摘要：Context. The giant radio galaxy PBC J2333.9-2343 shows different characteristics at different wavebands that are difficult to explain within the actual generic schemes of unification of active galactic nuclei (AGN). It is therefore a good candidate host for different phases of nuclear activity.
Aims. We aim at disentangling the nature of this AGN by using simultaneous multiwavelength data.
Methods. We obtained data in 2015 from the Very Long Baseline Array (VLBA), the San Pedro Mártir telescope, and the XMM-Newton observatories. This allows the study of the nuclear parts of the galaxy through its morphology and spectra and the analysis of the spectral energy distribution (SED). We also reanalysed previously-presented optical data from the San Pedro Mártir telescope from 2009 to provide a homogeneous comparison.
Results. At X-ray frequencies the source is unabsorbed. The optical spectra are of a type 1.9 AGN, both in 2009 and 2015, although showing a broader component in 2015. The VLBA radio images show an inverted spectrum with a self-absorbed, optically thick compact core (αc = 0.40, where Sν ∝ ν+ α) and a steep-spectrum, optically thin jet (αj,8−15 = −0.5). The SED resembles that of typical blazars and is best represented by an external Compton (EC) model with a viewing angle of approximately 3–6°. The apparent size of the large-scale structure of PBC J2333.9-2343 must correspond to an intrinsic deprojected value of approximately 7 Mpc for θv < 10°, and to >13 Mpc for θv < 5°, a value much larger than the biggest giant radio galaxy known, which is 4.5 Mpc.
Conclusions. The above arguments suggest that PBC J2333.9-2343 has undergone a new episode of nuclear activity and that the direction of the new jet has changed in the plane of the sky and is now pointing towards us. This changes this source from a radio galaxy to a blazar, a very exceptional case of restarting activity.
