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Thermal and Nonthermal Contributions
to theFlare X-ray Flux
Brian R. Dennis1, Kenneth J. H. Phillips1, 2, Richard A. Schwartz1, 3, Anne K. Tolbert1,3, and Hugh S. Hudson4
1NASA GSFC, 2NRC Senior Resident Research Associate3SSAI, 4SSL Berkeley
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IntroductionRHESSI provides high resolution imaging and spectroscopy X-ray observations in the critical energy range from a few keV to a few tens of keV. These observations allow the following classic ways to be used to differentiate between the thermal and nonthermal components of the X-ray flux:
– gradually vs. impulsively varying flux
– exponential vs. power-law spectra,
– extended coronal vs. compact footpoint sources
RHESSI also detects the iron-line complex at ~6.7 keV (1.9 Å).
– The peak energy is a function of temperature.
– The equivalent width (line-to-continuum ratio) is a function of temperature and iron abundance.
Mewe (1985/6) and Chianti (1997) give line and continuum spectra as functions of temperature and abundances.
These predictions are compared with RHESSI measurements for the X-flare on 21 April 2002.
ReferencesMewe, Gronenschild, van den Oord, 1985, (Paper V) A. & A. Suppl., 62, 197; Mewe, Lemen, and van den Oord, 1986, (Paper VI) A. & A. Suppl., 65, 511
Mewe, R., Kaastra, J. S., Liedahl, D. A., “Update of MEKA: MEKAL” (http://heasarc.gsfc.nasa.gov/docs/journal/meka6.html) Legacy, Journal of the High Energy Astrophysics Science Archive Research Center (HESARC), NASA GSFC, 6, 16, 1995.
Dere, K. P.; Landi, E.; Mason, H. E.; Monsignori Fossi, B. C.; Young, P. R., “CHIANTI - an atomic database for emission lines” A & A Supplement series, 125, 149-173, 1997.
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X-flare on 21 April 2002
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RHESSI Time History
• Soft X-ray flux (<12 keV) rises gradually suggesting thermal emission.
• Hard X-ray flux (>25 keV) shows impulsive peaks at ~01:16 UT suggesting nonthermal emission.
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RHESSI ContoursWhite: 6 – 12 keVBrown: 12 – 25 keVBlue: 25 – 50 keV
TRACE Image
1.95 nm
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RHESSI Imaging
• Compact hard X-ray sources (>25 keV) on TRACE ribbons suggest nonthermal emission at loop footpoints.
• Extended soft X-ray source (<25 keV) and diffuse TRACE emission in corona suggest thermal emission at ~20 MK.
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Shutters Out
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RHESSI Spectra
Thin Shutters In (A1)
Detector #4
1-minute accumulations
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RHESSI Spectra
Thin Shutters (A1)
Thin + Thick (A3)
Detector #4
1-minute accumulations
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RHESSI Spectra
• Steep spectra before ~01:00 UT suggest thermal emission.
• Flatter power-law spectrum above ~20 keV at 01:16 UT suggests nonthermal emission.
• Peak at ~6.7 keV (1.9 Å) is the iron-line complex from thermal plasma.
• Peak at ~8 keV is most probably the Fe/Ni complex.
• Peak at ~10.5 keV is most probably caused by K-escape from the germanium detectors.
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Mewe & Chianti Thermal Spectra
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Peak Position of Iron-line ComplexE
nerg
y in
keV
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Iron-line ComplexRHESSI Measurements of Peak Energy
A0 A1 A3
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Iron-line Complex - Peak Energy
• Detector-to-detector variations and higher-than-predicted peak energy must be instrument calibration issues.
• Variation of peak energy with time may be related to high detector count rates.
• Reason for sharp rise in peak energy determined from detector #8 after ~01:14 UT is unknown.
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Equivalent Width of Iron-Line Complex
Mewe_spec(coronal - Meyer 1985)
Chianti (coronal abundances)
Chianti (photospheric)
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Iron-line ComplexRHESSI Measurements of Equivalent Width
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Iron-line Complex – Equivalent Width
• Equivalent-width measurements should be relatively independent of detector energy calibration and sensitivity.
• Detector-to-detector differences seem to be related to energy resolution.
• Large differences exist between Mewe (1985/6) and Chianti (1997) predictions.
• Measured equivalent widths are higher than predicted by both Mewe and Chianti, even for coronal abundances (Fe abundance 3 x photospheric).
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Conclusions• RHESSI can differentiate between thermal and
nonthermal emissions.• Iron-line features are clearly seen in RHESSI spectra.• Detector response is still preliminary – work in
progress.• Mewe (1985/6) and Chianti (1997) models of the
thermal spectra differ in the predicted line intensities.• GOES temperatures may be inaccurate since they are
calculated using the older Mewe spectra.