Unveiling the nature of the γ-ray emitting active galactic nucleus PKS 0521-36

PKS 0521-36 is an active galactic nucleus (AGN) with uncertain classification. We investigate the properties of this source from radio to gamma-rays. The broad emission lines in the optical and ultraviolet bands and steep radio spectrum indicate a possible classification as an intermediate object between broad-line radio galaxies (BLRG) and steep spectrum radio quasars (SSRQ). On pc-scales PKS 0521-36 shows a knotty structure similar to misaligned AGN. The core dominance and the gamma-ray properties are similar to those estimated for other SSRQ and BLRG detected in gamma-rays, suggesting an intermediate viewing angle with respect to the observer. In this context the flaring activity detected from this source by Fermi-Large Area Telescope between 2010 June and 2012 February is very intriguing. We discuss the gamma-ray emission of this source in the framework of the structured jet scenario, comparing the spectral energy distribution (SED) of the flaring state in 2010 June with that of a low state. We present three alternative models corresponding to three different choices of the viewing angles theta(v) = 6 degrees, 15 degrees, and 20 degrees. We obtain a good fit for the first two cases, but the SED obtained with theta(v) = 15 degrees if observed at a small angle does not resemble that of a typical blazar since the synchrotron emission should dominate by a large factor (similar to 100) the inverse Compton component. This suggests that a viewing angle between 6 degrees and 15 degrees is preferred, with the rapid variability observed during gamma-ray flares favouring a smaller angle. However, we cannot rule out that PKS 0521-36 is the misaligned counterpart of a synchrotron-dominated blazar.