Multiwavelength Observations of the Gamma-Ray Blazar PKS 0528+134 in Quiescence

摘要

We present multiwavelength observations of the ultraluminous blazar-typeradio loud quasar PKS 0528+134 in quiescence during the period July to December2009. Significant flux variability on a time scale of several hours was foundin the optical regime, accompanied by a weak trend of spectral softening withincreasing flux. We suggest that this might be the signature of a contributionfrom the accretion disk at the blue end of the optical spectrum. The opticalflux is weakly polarized with rapid variations of the degree and direction ofpolarization, while the polarization of the 43 GHz radio core remains steady.Optical spectropolarimetry suggests a trend of increasing degree ofpolarization with increasing wavelength, providing additional evidence for anaccretion disc contribution towards the blue end of the optical spectrum. Weconstructed four SEDs indicating that even in the quiescent state, thebolometric luminosity of PKS 0528+134 is dominated by its gamma-ray emission. Aleptonic single-zone jet model produced acceptable fits to the SEDs withcontributions to the high-energy emission from synchrotron self-Comptonradiation and Comptonization of direct accretion disk emission. Fit parametersclose to equipartition were obtained. The moderate variability on long timescales implies the existence of on-going particle acceleration, while theobserved optical polarization variability seems to point towards a turbulentacceleration process. Turbulent particle acceleration at stationary featuresalong the jet therefore appears to be a viable possibility for the quiescentstate of PKS 0528+134.