Hard x and gamma ray polarization the ultimate dimension esa cosmic vision 2015 2025
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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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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 esa cosmic vision 2015 2025

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


Hard x and gamma ray polarization the ultimate dimension esa cosmic vision 2015 2025

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


Hard x and gamma ray polarization the ultimate dimension esa cosmic vision 2015 2025

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


Hard x and gamma ray polarization the ultimate dimension esa cosmic vision 2015 2025

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


Hard x and gamma ray polarization the ultimate dimension esa cosmic vision 2015 2025

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


Hard x and gamma ray polarization the ultimate dimension esa cosmic vision 2015 2025

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


Hard x and gamma ray polarization the ultimate dimension esa cosmic vision 2015 2025

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


Hard x and gamma ray polarization the ultimate dimension esa cosmic vision 2015 2025

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


Hard x and gamma ray polarization the ultimate dimension esa cosmic vision 2015 2025

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


Hard x and gamma ray polarization the ultimate dimension esa cosmic vision 2015 2025

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


Hard x and gamma ray polarization the ultimate dimension esa cosmic vision 2015 2025

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


Hard x and gamma ray polarization the ultimate dimension esa cosmic vision 2015 2025

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


Hard x and gamma ray polarization the ultimate dimension esa cosmic vision 2015 2025

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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