Multiwavelength Study of High-Redshift Blazars

Abstract

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 (${\Gamma }_{\gamma }\ge 2.2$) whereas those that have been observed in the X-ray band have hard X-ray spectra (${\Gamma }_{\mathrm{X}}=1.01-1.86$). The $\gamma$-ray flux of high-redshift blazars ranges from $4.84\times 10^{-10}$ to $1.50\times 10^{-7}\mathrm{photon}\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$ and the luminosity is within $(0.10-5.54)\times 10^{48}\mathrm{erg}{\mathrm{s}}^{-1}$ which during the $\gamma$-ray flares increases up to $(0.1-1)\times 10^{50}\mathrm{erg}{\mathrm{s}}^{-1}$. 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 $\le 0.05$ 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 $L_{\mathrm{d}}\simeq (1.09-10.94)\times 10^{46}\mathrm{erg}{\mathrm{s}}^{-1}$ and $(1.69-5.35)\times 10^9{M}_{\odot }$, respectively.

Accepted for publication in MNRAS

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