RHESSI Spectroscopy - OSPEX User Guide

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==Links to documentation for fitting RHESSI Data==
==Links to documentation for fitting RHESSI Data==
===[[Creating a Spectrum File Using the HESSI GUI]]===
===[[Creating a Spectrum File Using the HESSI GUI]]===
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This document goes through the steps necessary to create a spectrum file using either the HESSI GUI or the IDL Command Line. The documentation is for creating a spectrum file and a spectrometer response matrix (SRM) file for a single detector, but is easily expandable to creating files for multiple detectors.  
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This document goes through the steps necessary to create a spectrum file using either the HESSI GUI or the IDL Command Line. The documentation is for creating a spectrum file and a spectrometer response matrix (SRM) file for a single detector, but is easily expandable to creating files for multiple detectors. There is also [http://beauty.nascom.nasa.gov/~zarro/php/ssw_print.php?file=$SSW/hessi/idl/spectra/hsi_spectrum_sep_det_files.pro an ssw routine] which can be used to create single detector spectrum files. Each file is created with native 1/3 keV energy bins for the detector and 4 second time binning. It covers the whole daytime part of the orbit containing the flare plus the available before and after nighttime data for use in background estimation. Examples of calling the routine from the command line are given in the file header.
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::*[http://beauty.nascom.nasa.gov/~zarro/php/ssw_print.php?file=$SSW/hessi/idl/spectra/hsi_spectrum_sep_det_files.pro An SSW routine to create separate detector spectrum files]
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===[[OSPEX GUI Users Guide|User Guide for OSPEX]]===
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:::An ssw routine that creates a RHESSI spectrum and SRM file for the front segment of each detector given a time interval or flare number. Each file is created with native 1/3 keV energy bins for the detector and 4 second time binning. It covers the whole daytime part of the orbit containing the flare plus the available before and after nighttime data for use in background estimation. Examples of calling the routine from the command line are given in the file header.
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Using a spectrum file and SRM file this document goes through the steps necessary for using the OSPEX GUI to fit a RHESSI spectrum, including defining background, time intervals, a fit model and doing a fit. This section also includes information on individual [[Fit function components]], [[Fitting multiple intervals]], [[View fit results|Viewing Fit Results]], and [[Fitting a new detector|using the final fit results from one detector to fit another]].  
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===[[OSPEX GUI Users Guide|OSPEX GUI User Guide]]===
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Using a spectrum file and SRM file this document goes through the steps necessary for using the OSPEX GUI to fit a RHESSI spectrum, including defining background, time intervals, a fit model and doing a fit.
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:A list of fit function components that can be used to create a model for RHESSI data in OSPEX, with a brief explanation of each component. This is not a complete list of all available components. See [[Fit function components]] for a complete list.
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::*[[Vth - Variable Thermal|Vth - Variable Thermal]]
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:::The variable thermal fit model assumes that the X-ray source is all at the same temperature. The following three parameters can be fixed or allowed to vary during the fitting procedure-
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# Emission measure in units of 10^49 cm^(-3),  
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# Temperature in keV, and
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# Abundance of Fe relative the coronal abundance used in CHIANTI. (The relative abundance of Ni is assumed to be the same as that for Fe for the purposes of calculating the flux in the Fe-Ni complex at ~8 keV.)
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::*[[Bpow - Broken Power Law|Bpow - Broken Power law]]
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:::This function assumes that the spectrum of the X-ray photons incident on RHESSI is a broken power law with the following parameters:
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# Photon flux of the low energy power law at the pivot energy (50 keV by default),
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# Break energy in keV,
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# Spectral index below the break energy, and
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# Spectral index above the break energy.
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This is the simplest model available to fit the non-thermal emission during a flare. 
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::*[[Thick2 - Thick-Target Bremsstrahlung Version 2| Thick Target]]
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:::The thick-target fit model component assumes that the spectrum of the X-ray emitting electrons incident on the thick target is a double power law with both a low-energy and a high-energy cutoff. The non-thermal bremsstrahlung photon spectrum is calculated from the interaction of these energetic electrons as they stop in a cold thick target. The available parameters are as follows:
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# Total electron flux integrated  over all energies,
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# Low energy cutoff in keV
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# High energy cutoff in keV
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# Break energy in keV
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# Powqer-law spectral index below the break energy
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# Power-law spectral index above the braeak energy.
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::*[[Drm mod - Pseudo function for fine tuning RHESSI DRM parameters| DRM Mod]]
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:::The drm_mod function adjusts the gain offset and resolution of a fit model to account for small differences in those quantities from the RHESSI calibration measurements. This function can only be used effectively when the Fe-line complex at 6.7 keV can be seen in the count-flux spectrum.  Also it can only be used with single detector spectrum files.
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::*[[Pileup mod - Pseudo function for correcting pileup|Pileup Mod]]
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:::The pileup_mod function calculates the expected pulse pileup contribution to the count-flux spectrum for the assumed photon spectrum. The default parameters provide the expected count-rate dependency of this component. It's effect is small at low count rates and increases with higher count rate. Any change from the default parameters should be made with caution. Note that pileup_mod can only be used with single detector spectrum files.
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:[[Fitting multiple intervals|Fitting Multiple Intervals]]
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::Fitting Multiple Intervals describes the methods available through the OSPEX GUI to loop through and fit multiple time intervals defined by the user.
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:[[View fit results|Viewing Fit Results]]
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::View Fit Results describes the GUI for viewing the parameters from doing fits of multiple intervals as a function of time or interval number. It also contains an explanation of calculating the non-thermal energy flux from thick-target parameters. 
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:[[Fitting a new detector|Using final fit results from one detector as starting parameters for another]]
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::Fitting a new detector contains instructions on editing an OSPEX script created after doing fits for a single detector in order to use the same time intervals, background selections, and final fit parameters as initial parameters when fitting spectra for a different detector.
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Latest revision as of 19:27, 9 June 2010

