Kinematics of the M87 jet in the collimation zone: gradual acceleration and velocity stratification. (arXiv:1911.02279v1 [astro-ph.HE])

Kinematics of the M87 jet in the collimation zone: gradual acceleration and velocity stratification. (arXiv:1911.02279v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Park_J/0/1/0/all/0/1">Jongho Park</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hada_K/0/1/0/all/0/1">Kazuhiro Hada</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kino_M/0/1/0/all/0/1">Motoki Kino</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nakamura_M/0/1/0/all/0/1">Masanori Nakamura</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hodgson_J/0/1/0/all/0/1">Jeffrey Hodgson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ro_H/0/1/0/all/0/1">Hyunwook Ro</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cui_Y/0/1/0/all/0/1">Yuzhu Cui</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Asada_K/0/1/0/all/0/1">Keiichi Asada</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Algaba_J/0/1/0/all/0/1">Juan-Carlos Algaba</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sawada_Satoh_S/0/1/0/all/0/1">Satoko Sawada-Satoh</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lee_S/0/1/0/all/0/1">Sang-Sung Lee</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cho_I/0/1/0/all/0/1">Ilje Cho</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shen_Z/0/1/0/all/0/1">Zhiqiang Shen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jiang_W/0/1/0/all/0/1">Wu Jiang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Trippe_S/0/1/0/all/0/1">Sascha Trippe</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Niinuma_K/0/1/0/all/0/1">Kotaro Niinuma</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sohn_B/0/1/0/all/0/1">Bong Won Sohn</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jung_T/0/1/0/all/0/1">Taehyun Jung</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhao_G/0/1/0/all/0/1">Guang-Yao Zhao</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wajima_K/0/1/0/all/0/1">Kiyoaki Wajima</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tazaki_F/0/1/0/all/0/1">Fumie Tazaki</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Honma_M/0/1/0/all/0/1">Mareki Honma</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+An_T/0/1/0/all/0/1">Tao An</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Akiyama_K/0/1/0/all/0/1">Kazunori Akiyama</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Byun_D/0/1/0/all/0/1">Do-Young Byun</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kim_J/0/1/0/all/0/1">Jongsoo Kim</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhang_Y/0/1/0/all/0/1">Yingkang Zhang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cheng_X/0/1/0/all/0/1">Xiaopeng Cheng</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kobayashi_H/0/1/0/all/0/1">Hideyuki Kobayashi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shibata_K/0/1/0/all/0/1">Katsunori M. Shibata</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lee_J/0/1/0/all/0/1">Jee Won Lee</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Roh_D/0/1/0/all/0/1">Duk-Gyoo Roh</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Oh_S/0/1/0/all/0/1">Se-Jin Oh</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Yeom_J/0/1/0/all/0/1">Jae-Hwan Yeom</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Jung_D/0/1/0/all/0/1">Dong-Kyu Jung</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Oh_C/0/1/0/all/0/1">Chungsik Oh</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kim_H/0/1/0/all/0/1">Hyo-Ryoung Kim</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hwang_J/0/1/0/all/0/1">Ju-Yeon Hwang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hagiwara_Y/0/1/0/all/0/1">Yoshiaki Hagiwara</a>

We study the kinematics of the M87 jet using the first year data of the KVN
and VERA Array (KaVA) large program, which has densely monitored the jet at 22
and 43 GHz since 2016. We find that the apparent jet speeds generally increase
from $approx0.3c$ at $approx0.5$ mas from the jet base to $approx2.7c$ at
$approx20$ mas, indicating that the jet is accelerated from subluminal to
superluminal speeds on these scales. We perform a complementary jet kinematic
analysis by using archival Very Long Baseline Array monitoring data observed in
$2005-2009$ at 1.7 GHz and find that the jet is moving at relativistic speeds
up to $approx5.8c$ at distances of $200-410$ mas. We combine the two kinematic
results and find that the jet is gradually accelerated over a broad distance
range that coincides with the jet collimation zone, implying that conversion of
Poynting flux to kinetic energy flux takes place. If the jet emission consists
of a single streamline, the observed trend of jet acceleration ($Gammapropto
z^{0.16pm0.01}$) is relatively slow compared to models of a highly magnetized
jet. This indicates that Poynting flux conversion through the differential
collimation of poloidal magnetic fields may be less efficient than expected.
However, we find a non-negligible dispersion in the observed speeds for a given
jet distance, making it difficult to describe the jet velocity field with a
single power-law acceleration function. We discuss the possibility that the jet
emission consists of multiple streamlines following different acceleration
profiles, resulting in jet velocity stratification.

We study the kinematics of the M87 jet using the first year data of the KVN
and VERA Array (KaVA) large program, which has densely monitored the jet at 22
and 43 GHz since 2016. We find that the apparent jet speeds generally increase
from $approx0.3c$ at $approx0.5$ mas from the jet base to $approx2.7c$ at
$approx20$ mas, indicating that the jet is accelerated from subluminal to
superluminal speeds on these scales. We perform a complementary jet kinematic
analysis by using archival Very Long Baseline Array monitoring data observed in
$2005-2009$ at 1.7 GHz and find that the jet is moving at relativistic speeds
up to $approx5.8c$ at distances of $200-410$ mas. We combine the two kinematic
results and find that the jet is gradually accelerated over a broad distance
range that coincides with the jet collimation zone, implying that conversion of
Poynting flux to kinetic energy flux takes place. If the jet emission consists
of a single streamline, the observed trend of jet acceleration ($Gammapropto
z^{0.16pm0.01}$) is relatively slow compared to models of a highly magnetized
jet. This indicates that Poynting flux conversion through the differential
collimation of poloidal magnetic fields may be less efficient than expected.
However, we find a non-negligible dispersion in the observed speeds for a given
jet distance, making it difficult to describe the jet velocity field with a
single power-law acceleration function. We discuss the possibility that the jet
emission consists of multiple streamlines following different acceleration
profiles, resulting in jet velocity stratification.

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