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Polarization from celestial sources may derive from:

Explore the importance of X-ray astrophysical polarimetry and its application in studying celestial sources and strong field gravity near compact sources. Learn about the principles of polarization and how micro-pattern gas detectors can provide valuable data.

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Polarization from celestial sources may derive from:

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  1. X-Ray Polarimetry with a Micro Pattern Gas Detector with Pixel Read-outRonaldo BellazziniINFN - Pisa

  2. Why X-ray Astrophysical Polarimetry? • Polarization from celestial sources 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) • Vacuum polarization and birefringence through extreme magnetic fields • (Gnedin et al., 1978; Ventura, 1979; Mészáros & Ventura, 1979)

  3. Polarization from Supernova Remnants: The Crab case Radio (VLA) Infrared (Keck) Optical (Palomar) X-rays (Chandra) Crab-Nebula shows the same degree and angle of polarization from radio to X-rays and this is a signature of synchrotron emission.

  4. X-ray polarimetry offers a definitive test of strong field gravity near very compact sources: Black Hole binaries, Neutron Stars and microquasars... Unlike spectral data, polarization data are strongly affected by general relativistic effects. For example: A BH is surrounded by an optically thick and geometrically thin accretion disk. Heigher energy photons come from smaller disk radii. As a consequence, as the photon energy increases from 1 to 10 KeV, the plane of linear polarization will swing smoothly trough an angle of ˜27° for a 9 Solar Mass BH and 40º for an extreme Kerr BH (for an inclination of 41º). This effect is due to the strong gravitational bending of light rays.

  5. Polarimetry would add to energy and time two further observable quantities, the amount and the angle of polarization, constraining any model and interpretation: a theoretical/observational breakthrough.”P. Meszaros et al. 1988 • Simulated view of an accretion disk around a black hole as it appears to a distant observer • Light bending makes visible the bottom part of a disk. • - Doppler boosting produce an increased intensity of one-side Accreting X-ray Pulsar

  6. The photo-electric effect is very sensitive to photon polarization Heitler W.,The Quantum Theory of Radiation Polarization information is derived from the track of the photoelectrons imaged by a finely subdivided gas detector

  7. Dependence of polar angle of photo-electron in Ne

  8. Basics of photoeffect in gases The photoelectron is slowed by ionizing collisions with outer electrons of the atoms of the medium. The energy loss increases with decreasing kinetic energy (Bethe law for low energy). Electrons are also scattered by charges in the nuclei with no significant energy loss . This follows the screened Rutherford law : While scattering crucially depends on the atomic number, slowing down is only moderately dependent. The photoelectron leaves in the absorber a string of electron/ion pairs, marking the path from its creation to the stopping point. We call this cluster a “track”: in the initial part of this track resides the information on the original electron direction and thence the key to derive the polarization of the photon. This dependence is preserved if the track is projected onto a plane perpendicular to the radiation.

  9. Projection of MC photoelectron tracks

  10. The micro-pattern gas detector scheme

  11. The overall detector assembly and read-out electronics

  12. The anode charge collection plane

  13. Microscope picture of the GEM structure Microscope picture of the pixelized read-out

  14. X photon (E) conversion GEM gain collection pixel PCB a E 20 ns X ray gas mixture: - Ne / Ar / Kr ….. - Methane/ Ethane C02, DME…... Electric field structure

  15. 5.9 KeV electrons

  16. 5.9 KeV unpolarized source 5.4 KeV polarized source MDP scales as: for bright sources for faint sources Angular distribution Modulation factor = (Cmax – Cmin)/ (Cmax + Cmin) ˜50% at 6 KeV

  17. 5.9 KeV unpolarized source 5.4 KeV polarized source Scatter plot of the baricenters relative to the reconstructed impact point No rotation of the detector is needed!

  18. 5.9 KeV unpolarized source 5.4 KeV polarized source

  19. Red line – direction of the photoelectrons using the baricenter information Green line – reconstructed direction using impact point

  20. Imaging capability Baricenters Impact points

  21. Present and optimized configuration for astrophysical applications

  22. Next technological step PCB read-out anodes VLSI pixel chip from digital X-ray camera

  23. According to Nature….. “ the work is highly significant for high energy astrophysics and astronomy in general. X-ray polarimetry is a unique probe of particle acceleration in the universe. It will provide a new tool for studying the fascinating and poorly understood jet sources. The instrumentation described here will very likely revolutionize this area of study …..”

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