Low optical polarisation at the core of the optically-thin jet of M87

Abstract

We study the optical linear and circular polarisation in the optically-thin regime of the core and jet of M87. Observations were acquired two days before the Event Horizon Telescope (EHT) campaign in early April 2017. A high degree ($\sim 20$ per cent) of linear polarisation (P${}_{\mathrm{lin}}$) is detected in the bright jet knots resolved at $\sim 10\mathrm{arcsec}$ to $23\mathrm{arcsec}$ ($0.8$-$1.8\mathrm{kpc}$) from the centre, whereas the nucleus and inner jet show P${}_{\mathrm{lin}}\lesssim 5$ per cent. The position angle of the linear polarisation shifts by $\sim 90$ degrees from each knot to the adjacent ones, with the core angle perpendicular to the first knot. The nucleus was in a low level of activity (P${}_{\mathrm{lin}}\sim 2$-$3$ per cent), and no emission was detected from HST-1. No circular polarisation was detected either in the nucleus or the jet above a $3\sigma$ level of P${}_{\mathrm{circ}}\le 1.5$ per cent, discarding the conversion of P${}_{\mathrm{lin}}$ into P${}_{\mathrm{circ}}$. A disordered magnetic field configuration or a mix of unresolved knots polarised along axes with different orientations could explain the low P${}_{\mathrm{lin}}$. The latter implies a smaller size of the core knots, in line with current interferometric observations. Polarimetry with EHT can probe this scenario in the future. A steep increase of both P${}_{\mathrm{lin}}$ and P${}_{\mathrm{circ}}$ with increasing frequency is expected for the optically-thin domain, above the turnover point. This work describes the methodology to recover the four Stokes parameters using a $\lambda /4$ wave-plate polarimeter.

Accepted for publication in MNRAS. 10 pages, 8 figures

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