Thick2 - Thick Target Bremsstrahlung Version 2

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Thick Target Bremsstralung

Introduction

The Thick Target model component can be used to calculate the non-thermal photon spectrum (at X-Ray and γ-Ray energies) due to the interactions of energetic electrons with a thick target plasma. The model makes an assumption that there is no angular dependence for the electron distribution function or radiation, therefore they are isotropic.

Parameters

thick2 - Thick-Target Bremsstrahlung x-ray/gamma-ray spectrum from an isotropic electron distribution Version 2 (~10-100 times faster than Version 1, with a relative error of ~1.e-4) (see [1])

Fitting with Thick Target

The thick target Bremsstralung model adjusts the parameters of the electron spectrum interacting with the plasma, to produce the resulting X-Ray (or γ-Ray) photon spectrum.The model of the electron spectrum is a double power law. In order to reduce it to a single power law the break energy, a[2], can be set to a value below the low energy cutoff or above the high energy cutoff.

Below is an example of the photon spectrum derived from using the thick target bremsstrahlung model for a time interval during the 21Apr2002 flare:

The thick target component of a RHESSI fit modeling the non-thermal part the energy spectrum. The top panel shows count flux vs. energy, the bottom panel shows photon flux v. energy

The fit parameters of the thick target model describe the characteristics of the electron distribution function, by adjusting those parameters the resultant photon flux is changed. It is important to keep this in mind, the electron distribution function is not the same as the photon flux shown above. The parameters for the thick target component shown in the plot above are:

where a[1], a[2], etc... are described in the parameters section. These parameters describe an electron distribution in energy. In this case the distribution has no electrons below 19.1keV and above 32000keV. These are sharp cutoffs in the electron distribution, however because of the bremsstralung mechanism by which the non-thermal photon spectrum is created this electron distribution does create photons below the cutoff energy. The high energy cutoff is very large, consequently it does not affect the spectrum very much because it is only being fit to 40keV. For this same reason setting the break energy to 600keV has very little effect as well. Even though it does not technically meet the requirement of the break energy being greater than the high energy cutoff, because both values are so large compared to the high end of the energy range being fit the electron distribution effectively reduces to a power law with a low energy cutoff of 19.1keV and a delta of 7.4.

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