The broad band energy distribution of the misaligned gamma-ray blazar PKS 0521-365

Abstract

Simultaneous UV and X-ray observations of the blazar PKS 0521-365 are combined with radio, IR, optical and gamma-ray data taken at different epochs to study the overall emission from 10^9 to 10^23 Hz. The energy distribution declines from the IR to the UV range (alpha_opt-UV = 1.4 +/- 0.1), buthardens at higher frequencies. The remarkably flat X-ray spectrum (energy index alpha = 0.68 +/- 0.12 in the range 0.2-7 keV) suggests a significant contribution by the inverse Compton process in this band. This is supported by the detection of the source in gamma-rays by the EGRET experiment. The full radio-to-gamma-ray energy distribution is discussed. Various arguments indicate that the jet is not closely aligned with the line of sight (theta ~ 30 deg), implying that the observed radiation is weakly boosted. This allows a direct estimate of the importance of different sources of photons available for upscattering. We conclude that in this source the synchrotron photons are likely to be the seeds for the inverse Compton process.