Constraining the shear acceleration model for the X-ray emission of large-scale extragalactic jets. (arXiv:2011.03264v2 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Tavecchio_F/0/1/0/all/0/1">F. Tavecchio</a>
The nature of the intense X-ray emission from powerful extragalactic jets at
large ($>1$ kpc) scale is still debated. The scenario that invokes the inverse
Compton scattering of the CMB by electrons is challenged by the lack of
gamma-ray emission in the GeV band. An alternative assumes synchrotron emission
by a distinct population of ultra-high energy electrons. Here we present a
concrete attempt to apply this scenario, exploring the specific model in which
the ultra-high energy electrons are accelerated in a shear layer surrounding
the jet. We limit the study to non-relativistic flows and particle acceleration
is treated by a Fokker-Planck equation. The observed relation between low
energy (radio, optical) and X-ray emission prompts us to assume that the
required population of pre-accelerated particles is provided by a shock
responsible for the acceleration of the electrons emitting at low frequencies.
We apply the model to the emission of the principal knots of the jets of PKS
0637-752 and PKS 1136-135, two of the best studied objects. For the set of
fiducial parameters adopted, the condition that the jet power does not exceeds
a limiting value of $10^{48}$ erg s$^{-1}$ constrains the magnetic field above
$10$ $mu$G and indicates moderate beaming ($deltasimeq 2$) for PKS 0637-752.
For both sources, the requirement that acceleration of the electrons proceeds
faster than radiative cooling can be met if the magnetic turbulence in the
shear layer follows a Kolmogorov spectrum, $I(k)propto k^{-q}$ with $q=5/3$,
but cannot satisfied in the Bohm-like case ($q=1$).
The nature of the intense X-ray emission from powerful extragalactic jets at
large ($>1$ kpc) scale is still debated. The scenario that invokes the inverse
Compton scattering of the CMB by electrons is challenged by the lack of
gamma-ray emission in the GeV band. An alternative assumes synchrotron emission
by a distinct population of ultra-high energy electrons. Here we present a
concrete attempt to apply this scenario, exploring the specific model in which
the ultra-high energy electrons are accelerated in a shear layer surrounding
the jet. We limit the study to non-relativistic flows and particle acceleration
is treated by a Fokker-Planck equation. The observed relation between low
energy (radio, optical) and X-ray emission prompts us to assume that the
required population of pre-accelerated particles is provided by a shock
responsible for the acceleration of the electrons emitting at low frequencies.
We apply the model to the emission of the principal knots of the jets of PKS
0637-752 and PKS 1136-135, two of the best studied objects. For the set of
fiducial parameters adopted, the condition that the jet power does not exceeds
a limiting value of $10^{48}$ erg s$^{-1}$ constrains the magnetic field above
$10$ $mu$G and indicates moderate beaming ($deltasimeq 2$) for PKS 0637-752.
For both sources, the requirement that acceleration of the electrons proceeds
faster than radiative cooling can be met if the magnetic turbulence in the
shear layer follows a Kolmogorov spectrum, $I(k)propto k^{-q}$ with $q=5/3$,
but cannot satisfied in the Bohm-like case ($q=1$).
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