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Evolving X-ray Polarimetry towards high energy and solar science Sergio Fabiani Universit degli Studi di Roma Tor Vergata INAF / IAPS I A P S Istituto di Astrofisica e Planetologia Spaziali

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OUTLINE Polarimetry Basics Solar Flares X-ray Emission Solar Flares X-ray Polarization Photoelectric Polarimeter (Gas Pixel Detector Low Energy: 2-35 keV) Compton Polarimeter (High Energy : starting from 20 keV) Conclusions

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POLARIMETRY BASICS Polarimetry = Analyser + Detector Axis Analyser : For analysing different angles of polarization with respect to an axis Detector : For detecting photons for each angle For 100 % polarized radiation we define the MODULAITON FACTOR Unpolarized radiation same probability for all angles flat response Polarized radiation different probability for different angles Modulated response

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POLARIMETRY BASICS Polarization Degree Minimum Detectable Polarization (at 99% confidence level) S : source rate B : background rate T : integration time If S >> B (source dominated) N of photons needed to achieve a value of MDP For MDP=1%, with =0.5 We need to detect 736 *10^3 photons A LOT OF COUNTS !!

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SOLAR FLARES X-RAY EMISSION http://solarb.msfc.nasa.gov/news/07192008.html http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=14 Magnetic reconnection Heating of plasma Acceleration of electrons Bremmsstrahlung emission Compton back scattering Polarimetry can give information about: Magnetic Field Directivity of accelerated electrons Plasma emitting source geometry

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SOLAR FLARES X-RAY EMISSION Flares are classified according to the order of magnitude of the peak burst intensity (I) measured at the earth in the 1-8 Angstrom wavelength band (about 1.55 12.4 keV). B I < 10 -6 W/m^2 C 10 -6 < = I < 10 -5 W/m^2 M 10 -5 < = I < 10 -4 W/m^2 X I > = 10 -4 W/m^2

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Thermal bremsstrahlung with a low degree of polarization expected (few per cent) Non-thermal bremsstrahlung expected to be highly polarized up to 40-50 % SOLAR FLARES X-RAY POLARIZATION The RHESSI satellite didn't give a clear result !! [Suarez-Garcia et a. 2006l] [ Zharkova et al. (2010) ] [ X1.5 class flare by Karlicky et al. (2004)] RHESSI results [Emslie & Brown (1980)]

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Gas Pixel Detector Photoelectric polarimeter: polarimetry, image, spectrum, timing 2-35 keV with different gas mixtures He - DME gas mixture (2-10 keV) Ar - DME gas mixture (10-35 keV)

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MDP for flare spectrum previously shown (Dt=16 s) 1 cm^2 GPD collecting effective area Ar (60%) - DME (40%) Pressure 3 bar Gas cell thickness 3 cm SOME ESTIMATION FOR GPD MDP a 1 / (Collecting Effective Area) For achieving low MDP large collecting area is needed Two option for preserving imaging capability: GPD + Coded Mask Aperture (1cm^2) x N : Array option GPD + X-ray telescope (at least some tens of cm^2) [Fabiani et al. (2012)]

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COMPTON POLARIMETER SCHEME Loss of imaging capability if a monolithic scintillator is employed but there is good light collection which allows a good signal detection, For preserving imaging capability could be employed as scatterer a bundle of scintillating fibers coupled with a position sensitive detector. Usual cladded fibers give rise to a large light loss there is good collection only for light photons which undergo total internal reflection. E incoming photon energy E scattered photon energy Scattering and loss of energy converted into light within the scintillator Absorption Coincidence for background reduction

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15 20 35 ( keV) OR WHAT TO DO Telescope Coded Mask Aperture Telescope GPD Compton

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CONCLUSIONS Solar Flares X-ray emission in a wide energy band allows to study: different polarization properties (thermal vs non thermal emission) polarization maps of solar flares with the GPD imaging capabilities At the present many controversial results have been achieved (not only RHESSI results ) Work in progress for characterization and development of instrumentation for X-ray polarimetry covering a wide energy band Photoelectric (2-35 keV) Compton (starting from 20 keV)

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RHESSI [gamma-rays (blue) and X-rays (red)] and TRACE [UV image]View of January 20, 2005 Solar Flare. (http://solarb.msfc.nasa.gov/science/multimedia.html)

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RHESSI. Rotating platform (15 rpm), solar hard X-imaging and spectroscopy. Two different techniques: 1.high energy (> 100 keV) software determination of coincidence event between 9 Germanium detectors. 2.Low energy (< 100 keV) it uses the scattering from a passive Be block collimated toward the sun. The bottom section of the Germanium detectors collects the photons scattered by the Beryllium block.