  The most powerful blazars are the flat spectrum radio quasars whose emission
is dominated by a Compton component peaking between a few hundred keV and a few
hundred MeV. We selected two bright blazars, PKS 2149-306 at redshift z=2.345
and S5 0836+710 at z=2.172, in order to observe them in the hard X-ray band
with the NuSTAR satellite. In this band the Compton component is rapidly rising
almost up to the peak of the emission. Simultaneous soft-X-rays and UV-optical
observations were performed with the Swift satellite, while near-infrared (NIR)
data were obtained with the REM telescope. To study their variability, we
repeated these observations for both sources on a timescale of a few months.
While no fast variability was detected during a single observation, both
sources were found to be variable in the X-ray band, up to 50%, between the two
observations, with larger variability at higher energies. No variability was
detected in the optical/NIR band. These data together with Fermi-LAT, WISE and
other literature data are then used to study the overall spectral energy
distributions (SEDs) of these blazars. Although the jet non-thermal emission
dominates the SED, it leaves the UV band unhidden, allowing us to detect the
thermal emission of the disc and to estimate the mass of the black hole. The
non-thermal emission is well reproduced by a one-zone leptonic model. The
non-thermal radiative processes are synchrotron, self-Compton and external
Compton using seed photons from both the broad-line region (BLR) and the torus.
We find that our data are better reproduced if we assume that the location of
the dissipation region of the jet, R_diss, is in-between the torus, (at
R_torus), and the BLR (R_torus>R_diss>R_BLR). The observed variability is
explained by changing a minimum number of model parameters by a very small
amount.
