Kavli Affiliate: Feng Yuan
| First 5 Authors: Sijia Peng, Ru-Sen Lu, Ciriaco Goddi, Thomas P. Krichbaum, Zhiyuan Li
| Summary:
Faraday rotation is an important probe of the magnetic fields and magnetized
plasma around active galactic nuclei (AGN) jets. We present a Faraday rotation
measure image of the M87 jet between 85.2 GHz and 101.3 GHz with a resolution
of ~2" with the Atacama Large Millimeter/submillimeter Array (ALMA). We found
that the rotation measure (RM) of the M87 core is $rm (4.5pm
0.4)times10^{4} rad m^{-2}$ with a low linear polarization fraction of $rm
(0.88pm 0.08)%$. The spatial RM gradient in the M87 jet spans a wide range
from $sim -2times10^4rm~rad m^{-2}$ to $sim 3times10^4rm~rad m^{-2}$
with a typical uncertainty of $0.3times10^4rm~rad m^{-2}$. A comparison with
previous RM measurements of the core suggests that the Faraday rotation of the
core may originate very close to the super massive black hole (SMBH). Both an
internal origin and an external screen with a rapidly varying emitting source
could be possible. As for the jet, the RM gradient indicates a helical
configuration of the magnetic field that persists up to kpc scale. Combined
with the kpc-scale RM measurements at lower frequencies, we found that RM is
frequency-dependent in the jet. One possible scenario to explain this
dependence is that the kpc-scale jet has a trumpet-like shape and the jet coil
unwinds near its end.
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