Download Polarization radiation from the accretion disk and

Polarization radiation from the accretion disk
and proto-planetary formation mechanisms
and application diagnostics processes in them
Prof. Lachezar FILIPOV
Bulgarian Academy of Sciences
Space and Technology Research Institute
Head of Astrophysical and Space Dynamics Department
The idea of this report is to apply the study of
the polarization of the radiation from different
accretion disks as a method for describing the
structure and dynamics of different formations
in them.
Nonlinear Physics of Astrophysical Discs
• The idea is we study nonlinear physics of
accretion discs, which gives us the
existence of a wide range of structures.
• The question of how to use the sensitivity
of the polarization to use it in the study of
these patterns , their dynamics and
evolution.
DIFINITION
Light polarization - the orientation of the electric field
vector and the magnetic induction of a light wave in a
plane perpendicular to the light beam. Polarization is
usually the reflection and refraction of light, as well as
the propagation of light in an anisotropic medium.
Distinguish between linear, circular and elliptical
polarization of light.
DIFINITION-!
Something more general would like to set as a
polarization-division of the radiation corresponding to
the interaction (reflection, refraction or ?) with the
environment (dust, plasma, field or any other form of
energy)
Polarization of light: definitions and terms
Harmonic electric field monochromatic plane wave:
Ax, Ay - two orthogonal components (fashion) e. vector,  - phase difference
Ax  Ay,  = 0 - linear polarization
Ax = Ay,  = / 2 - circular
polarization
Ax Ay,
0,  / 2 - is elliptic
polarization
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Polarized radiation in astrophysical objects
Main sources of polarized radiation :
•
•
•
•
•
Light scattering in the gas and dust shells around stars
Polarization of starlight by interstellar dust (interstellar polarization)
Polarization of light in stellar atmospheres in the presence of magnetic field
(Zeeman effect)
Polarized radiation of electrons moving in a magnetic field (cyclotron and
synchrotron radiation)
Polarization and strong gravity effects around black-holes
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I will not discuss in detail the processes that underlie the
mechanisms of polarization, they were very well developed
mentioned several of the submissions in the past two days.
• Roberto Casini-light and mater
• Luca Belluzzi - scattering
• Richard Ignace - circumstellar media
• Michal Dovcak- strong gravity
• Frederic Marin- STOKES code
• Marko Stalelski – SKIRT code – AGN dust
Generalities
Polarization = phenomenon connected with the transversality character of electromagnetic
waves.
In principle, polarization can be defined for any kind of transverse waves
(elastic waves in a solid, seismic waves, waves in a guitar string, etc.
Longitudinal waves “have no polarization”.
For studying polarization, more than for any other discipline of physics, the
famous words of Galileo still sound extremely approriate:
“The Universe is written in mathematical language, and its characters are triangles,
circles, and other geometrical figures, without which it is humanly impossible to
understand a single word.”
General challenge of astronomical polarimetry: to lower the sensitivity limits
of the available instruments to smaller and smaller values (10-2, 10-3, 10-4, 10-5…..). It has to be
remarked that this is “a never ending story”…..
Scattering polarization
One of the most important phenomena in the physics of polarization concerns the polarization
properties of scattered radiation. Scattering can take place either on free electrons (Thomson
scattering or Compton scattering, according to the energy of the incident photon, as compared
to the rest mass of the electron), on bound electrons (Rayleigh sctattering on atoms or
molecules).
The physical laws controlling such properties have been initially derived from classical physics
and they have been later generalized to handle relativistic and quantum effects.
In classical terms, the laws of scattering polarization can be simply derived by first considering
the acceleration of the electron, resulting from the electric field of the incident, polarized
electromagnetic radiation beam, and by then evaluating the polarization of the radiation emitted
by the accelerated electron according to the standard theory based on the Liénard & Wiechart
potentials. By a similar procedure it is possible to derive the theoretical polarization of
bremsstrahlung, cyclotron and synchrotron radiation. In these cases the acceleration of the
electron is just produced either by the collision or by the Lorentz force due to the magnetic field.
Rayleigh Scattering
Unfortunately, when modelling astronomical objects, we are often (not to
say always) confronted with very complicated geometrical scenarios.
