Low optical polarisation at the core of the optically-thin jet of M87. (arXiv:2005.14719v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Fresco_A/0/1/0/all/0/1">A.Y. Fresco</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fernandez_Ontiveros_J/0/1/0/all/0/1">J.A. Fernández-Ontiveros</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Prieto_M/0/1/0/all/0/1">M.A. Prieto</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Acosta_Pulido_J/0/1/0/all/0/1">J.A. Acosta-Pulido</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Merloni_A/0/1/0/all/0/1">A. Merloni</a>
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$_{rm lin}$) is detected in the
bright jet knots resolved at $sim 10, rm{arcsec}$ to $23, rm{arcsec}$
($0.8$-$1.8, rm{kpc}$) from the centre, whereas the nucleus and inner jet
show P$_{rm lin} lesssim 5$ 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$_{rm lin} sim 2$-$3$ per cent), and no emission was
detected from HST-1. No circular polarisation was detected either in the
nucleus or the jet above a $3sigma$ level of P$_{rm circ} leq 1.5$ per cent,
discarding the conversion of P$_{rm lin}$ into P$_{rm circ}$. A disordered
magnetic field configuration or a mix of unresolved knots polarised along axes
with different orientations could explain the low P$_{rm lin}$. 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$_{rm lin}$ and P$_{rm circ}$ 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.
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$_{rm lin}$) is detected in the
bright jet knots resolved at $sim 10, rm{arcsec}$ to $23, rm{arcsec}$
($0.8$-$1.8, rm{kpc}$) from the centre, whereas the nucleus and inner jet
show P$_{rm lin} lesssim 5$ 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$_{rm lin} sim 2$-$3$ per cent), and no emission was
detected from HST-1. No circular polarisation was detected either in the
nucleus or the jet above a $3sigma$ level of P$_{rm circ} leq 1.5$ per cent,
discarding the conversion of P$_{rm lin}$ into P$_{rm circ}$. A disordered
magnetic field configuration or a mix of unresolved knots polarised along axes
with different orientations could explain the low P$_{rm lin}$. 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$_{rm lin}$ and P$_{rm circ}$ 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.
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