Anatomy of a blazar in the (sub-)millimeter: ALMA Observations of PKS 0521-365

Abstract

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. We analyze 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 is analyzed in band 3, 6 and 7 and combined. The whole set of data is combined and wavelet filtered to obtain a deep image reaching a dynamic range of 47000. The individual emission flux is measured at different date over a period of 11 months in various components. Finally we analyze the Spectral Energy Distribution (SED) in each different component, including the radio jet and counter jet. 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 1.6$. The HS is at a distance of 19 kpc from the center. The SED analysis shows a strong variation of the core spectral index, especially in band 3. The two components in the radio jet have roughly a flat spectral index in band 6 and 7. Using these ALMA data the different weak and extended components are detected. 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 favoured. Further observations at higher spatial resolution are needed to confirm that hypothesis.

arXiv comment14 pages, 15 figures, accepted for publication in A&A

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