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 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

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

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 !!

SOLAR FLARES X-RAY EMISSION 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

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 < W/m^2 C < = I < W/m^2 M < = I < W/m^2 X I > = W/m^2

Thermal bremsstrahlung with a low degree of polarization expected (few per cent) Non-thermal bremsstrahlung expected to be highly polarized up to % 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)]

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)

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)]

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

( keV) OR WHAT TO DO… Telescope Coded Mask Aperture Telescope GPD Compton

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)

RHESSI [gamma-rays (blue) and X-rays (red)] and TRACE [UV image]View of January 20, 2005 Solar Flare. (

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.