Multiwavelength Study of High-Redshift Blazars
  High-redshift blazars are among the most powerful objects in the Universe.
The spectral and temporal properties of thirty-three distant blazars ($z>2.5$)
detected in the high energy $\gamma$-ray band are investigated by analyzing the
{\it Fermi}-LAT and {\it Swift} UVOT/ XRT data. The considered sources have
soft time averaged $\gamma$-ray spectra ($\Gamma_{\rm \gamma}\geq2.2$) whereas
those that have been observed in the X-ray band have hard X-ray spectra
($\Gamma_{\rm X}=1.01-1.86$). The $\gamma$-ray flux of high-redshift blazars
ranges from $4.84\times10^{-10}$ to $1.50\times10^{-7}\:{\rm
photon\:cm^{-2}\:s^{-1}}$ and the luminosity is within
$(0.10-5.54)\times10^{48}\:{\rm erg\:s^{-1}}$ which during the $\gamma$-ray
flares increases up to $(0.1-1)\times10^{50}\:{\rm erg\:s^{-1}}$. In the X-ray
band, only the emission of PKS 0438-43, B2 0743+25 and TXS 0222+185 is found to
vary in different Swift XRT observations whereas in the $\gamma$-ray band, the
emission is variable for fourteen sources: the flux of B3 1343+451 and PKS
0537-286 changes in sub-day scales, that of PKS 0347-211 and PKS 0451-28 in day
scales, while the$\gamma$-ray variability of the others is in week or month
scales. The properties of distant blazar jets are derived by modeling the
multiwavelength spectral energy distributions within a one-zone leptonic
scenario assuming that the X-ray and $\gamma$-ray emissions are produced from
inverse Compton scattering of synchrotron and dusty torus photons. From the
fitting, the emission region size is found to be $\leq0.05$ pc and the magnetic
field and the Doppler factor are correspondingly within $0.10-1.74$ G and
$10.0-27.4$. By modeling the optical-UV excess, we found that the central black
hole masses and accretion disk luminosities are within $L_{\rm
d}\simeq(1.09-10.94)\times10^{46}\:{\rm erg \: s^{-1}}$ and
$(1.69-5.35)\times10^{9}\:M_\odot$, respectively.
