The high-redshift blazar S5 0836+71: A broadband study

A broadband study of the high-redshift blazar S5 0836+71 (z = 2.172) is presented. Multi-frequency light curves show multiple episodes of X-ray and γ-ray flares, while optical-UV fluxes show little variations. During the GeV outburst, the highest γ-ray flux measured is (5.22 ± 1.10) × 10⁻⁶$phc{m}^{-2}{s}^{-1}$ in the range of 0.1−300 GeV, which corresponds to an isotropic γ-ray luminosity of (1.62 ± 0.44) × 10⁵⁰ erg s${}^{-1}$, thereby making this one of the most luminous γ-ray flares ever observed from any blazar. A fast γ-ray flux rising time of ∼3 hr is also noticed, which is probably the first measurement of hour-scale variability detected from a high-redshift ($z>$ 2) blazar. The various activity states of S5 0836+71 are reproduced under the assumption of the single-zone leptonic emission model. In all states, the emission region is located inside the broad-line region, and the optical-UV radiation is dominated by the accretion disk emission. The modeling parameters suggest the enhancement in bulk Lorentz factor as a primary cause of the γ-ray flare. The high X-ray activity with a less variable γ-ray counterpart can be due to the emission region being located relatively closer to the black hole where the dominating energy density of the disk emission results in higher X-ray flux due to inverse-Compton scattering of disk photons.