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Hard X and Gamma-ray Polarization: the ultimate dimension (ESA Cosmic Vision 2015-2025)

Hard X and Gamma-ray Polarization: the ultimate dimension (ESA Cosmic Vision 2015-2025). or the Compton Scattering polarimetery challenges. Ezio Caroli, INAF/IASF – Sezione di Bologna. Hard X and Gamma-ray Polarization: the ultimate dimension.

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Hard X and Gamma-ray Polarization: the ultimate dimension (ESA Cosmic Vision 2015-2025)

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  1. Hard X and Gamma-ray Polarization: the ultimate dimension(ESA Cosmic Vision 2015-2025) or the Compton Scattering polarimetery challenges Ezio Caroli, INAF/IASF – Sezione di Bologna

  2. Hard X and Gamma-ray Polarization: the ultimate dimension Conventional analysis (imaging, spectroscopy and timing) of the high energy radiation from cosmic ray sources often provides two or more different models that successfully explain the observations; The combined measurement of polarization angle and degree of linear polarization can provide vital extra informationto discrimate among the models. • Solar FlaresSolar flare emission is contaminated by thermal emission at lower energies and by lines above 1 MeV, the best energy range for polarimetry investigation from solar flares is from 0.1-1 MeV.Models for solar flares predict polarization levels as high as 20 or 30% in this energy band. • Gamma-ray Bursts. Since the peak energy of the GRB spectrum is narrowly distributed at ~ 250 keV, Compton polarimetry is clearly a requirement for this topic. A high level of polarization (>50%) is expected in GRB if the prompt emission, as recently suggested, is the result of a relativistic expansion of strongly magnetized electron-positron plasmas; in the standard (internal-external shock) model, polarization levels of about 10-20% are expected • Other possible sourcesPulsars: hard X-ray polarimetry to understand the extent to which gamma-ray photons are related to those at longer wavelengths (e.g. a polarisation level >20% is expected from the CRAB pulsar). Soft Gamma-ray Repeaters:one by-product of magnetic photon splitting (50-500 keV) is that the reprocessed photons would exhibit a polarization level of ~25%. Massive Black Hole: the geometry of the the accretion disk; For optical thin disks polarization levels as high as 30-60% are possible while in the optically thick regime lower levels (~10%) are predicted. E. Caroli, INAF/IASF-Sezione di Bologna

  3. Polarisation direction  Q factor  (°) Hard X and Gamma-ray Polarization: the ultimate dimension Klein-Nishina cross-section for linearly polarized photons: E. Caroli, INAF/IASF-Sezione di Bologna

  4. Hard X and Gamma-ray Polarization: the ultimate dimension • Advantages of a ‘thick’ pixel detectors: • Each element of the dectection plane is both a scatterer and a detector. Therefore all the sensitive area is used as polarimeter • The geometry of the detector select only events with a scattering angle close to 90º. For these scattering angles we have the best modulation factor. E. Caroli, INAF/IASF-Sezione di Bologna

  5. Thickness (mm) Pixel (mm2) Bias (V/mm) Resistivity (.cm) D.C. (nA) 3.4 – 5 – 7.5 2.5  2.5 ~100 1 – 5 x109 20 - 40 CSP Model Eurorad PR-304 Sensitivity 2 V/pC Sensitivity vs photon energy 70 mV/MeV Rise time < 200 ns Equivalent noise < 3 keV Bias +/- 12 V Hard X and Gamma-ray Polarization: the ultimate dimension The POLCA pixellated CdTe detectors Experiment at the beam line ID15 of the ESRF in July 2002 E. Caroli, INAF/IASF-Sezione di Bologna

  6. Allows extrapolation to a 7x7 matrix Hard X and Gamma-ray Polarization: the ultimate dimension POLCA: Experiment at the beam line ID15 of the ESRF in July 2002 • Corner pixel irradiaton • Rotation of the 4x4 matrix • 90° double events distribution symmetry E. Caroli, INAF/IASF-Sezione di Bologna

  7. Hard X and Gamma-ray Polarization: the ultimate dimension The single events are used to correct the double counts maps for pixel response non uniformity in order to reduce sistematics effects on the evaluation of the Q factor: Nx/y = Mx/y x Nt/Nsxy M = detected double events Nt = Total events Ns = Single events Single events Double events E. Caroli, INAF/IASF-Sezione di Bologna

  8. Hard X and Gamma-ray Polarization: the ultimate dimension POLCA experiment POLCA 7.5 mm 300 keV Monte Carlo MC 7.5 mm 300 keV E. Caroli, INAF/IASF-Sezione di Bologna

  9. Hard X and Gamma-ray Polarization: the ultimate dimension Q Factor in function of energy for CIPHER telescope in the following cases: excluding only central pixel double events, excluding additional first order pixels double events, and additionally second order pixels double events E. Caroli, INAF/IASF-Sezione di Bologna

  10. Hard X and Gamma-ray Polarization: the ultimate dimension Minimum Detectable Polarisation The MDP calculated for the CIPHER (160 cm2) telescope when irradiated by a Crab emission flux in presence of a background noise measured by HEXIS instrument (full active shielding configuration). MDP=5% in 10000 s. E. Caroli, INAF/IASF-Sezione di Bologna

  11. Hard X and Gamma-ray Polarization: the ultimate dimension Polarimetric Sensitivity for CIPHER geometry The polarimetric sensitivity is (3σ, 15000 s): ~20 mCrab over the 40-1000 keV range Crab (10% polarized), 3 hours E. Caroli, INAF/IASF-Sezione di Bologna

  12. Hard X and Gamma-ray Polarization: the ultimate dimension Laue Lens Telescope The lens does not affect the linear polarisation status of the incidence beam (Frontera et al., SPIE 1995) • Focal plane detector requirements: • Sensitive area ≈ 50 cm2 • Efficient at high energy (from 60 to 400 keV) therefore a CdTe thickness from 3 to 10 mm is required, • Pixel size at mm level because the expected point spread function is about 10 mm (FWHM) for photon energies of about 200 keV. • A possible implementation: 32×32 CdTe pixels, where each pixel has a 2×2 mm2 surface area and pixel thickness limits from a least 3 mm up to 10 mm. E. Caroli, INAF/IASF-Sezione di Bologna

  13. Hard X and Gamma-ray Polarization: the ultimate dimension Lens aperture:400 cm2; Focal length 10 m Double events map for a Crab like incidence (100% polarized) flux in the 60-400 keV range Polarisation direction 100 1000 10000 E. Caroli, INAF/IASF-Sezione di Bologna Polarization direction Polarization direction Polarization direction Laue

  14. Hard X and Gamma-ray Polarization: the ultimate dimension Conclusion • Wide field telescope configuration: both experiments (like POLCA) and simulations suggest that with a thick (5-10 mm) CdTe/CZT pixel (few mm2) detector modulation factor up to 0.5 can be achieved in the range 100-500 keV. These results allow to predict that an MDP at level of 1-2% can be obtained with 500 cm2 in about 30 h exposure for a 100 mCrab source flux. • Gamma ray lens configuration: the implementation of the laue lens technique can provide an improvement in sensitivity of about 2 order of magnitude with respect to current hard X and soft gamma ray telescope. A polarimeter in the focal plane of this kind of telescope can attain unprecedented performance: e.g. with a collecting area of 300 cm2 and 10 m focal length MDP ≈ 0.2% can be achieved in 30 hr exposure for a 100 mCrab source flux. Both mission configuration can be evisaged for the next decades, the choice depending on the scienfitic requirements and objectives E. Caroli, INAF/IASF-Sezione di Bologna

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