Contents

Introduction

This is a general introduction to X-ray spectroscopy from RHESSI front-segment data using the Object-oriented version of the SPectrum EXecutive (OSPEX). It covers the steps necessary to fit RHESSI count-rate spectra, starting with the creation of the spectrum and the Spectrometer Response Matrix (SRM) fits files and going through the processes of defining the background spectrum, the time intervals, and the fit functions to be used. The OSPEX - OBJECT SPECTRAL EXECUTIVE GUIDE contains additional information on using OSPEX. Note that similar techniques can also be used for rear-segment data but the detailed analysis of gamma-ray spectra is not covered here.

Much work has already been done by fitting spectra to data from multiple detectors simultaneously. Detectors are chosen for inclusion in the analysis based on their energy coverage and the energy range of interest. Detector #2, for example is generally not usable below ~20 keV, detector #7 below ~10 keV, and detector #8 is occasionally contaminated by pickup from the spacecraft transmitter during passes over a ground stations. This type of analysis is generally adequate above ~20 keV and provides for the greatest sensitivity, but it does not allow various differences between detectors to be taken into account. It should not be used where the highest spectral accuracy is desired or where it is important to estimate uncertainties on the fitted spectral parameters that depend on the systematic differences between detectors.

The general spectral analysis philosophy adopted here is to analyze the count-rate spectra for each of the nine RHESSI front segments independently. Only in this way can the different characteristics of the nine detectors be fully accounted for. Currently, the following detector characteristics can only be taken into account by analyzing the data from each detector separately:

  1. native 1/3 keV energy bins that have different energy edges for each detector,
  2. conversion factors from the detector pulse amplitude to energy loss in keV,
  3. FWHM energy resolution,
  4. relative central-to-total attenuation of each attenuator, and
  5. pulse pile-up.

A further advantage of analyzing spectra from each detector separately is that each spectrum so obtained is an independent estimate of the true solar spectrum. Thus, the scatter of the values obtained for any one of the various spectral fit parameters gives perhaps the best indication of the uncertainties in that parameter. This is superior to the uncertainties derived from the least-squares fitting routine used in OSPEX since it includes any systematic uncertainties in the response of each detector in addition to the statistical uncertainties considered in OSPEX.

Links to documentation for fitting RHESSI Data

Creating a Spectrum File Using the HESSI GUI

This document goes through the steps necessary to create a spectrum file using either the HESSI GUI or the IDL Command Line. The documentation is for creating a spectrum file and a spectrometer response matrix (SRM) file for a single detector, but is easily expandable to creating files for multiple detectors. There is also an ssw routine which can be used to create single detector spectrum files. Each file is created with native 1/3 keV energy bins for the detector and 4 second time binning. It covers the whole daytime part of the orbit containing the flare plus the available before and after nighttime data for use in background estimation. Examples of calling the routine from the command line are given in the file header.

User Guide for OSPEX

Using a spectrum file and SRM file this document goes through the steps necessary for using the OSPEX GUI to fit a RHESSI spectrum, including defining background, time intervals, a fit model and doing a fit. This section also includes information on individual Fit function components, Fitting multiple intervals, Viewing Fit Results, and using the final fit results from one detector to fit another.


Contact Andy Gopie with any questions, corrections, or suggestions

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