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. 17 March 2005 E. Caroli, INAF/IASF-Sezione di Bologna 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 information to discrimate among the models. Solar FlaresSolar Flares Solar 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 sourcesOther possible sources Pulsars: 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. Hard X and Gamma-ray Polarization: the ultimate dimension

3. 17 March 2005 E. Caroli, INAF/IASF-Sezione di Bologna Hard X and Gamma-ray Polarization: the ultimate dimension Klein-Nishina cross-section for linearly polarized photons:   (°) Q factor  Polarisation direction

4. 17 March 2005 E. Caroli, INAF/IASF-Sezione di Bologna 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.

5. 17 March 2005 E. Caroli, INAF/IASF-Sezione di Bologna Hard X and Gamma-ray Polarization: the ultimate dimension 20 - 40 1 – 5 x10 9 ~100 2.5  2.5 3.4 – 5 – 7.5 D.C. (nA) Resistivity ( .cm) Bias (V/mm) Pixel (mm 2 ) Thickness (mm) +/- 12 VBias < 3 keVEquivalent noise < 200 nsRise time 70 mV/MeVSensitivity vs photon energy 2 V/pCSensitivity CSP Model Eurorad PR-304 The POLCA pixellated CdTe detectors Experiment at the beam line ID15 of the ESRF in July 2002

6. 17 March 2005 E. Caroli, INAF/IASF-Sezione di Bologna Hard X and Gamma-ray Polarization: the ultimate dimension Corner pixel irradiaton Rotation of the 4x4 matrix 90° double events distribution symmetry Allows extrapolation to a 7x7 matrix POLCA: Experiment at the beam line ID15 of the ESRF in July 2002

7. 17 March 2005 E. Caroli, INAF/IASF-Sezione di Bologna Hard X and Gamma-ray Polarization: the ultimate dimension Single eventsDouble events 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: N x/y = M x/y x N t /N sxy M = detected double events N t = Total events N s = Single events

8. 17 March 2005 E. Caroli, INAF/IASF-Sezione di Bologna Hard X and Gamma-ray Polarization: the ultimate dimension POLCA experiment Monte Carlo POLCA 7.5 mm 300 keV MC 7.5 mm 300 keV

9. 17 March 2005 E. Caroli, INAF/IASF-Sezione di Bologna 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

10. 17 March 2005 E. Caroli, INAF/IASF-Sezione di Bologna Hard X and Gamma-ray Polarization: the ultimate dimension The MDP calculated for the CIPHER (160 cm 2 ) 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. Minimum Detectable Polarisation

11. 17 March 2005 E. Caroli, INAF/IASF-Sezione di Bologna Hard X and Gamma-ray Polarization: the ultimate dimension The polarimetric sensitivity is (3σ, 15000 s): ~20 mCrab over the 40-1000 keV range Polarimetric Sensitivity for CIPHER geometry Crab (10% polarized), 3 hours

12. 17 March 2005 E. Caroli, INAF/IASF-Sezione di Bologna Hard X and Gamma-ray Polarization: the ultimate dimension Focal plane detector requirements: Sensitive area ≈ 50 cm 2 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. Laue Lens Telescope The lens does not affect the linear polarisation status of the incidence beam (Frontera et al., SPIE 1995)

13. 17 March 2005 E. Caroli, INAF/IASF-Sezione di Bologna Hard X and Gamma-ray Polarization: the ultimate dimension Laue Polarizationdirection Polarizationdirection Polarizationdirection Lens aperture:  400 cm 2 ; Focal length 10 m Double events map for a Crab like incidence (100% polarized) flux in the 60-400 keV range Polarisation direction 100100010000

14. 17 March 2005 E. Caroli, INAF/IASF-Sezione di Bologna 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 mm 2 ) 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 cm 2 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 cm 2 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