XRay Polarimetry with Micro Pattern Gas Detectors A

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

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

X-Ray Polarimetry with Micro Pattern Gas Detectors Polarization from Supernova Remnants: The Crab case Radio Infrared (VLA) (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. SAMBA – Trieste (Italy) 27/29 May 2002 R. Bellazzini - INFN Pisa

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

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

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

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

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

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

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

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

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

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

X-Ray Polarimetry with Micro Pattern Gas Detectors X ray gas mixture: - Ne / Ar / Kr …. . - Methane/ Ethane C 02, DME…. . . X photon (E) Electric field structure conversion gain collection GEM pixel PCB a E 20 ns SAMBA – Trieste (Italy) 27/29 May 2002 R. Bellazzini - INFN Pisa

X-Ray Polarimetry with Micro Pattern Gas Detectors 5. 9 Ke. V electrons SAMBA – Trieste (Italy) 27/29 May 2002 R. Bellazzini - INFN Pisa

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

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

X-Ray Polarimetry with Micro Pattern Gas Detectors Angular distribution 5. 9 Ke. V unpolarized source 5. 4 Ke. V polarized source Modulation factor = (Cmax – Cmin)/ (Cmax + Cmin) ˜ 50% at 6 Ke. V MDP scales as: SAMBA – Trieste (Italy) 27/29 May 2002 for bright sources for faint sources R. Bellazzini - INFN Pisa

X-Ray Polarimetry with Micro Pattern Gas Detectors Scatter plot of the baricenters relative to the reconstructed impact point 5. 9 Ke. V unpolarized source 5. 4 Ke. V polarized source No rotation of the detector is needed! SAMBA – Trieste (Italy) 27/29 May 2002 R. Bellazzini - INFN Pisa

X-Ray Polarimetry with Micro Pattern Gas Detectors 5. 9 Ke. V unpolarized source SAMBA – Trieste (Italy) 27/29 May 2002 5. 4 Ke. V polarized source R. Bellazzini - INFN Pisa

X-Ray Polarimetry with Micro Pattern Gas Detectors Real photoelectron track 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 SAMBA – Trieste (Italy) 27/29 May 2002 Modulation factor dependance on the track length used in the reconstruction. R. Bellazzini - INFN Pisa

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

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

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

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

X-Ray Polarimetry with Micro Pattern Gas Detectors 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 mm pith readout plane and 1 cm absorption thickness SAMBA – Trieste (Italy) 27/29 May 2002 R. Bellazzini - INFN Pisa

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

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

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

X-Ray Polarimetry with Micro Pattern Gas Detectors 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 m. Crab sources. This sensitivity will allow polarimetry measurements to be made on thousands of galactic and extragalactic sources: a real breakthrough in X-ray astronomy. SAMBA – Trieste (Italy) 27/29 May 2002 R. Bellazzini - INFN Pisa