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 \gamma -ray flares , while optical-UV fluxes show little variations .
During the GeV outburst , the highest \gamma -ray flux measured is ( 5.22 \pm 1.10 ) \times 10 ^ { -6 } { ph cm ^ { -2 } s ^ { -1 } } in the range of 0.1 - 300 GeV , which corresponds to an isotropic \gamma -ray luminosity of ( 1.62 \pm 0.44 ) \times 10 ^ { 50 } erg s ^ { -1 } , thereby making this as one of the most luminous \gamma -ray flare ever observed from any blazar .
A fast \gamma -ray flux rising time of \sim 3 hours 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 single zone leptonic emission model .
In all the 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 suggests the enhancement in bulk Lorentz factor as a primary cause of the \gamma -ray flare .
The high X-ray activity with less variable \gamma -ray counterpart can be due to emission region to be 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 .