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26 April 2024 |
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The stratified two-sided jet of Cygnus A. Acceleration and collimation | | B. Boccardi
; T.P. Krichbaum
; U. Bach
; F. Mertens
; E. Ros
; W. Alef
; J.A. Zensus
; | | Date: |
21 Sep 2015 | | Abstract: | High-resolution Very-Long-Baseline Interferometry observations of
relativistic jets are essential to constrain fundamental parameters of jet
formation models. At a distance of 249 Mpc, Cygnus A is a unique target for
such studies, being the only Fanaroff-Riley Class II radio galaxy for which a
detailed sub-parsec scale imaging of the base of both jet and counter-jet can
be obtained. Observing at millimeter wavelengths unveils those regions which
appear self-absorbed at longer wavelengths and enables an extremely sharp view
towards the nucleus to be obtained. We performed 7 mm Global VLBI observations,
achieving ultra-high resolution imaging on scales down to 90 $mu$as. This
resolution corresponds to a linear scale of only ${sim}$400 Schwarzschild
radii (for $M_{mathrm{BH}}=2.5 imes 10^9 M_{odot}$). We studied the
kinematic properties of the main emission features of the two-sided flow and
probed its transverse structure through a pixel-based analysis. We suggest that
a fast and a slow layer, with different acceleration gradients, exist in the
flow. The extension of the acceleration region is large (${sim} 10^4
R_{mathrm{S}}$), indicating that the jet is magnetically-driven. The limb
brightening of both jet and counter-jet and their large opening angles
($phi_mathrm{J}{sim} 10^{circ}$) strongly favor a spine-sheath structure.
In the acceleration zone, the flow has a parabolic shape ($rpropto z^{0.55pm
0.07}$). The acceleration gradients and the collimation profile are consistent
with the expectations for a jet in "equilibrium’’ (Lyubarsky 2009), achieved in
the presence of a mild gradient of the external pressure ($ppropto z^{-k},
kleq2$).} | | Source: | arXiv, 1509.6250 | | Services: | Forum | Review | PDF | Favorites | |
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