50 research outputs found
Linear and Circular Polarization Properties of Jets
I discuss the transfer of polarized synchrotron radiation in relativistic
jets. I argue that the main mechanism responsible for the circular polarization
properties of compact synchrotron sources is likely to be Faraday conversion
and that, contrary to common expectation, a significant rate of Faraday
rotation does not necessarily imply strong depolarization. The long-term
persistence of the sign of circular polarization, observed in some sources, is
most likely due to a small net magnetic flux generated in the central engine,
carried along the jet axis and superimposed on a highly turbulent magnetic
field. I show that the mean levels of circular and linear polarizations depend
on the number of field reversals along the line of sight and that the gradient
in Faraday rotation across turbulent regions can lead to "correlation
depolarization''. The model is potentially applicable to a wide range of
synchrotron sources. In particular, I demonstrate how the model can naturally
explain the excess of circular over linear polarization in the Galactic Center
(Sgr A*) and the low-luminosity AGN M81*.Comment: Invited Talk, to appear in "Circular Polarisation in Relativistic Jet
Sources", Astrophysics and Space Science, Fender R.P. and Macquart J.-P.
(Eds
Cosmic ray feedback in galaxies and galaxy clusters -- A pedagogical introduction and a topical review of the acceleration, transport, observables, and dynamical impact of cosmic rays
Understanding the physical mechanisms that control galaxy formation is a
fundamental challenge in contemporary astrophysics. Recent advances in the
field of astrophysical feedback strongly suggest that cosmic rays (CRs) may be
crucially important for our understanding of cosmological galaxy formation and
evolution. The appealing features of CRs are their relatively long cooling
times and relatively strong dynamical coupling to the gas. In galaxies, CRs can
be close to equipartition with the thermal, magnetic, and turbulent energy
density in the interstellar medium, and can be dynamically very important in
driving large-scale galactic winds. Similarly, CRs may provide a significant
contribution to the pressure in the circumgalactic medium. In galaxy clusters,
CRs may play a key role in addressing the classic cooling flow problem by
facilitating efficient heating of the intracluster medium and preventing
excessive star formation. Overall, the underlying physics of CR interactions
with plasmas exhibit broad parallels across the entire range of scales
characteristic of the interstellar, circumgalactic, and intracluster media.
Here we present a review of the state-of-the-art of this field and provide a
pedagogical introduction to cosmic ray plasma physics, including the physics of
wave-particle interactions, acceleration processes, CR spatial and spectral
transport, and important cooling processes. The field is ripe for discovery and
will remain the subject of intense theoretical, computational, and
observational research over the next decade with profound implications for the
interpretation of the observations of stellar and supermassive black hole
feedback spanning the entire width of the electromagnetic spectrum and
multi-messenger data.Comment: invited A&ARv review; revised version; accepted for publication; 238
page
The Efficiency of Magnetic Field Amplification at Shocks by Turbulence
Turbulent dynamo field amplification has often been invoked to explain the
strong field strengths in thin rims in supernova shocks (G)
and in radio relics in galaxy clusters (G). We present high
resolution MHD simulations of the interaction between pre-shock turbulence,
clumping and shocks, to quantify the conditions under which turbulent dynamo
amplification can be significant. We demonstrate numerically converged field
amplification which scales with Alfv\'en Mach number, , up to . This implies that the
post-shock field strength is relatively independent of the seed field.
Amplification is dominated by compression at low , and
stretching (turbulent amplification) at high . For high
, the -field grows exponentially and saturates at
equipartition with turbulence, while the vorticity jumps sharply at the shock
and subsequently decays; the resulting field is orientated predominately along
the shock normal (an effect only apparent in 3D and not 2D). This agrees with
the radial field bias seen in supernova remnants. By contrast, for low
, field amplification is mostly compressional, relatively
modest, and results in a predominantly perpendicular field. The latter is
consistent with the polarization seen in radio relics. Our results are
relatively robust to the assumed level of gas clumping. Our results imply that
the turbulent dynamo may be important for supernovae, but is only consistent
with the field strength, and not geometry, for cluster radio relics. For the
latter, this implies strong pre-existing -fields in the ambient cluster
outskirts.Comment: 15 pages, 11 figures, published version on MNRA
Heating, conduction and minimum temperatures in cooling flows
There is mounting observational evidence from Chandra for strong interaction
between keV gas and AGN in cooling flows. It is now widely accepted that the
temperatures of cluster cores are maintained at a level of 1 keV and that the
mass deposition rates are lower than earlier ROSAT/Einstein values. Recent
theoretical results suggest that thermal conduction can be very efficient even
in magnetized plasmas. Motivated by these discoveries, we consider a ``double
heating model'' which incorporates the effects of simultaneous heating by both
the central AGN and thermal conduction from the hot outer layers of clusters.
Using hydrodynamical simulations, we demonstrate that there exists a family of
solutions that does not suffer from the cooling catastrophe. In these cases,
clusters relax to a stable final state, which is characterized by minimum
temperatures of order 1 keV and density and temperature profiles consistent
with observations. Moreover, the accretion rates are much reduced, thereby
reducing the need for excessive mass deposition rates required by the standard
cooling flow models.Comment: 7 pages, 2 figures, minor changes, accepted for The Astrophysical
Journa