| Summary: | The blazar PKS 0208-512 was in the lowest <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>γ</mi></semantics></math></inline-formula>-ray brightness state during the initial 10 years of observations with the <i>Fermi</i> Gamma-ray Space Telescope (<i>Fermi</i>), which was an order of magnitude lower than its flux state during the EGRET era (1991–2000). The weekly averaged maximum <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>γ</mi></semantics></math></inline-formula>-ray flux of this source during the first 10 years of <i>Fermi</i> observation is nearly a factor of 3 lower than the highest flux observed by EGRET in a single epoch. During the period 2018–2020, the source showed a large <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mi>γ</mi></semantics></math></inline-formula>-ray flare, with the average brightness similar to the period 1991–2000. We observed the source with <i>AstroSat</i>, during its low and high activity states, respectively. We carried out broad-band spectral energy distribution (SED) modeling of the source using a one-zone leptonic emission model during its various brightness states. From the SED modeling, we found that there was an inefficient conversion from the bulk energy to the particle energy during the long-term low-activity states as compared to the high flux state during the EGRET era and the later part of <i>Fermi</i> observation.
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