A complete disclosure of the hidden type-1 AGN in NGC 1068 thanks to 52 yr of broad-band polarimetric observation

Abstract

We create the first broadband polarization spectrum of an active galactic nucleus (AGN) by compiling the 0.1 - 100 microns, 4.9 GHz and 15 GHz continuum polarization of NGC 1068 from more than 50 years of observations. Despite the diversity of instruments and apertures, the observed spectrum of linear continuum polarization has distinctive wavelength-dependent signatures that can be related to the AGN and host galaxy physics. The impact of the Big Blue bump and infrared bump, together with electron, Mie scattering, dichroism and synchrotron emission are naturally highlighted in polarization, allowing us to reveal the type-1 AGN core inside this type-2 object with unprecedented precision. In order to isolate the AGN component, we reconstruct the spectral energy distribution of NGC 1068 and estimate the fraction of diluting light in the observed continuum flux. This allows us to clearly and independently show that, in the case of NGC 1068, Thomson scattering is the dominant mechanism for the polarization in the optical band. We also investigate the effect of aperture on the observed polarization and confirm previous findings on the extension of the narrow line region of NGC 1068 and on the B-band and K-band polarization from the host. Finally, we do not detect statistically significant aperture-corrected polarimetric variations over the last 52 years, suggesting that the parsec-scale morphological and magnetic geometries probably remained stable for more than half a century.