.38 _ { -0.03 } ^ { +0.05 } ) and a soft component , approximated by a blackbody with kT _ { BB } = 0.66 _ { -0.04 } ^ { +0.05 } keV ( rest frame ) .
If the spectrum of 4C+25.05 is interpreted in the context of bulk Comptonization by cold electrons of broad-line region photons emitted in the direction of the jet , such an unusual temperature implies a bulk Lorentz factor of the jet of \Gamma _ { bulk } \sim 11.7 .
Bulk Comptonization is expected to be ubiquitous on physical grounds , yet no clear signature of it has been found so far , possibly due to its transient nature and the lack of high-quality , broad-band X-ray spectra .