In this study we present linear and circular polarisation maps in the optical rangeof the core and the jet in M87. The observations were acquired two days before theEvent Horizon Telescope campaign in early April 2017. A high degree (∼18%) of linearpolarisation is detected in three knots resolved along the jet from∼10′′up to23′′(0.8–1.8 kpc) from the centre, while the nucleus and the inner jet linear polarisation remainsone order of magnitude below (∼5%). The position angle of the linear polarisationshifts by∼90◦from each of these knots to the adjacent ones, being the core polarisationangle perpendicular to that of the first knot. No circular polarisation signal is detectedeither in the nucleus or along the jet at an rms level of0.5%, for those reasons wediscard the conversion of linear into circular polarisation as a possible explanation forthe low degree of linear polarisation within the inner∼10′′(e.g. via Faraday rotation).This suggests that the large difference in the linear polarisation degree between thenucleus and the extended jet is likely related to a disordered magnetic field distributionat the base of the jet or a mix of unresolved jet components in the nuclear region withdifferent linear polarisation angles. Further observations of the circular polarisationin M87 at higher angular resolution would be required to probe this scenario. In thiswork we also develop the full methodology to recover the four Stokes parameters usinga polarimeter with aλ/4wave-plate.
|Presenter status||eROSITA consortium member|