Anatomy of a blazar in the (sub-)millimeter: ALMA observations of PKS 0521-365⋆
1 Joint ALMA Observatory – ESO, Av. Alonso de Córdova, 3104 Santiago, Chile
2 Bosscha Observatory and Astronomy Research Division, FMIPA, Institut Teknologi Bandung, 40132 Jawa Barat, Indonesia
3 CAUP, Porto University, 4099-002 Porto, Portugal
4 National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA 22903, USA
Received: 8 August 2015
Accepted: 2 November 2015
Aims. We aim at analyzing the (sub-)millimeter emission in a nearby blazar, PKS 0521-365, to study the synchrotron and thermal emission in the different components detected at low frequency.
Methods. We analyzed the archive public data of the ALMA Cycle 0 where PKS 0521-365 is used as a calibrator. A total of 13 projects with 23 dataset were analyzed in Bands 3, 6, and 7 and combined. The whole set of data was combined and wavelet-filtered to obtain a deep image toward PKS 0521-365, reaching a dynamic range of 47 000. The individual emission flux was measured on different dates over a period of 11 months in various components. Finally we analyzed the spectral energy distribution (SED) in each different component, including the radio jet and counter jet.
Results. The point sources detected in the field follow a similar distribution to previous studies. The blazar flux shows large variation especially in Band 3. Different components are observed: core, radio jet, and newly detected counter jet, hot spot (HS), and a disky structure roughly perpendicular to the jet. The HS emission is formed by a point source surrounded by an extended emission. The viewing angle of the jet is about 30° with a Doppler factor of δ = 1.6. The HS is at a distance of 19 kpc from the center. The SED analysis shows a strong variation in the core spectral index, especially in Band 3. The two components in the radio jet have roughly a flat spectral index in Bands 6 and 7.
Conclusions. The different weak and extended components in PKS 0521-365 are detected with the ALMA data. The analysis of both jets constrains the geometrical distance of the HS to the center. The SED presents a different shape in time and frequency for each component. Finally, a new structure is detected roughly perpendicular to the radio jet. and a thermal emission origin is currently favored. Further observations at higher spatial resolution are needed to confirm that hypothesis.
Key words: galaxies: jets / galaxies: ISM / galaxies: active / BL Lacertae objects: individual: PKS 0521-365
FITS files for all the images are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (22.214.171.124) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/586/A70
© ESO, 2016