1. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa A Photoelectric Polarimeter based on a Micro Pattern Gas Detector for X-Ray Astronomy Ronaldo Bellazzini INFN - Pisa

2. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa 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) Why X-ray Astrophysical Polarimetry?

3. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa Polarization from Supernova Remnants: The Crab case Crab-Nebula shows the same degree and angle of polarization from radio to X-rays and this is a signature of synchrotron emission. Radio (VLA) Optical (Palomar) Infrared (Keck) X-rays (Chandra)

4. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa 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. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa 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 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 Accreting X-ray Pulsar

6. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa Heitler W.,The Quantum Theory of Radiation The photo-electric effect is very sensitive to photon polarization Polarization information is derived from the track of the photoelectrons imaged by a finely subdivided gas detector

7. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa Dependence of polar angle of photo-electron in Ne

8. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa 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. Basics of photoeffect in gases

9. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa Projection of MC photoelectron tracks

10. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa The micro-pattern gas detector scheme

11. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa The overall detector assembly and read-out electronics

12. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa The anode charge collection plane

13. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa Microscope picture of the GEM structure Microscope picture of the pixelized read-out

14. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa Electric field structure X ray gas mixture: - Ne / Ar / Kr ….. - Methane/ Ethane C02, DME…... 20 ns  E X photon (E) PCB GEM pixel conversion gain collection

15. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa 5.9 KeV electrons

16. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa

17. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa

18. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa Angular distribution 5.9 KeV unpolarized source 5.4 KeV polarized source Modulation factor = (Cmax – Cmin)/ (Cmax + Cmin) ˜ 50% at 6 KeV MDP scales as:for bright sources for faint sources

19. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa Scatter plot of the baricenters relative to the reconstructed impact point 5.9 KeV unpolarized source 5.4 KeV polarized source No rotation of the detector is needed!

20. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa 5.9 KeV unpolarized source 5.4 KeV polarized source

21. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa The path of the electron has been seriously affected by Coulomb scattering on a nucleus so that the mean direction of the track (red line) is quite different from the starting direction. A better reconstruction is made after identification of the conversion point and removal of the final part of the track Modulation factor dependance on the track length used in the reconstruction. Real photoelectron track

22. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa Red line – direction of the photoelectrons using the baricenter information Green line – reconstructed direction using impact point

23. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa

24. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa

25. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa Imaging capability BaricentersImpact points

26. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa Modulation factor as a function of X-ray energy (no cuts on the events) Simulation of a MPGD with 80/20 Neon-DME mixture, 100 m  pith readout plane and 1 cm absorption thickness

27. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa Present and optimized configuration for astrophysical applications

28. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa PCB read-out anodes Next technological step VLSI pixel chip from digital X-ray camera

29. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa XEUS: a telescope comprising a Detector Spacecraft that receives cosmic X-rays focused by a Mirror Spacecraft f.lying exactly 50 m in front of it

30. X-Ray Polarimetry with Micro Pattern Gas Detectors SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa Conclusions The performances obtained with the tested prototype have resulted much better than those of any actual traditional X-ray Polarimetry. In its improved configuration the MPGD target performance is the detection of 1% polarization for 1 mCrab sources. This sensitivity will allow polarimetry measurements to be made on thousands of galactic and extragalactic sources: a real breakthrough in X-ray astronomy.