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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 SIstituto di Astrofisica e Planetologia Spaziali1 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

2

POLARIMETRY BASICSPolarimetry = Analyser + DetectorAxis 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 FACTORUnpolarized radiation same probability for all angles flat response Polarized radiation different probability for different angles Modulated response

3 POLARIMETRY BASICS

Polarization Degree

Minimum Detectable Polarization (at 99% confidence level)S : source rateB : background rateT : integration time

If S >> B (source dominated)N of photons needed to achieve a value of MDPFor MDP=1%, with m=0.5

We need to detect 736 *10^3 photons

A LOT OF COUNTS !!

4SOLAR FLARES X-RAY EMISSION

http://solarb.msfc.nasa.gov/news/07192008.htmlhttp://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=14Magnetic reconnectionHeating of plasmaAcceleration of electronsBremmsstrahlung emissionCompton back scatteringPolarimetry can give information about:

Magnetic Field

Directivity of accelerated electrons

Plasma emitting source geometry5

SOLAR FLARES X-RAY EMISSIONFlares 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^2C 10-6 < = I < 10-5 W/m^2M 10-5 < = I < 10-4 W/m^2X I > = 10-4 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 40-50 %

SOLAR FLARES X-RAY POLARIZATIONThe 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)]7

Gas Pixel Detector

Photoelectric polarimeter: polarimetry, image, spectrum, timing

2-35 keV with different gas mixturesHe - 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 cmSOME ESTIMATION FOR GPDMDP a 1 / (Collecting Effective Area)For achieving low MDP large collecting area is neededTwo 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 energyScattering and loss of energy converted into light within the scintillatorAbsorptionCoincidence for background reduction10

15 20 35 ( keV)

ORWHAT TO DOTelescopeCoded Mask ApertureTelescopeGPDCompton11CONCLUSIONS 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)

12RHESSI [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:

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.