Evolving X-ray Polarimetry towards high energy and solar science Sergio Fabiani Università degli...
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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
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
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
Slide 2
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
Slide 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
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
Slide 4
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 !!
Slide 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
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
Slide 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). 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
Slide 7
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)]
Slide 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)
Slide 9
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)]
Slide 10
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
Slide 11
15 20 35 ( keV) OR WHAT TO DO Telescope Coded Mask Aperture
Telescope GPD Compton
Slide 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)
Slide 13
Slide 14
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)
Slide 15
Slide 16
Slide 17
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.