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Evolving X-ray Polarimetry towards high energy and solar science Sergio Fabiani

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

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Evolving X-ray Polarimetry towards high energy and solar science Sergio Fabiani

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  1. Evolving X-ray Polarimetry towards high energy and solar science Sergio Fabiani UniversitàdegliStudi di Roma “Tor Vergata” INAF / IAPS I A P S Istituto di Astrofisica e Planetologia Spaziali

  2. OUTLINE • PolarimetryBasics • 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

  3. 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 Unpolarized radiation → same probability for all angles → flat response Polarized radiation → different probability for different angles → Modulated response For 100 % polarized radiation we define the MODULAITON FACTOR

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

  5. 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 • Polarimetry can give information about: • Magnetic Field • Directivity of acceleratedelectrons • Plasma emitting source geometry • Magneticreconnection • Heating of plasma • Acceleration of electrons • Bremmsstrahlungemission • Compton back scattering

  6. 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).

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

  8. 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)

  9. SOME ESTIMATION FOR GPD • MDP for flare spectrum previouslyshown (Dt=16 s) • 1 cm^2 GPD collectingeffective area • Ar (60%) - DME (40%) • Pressure 3 bar • Gas cellthickness 3 cm [Fabiani et al. (2012)] For achievinglow MDP large collecting area isneeded… MDP a 1 / √ (CollectingEffective Area) • Two option for preservingimagingcapability: • GPD + CodedMask Aperture (1cm^2) x N : Array option • GPD + X-raytelescope (atleast some tens of cm^2)

  10. COMPTON POLARIMETER SCHEME Coincidence for background reduction Scattering and loss of energyconvertedinto light within the scintillator Absorption E incomingphotonenergy E’ scatteredphotonenergy • Loss of imagingcapabilityif a monolithicscintillator • isemployed… butthereisgood light collectionwhich • allows a goodsignaldetection, • For preservingimagingcapabilitycould be employedasscatterer a bundle of scintillatingfiberscoupled with a position sensitive detector. Usualcladdedfibersgive rise to a large light loss … thereisgoodcollectiononly for light photonswhichundergototalinternalreflection.

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

  12. 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)

  13. 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)

  14. RHESSI. Rotating platform (15 rpm), solar hard X-imaging and spectroscopy. Two different techniques: high energy (> 100 keV) software determination of coincidence event between 9 Germanium detectors. 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.

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