# Head of Astrophysical and Space Dynamics Department

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Head of Astrophysical and Space Dynamics Department
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

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

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 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).

Accretion Dynamics

Regular cataclysmic binary (left)
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 ( ~ 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 , 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 (Gnedin et al., 1978; Ventura, 1979; Mészáros & Ventura, 1979)

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 !