Obviously, we do not have the freedom, as usual in laboratory experiments,
to set things in such a way to get what is generally called the “good
geometry”. It is then very useful to introduce particular quantities that, from
one side, can handle in a compact way even the most complicated
geometrical situations, and, on the other hand, can keep track in the
mathematical description of the relevant symmetries of the problem.
More general theoretical schemes
The scattering laws previously illustrated are results that can be obtained as limiting case of a
general theoretical framework that has been developed mostly for the interpretation of solar
observations and for the diagnostics of solar magnetic fields in sunspots, active regions,
prominences, etc.
This general framework is referred to as the “Theory of the generation and transfer of polarized
radiation”. It aims at giving a self-consistent description of the polarized radiation field propagating
through an astrophysical plasma and of the statistical properties, in terms of populations and
“coherences”, of the atoms (or molecules) composing the plasma itself.
The “atomic system” is described in terms of its density matrix whereas the radiation field is
described through the Stokes parameters. By standard procedures of quantum electrodynamics
two sets of equations are obtained: the radiative transfer equations for polarized radition and the
statistical equilibrium
equations for the density matrix of the atomic system.
Accretion disks
- protostellar disks
- close binaries
- active galactic nuclei (AGNs)
T Tauri YSO, image by NASA
Illustration, D.Darling
Accretion disks
- protostellar disks
- close binaries
- active galactic nuclei (AGNs)
Illustration, NASA
Scheme of the model
polarized
unpolarized
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April 13, 2011
Magnetic fields in stars
A special group of interacting binary: cataclysmic binary (CVs).
Close pair of stars K-M dwarf + white dwarf. Orbital period - a few hours.
Dual system would fit within the orbit of the Moon (!)
Depending on the degree of magnetism of the white dwarf CVs are divided into "regular" (dwarf novae,
recurrent new nova variables) and magnetic (polars).
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Accretion Dynamics
Regular cataclysmic binary (left). Substance from red to white dwarf flows , forming around rapidly
rotating accretion disk . Temperature in the inner parts of the disk can reach 106 K. The accretion rate
is so high that the material does not have time to settle on the surface of the white dwarf. Accumulation
of material in the disk leads to its instability and explosion, observed as New flash .
Intermediate polars ( in the center ) . White dwarf has a strong magnetic field ( ~ 106 - 107 G), which
destroys the accretion disk near its surface . From a certain distance , the accretion of matter is
along the field lines of the white dwarf to its magnetic poles. White dwarf rotates around its axis
asynchronously ( faster than the orbits ) .
Polar Star or type AM Her ( right). The magnetic field of the white dwarf has a tension ~ 108 G. Disc
not formed at all, the accretion of matter is made directly to one of the poles by the accretion flow. Axial
rotation of the white dwarf is synchronized with the orbital .
Accretion disks
- protostellar disks
- close binaries
- active galactic nuclei (AGNs)
Quasar PKS 1127-145, image by Chandra
Illustration, NASA/ M.Weiss
Why X-ray Astrophysical Polarimetry ?
Polarization from celestial sources in X-rays may derive from:
•Emission processes themselves : cyclotron, synchrotron, non-thermal bremmstrahlung
(Westfold, 1959; Gnedin & Sunyaev, 1974; Rees, 1975)
•Scattering on aspherical accreting plasmas : disks, blobs, columns. (
Rees,
1975 Sunyaev & Titarchuk, 1985; Mészáros, P. et al. 1988, Sazonov 2002).
• Vacuum polarization
1979)
(Gnedin et al., 1978; Ventura, 1979; Mészáros & Ventura,
Accretion powered pulsars
Rotation powered pulsars & magnetars
X-ray polarimetry and strong gravity effects
around black-holes
Matter very close to the black hole experiences General and Special Relativity (due to
the large velocities involved) called Strong Gravity.
Galactic black holes
In galactic black-hole the disk emits in X-ray. The thermal emission by scattering becomes
polarized and the polarization signature of strong gravity is a continuous variation with
energy of the polarization degree and angle.
AGN
In AGNs the disk emits in UV and the X-ray polarization signature of strong gravity is
possible thanks to variability of the reflection component of the observed radiation.
X-ray polarisation in black hole accretion disk
Test of strong field gravity
CONCLUSION
Given the above, the ideas would be
very important to develop theoretical
and experimental methods for the
accurate diagnosis of relativistic
objects.
THANK YOU
FOR
YOUR ATTENTION !