Swift Observations of High‐Redshift Radio‐loud Quasars

Abstract

We report on Swift observations of four z>2 radio-loud quasars (0212+735, 0537-286, 0836+710, and 2149-307), classified as blazars. The sources, well-known emitters at soft-to-medium X-rays, were detected at >5 σ with the BAT experiment in 15-150 keV. No flux variability was detected within the XRT and BAT exposures, with the exception of 0836+710, which shows an increase by a factor of 4 in the 15-150 keV flux on a timescale of 1 month. The 0.3-10 keV spectra are well fitted by power-law models, with rather hard continua (photon indices Γ XRT ~1.3-1.5) similarly, the 15-150 keV spectra are described by power laws with Γ BAT ~1.3-1.8. The XRT data exhibit spectral curvature, which can be modeled in terms of either excess absorption along the line of sight or a downward-curved broken power law. In the former case, if the excess N H is at the rest frame of the source, columns of N z H =(0.3-6)×10 22 cm -2 are measured. Modeling of the SEDs of the four quasars shows that the emission at the higher frequencies, >~10 16 Hz, is dominated by the jet, while the steep optical-to-UV continua, observed with the UVOT, can be attributed to thermal emission from the accretion disk. The disk luminosity is between 1% and 10% of the jet power, similar to other powerful blazars.