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Line Features in RHESSI Spectra

Line Features in RHESSI Spectra. Kenneth J. H. Phillips Brian R. Dennis GSFC RHESSI Workshop Taos, NM 10 – 11 September 2003. Line Features in RHESSI Spectra. Two features from highly ionized Fe and Ni. ~6.7 keV Fe XXV lines and satellites Fe XVIII – Fe XXIV lines

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Line Features in RHESSI Spectra

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  1. Line Features in RHESSI Spectra Kenneth J. H. Phillips Brian R. Dennis GSFC RHESSI Workshop Taos, NM 10 – 11 September 2003

  2. Line Features in RHESSI Spectra Two features from highly ionized Fe and Ni. • ~6.7 keV • Fe XXV lines and satellites • Fe XVIII – Fe XXIV lines • Fe XXVI lines at higher T (>80MK) • ~8 keV • Fe XXV & Fe XXVI lines, more highly excited • Ni XXVII, Ni XXVIII lines

  3. The Fe-line and Fe/Ni-line Features • 6.7-keV feature resolved with crystal spectrometers in flares many times. • 8-keV feature hardly ever seen from flares with crystal spectrometers. • RHESSI is the first instrument (apart from NEAR-PIN) to see this part of the spectrum of flares. • RHESSI resolution (~0.8 keV FWHM for detector 4) allows for some diagnostic work.

  4. Chianti Spectrum, T=20MK Ca XIX Fe XXV Fe XXV Fe XXV +satellites Ni XXVII + sats Fe edge

  5. Chianti Spectrum, T=50MK Fe XXVI Fe XXV, XXVI high-n lines Ca XIX, XX lines Fe XXV + sats Ni XXVII + sats

  6. ~6.7-keV Fe-line FeatureComposition

  7. ~8-keV Fe-Ni-line FeatureComposition

  8. Temperature (T) Dependences • Line features are made up of many different lines. • Each line has different T dependence of intensity - G(T) functions. • Thus, line feature intensity varies with T relative to continuum. • Line-to-continuum ratio is best measured by “equivalent width”, i.e. width in keV of the line feature having intensity equal to continuum.

  9. Equivalent Width of Fe-line Feature ChiantiCoronal Fe abundance

  10. Temperature (T) Dependences • For multithermal plasma, must use differential emission measure, DEM(T). • Ken Phillips has used • DEM = K T-α • DEM = K exp(-T/T0) • Equivalent width vs. α and T0

  11. α Fe-line Equivalent Width vs α Equivalent Width in keV/A DEM = A T-α cm-3 K-1 Integral from 10 to 100 MK

  12. Fe-line Equiv. Width vs. T0 Equivalent Width in keV/B DEM = B exp (-T/T0) cm-3 K-1 Integral from 10 – 100 MK T0 (MK)

  13. Intensity Ratio of the 2 Features • The intensity ratio of the Fe-line to Fe/Ni-line features depends on T,but only weakly for T>40MK. • For T<30MK, the Fe/Ni-line feature is weak. • Thus, intensity ratio is useful asT-diagnostic for flares with T > 30 MK.

  14. Intensity ratio of the 2 features Chianti

  15. Fe-line Centroid Energy • As T increases, Fe XXIV satellites in the Fe-line feature decrease relative toFe XXV resonance line. • Thus, centroid energy of the Fe-line feature increases with T. • RHESSI’s small gain change with count rate make the modest energy change difficult to measure at present.

  16. Fe-line Feature Centroid Energy vs. T Stars – SMM/BCS Diamonds – Yohkoh/BCS Curve - Chianti

  17. Synthesizing X-ray Spectra • Chianti used for most plots here. • Comparisons between Chianti and SMM/BCS spectra show significant differences. Incorrect Fe XXIII line intensities in Chianti • Comparisons with APEC spectra also show differences. More lines included in APEC than in Chianti but may not be significant for RHESSI’s application.

  18. Other Atomic Codes • Currently, SPEX uses • Mewe et al. (1985) data, • Arnaud & Rothenflug (1985) ion fractions, • cosmic element abundances. • Mazzotta et al. (1998) ion fractions are better (rates based on better cross section data now available, some experimentally verified). • With APEC and Chianti, it’s possible to choose ion fraction calculation and abundances.

  19. Element Abundances in Flares • RHESSI line features depend only on Fe/H and Ni/H abundances. • Coronal Fe, Ni abundances are higher than photospheric by a factor of • 4 (Feldman et al.) • 1.5-2.0 (Fludra & Schmelz’s “hybrid” model) • 1 (Meyer). • Abundances in flares may vary • from flare to flare (Feldman et al.) • during flares (Sylwester et al.).

  20. Possible RHESSI Projects • Determine flare Fe abundances • continuum + line fits to RHESSI spectra • continuum slope gives T • line feature equivalent width gives Fe abundance. • Compare measured Fe abundance with nonthermal parameters, flare size, duration, etc. • Images in Fe line show location of high temperature plasma.

  21. Conclusions • Diagnostic potential of Fe and Fe/Ni line complexes in RHESSI flare spectra: • Equivalent width  Fe abundance with T from continuum • Fe to Fe/Ni ratio  T (~30 – 40 MK) • Fe feature centroid energy  T (problems) • Old version of Mewe code used in SPEX is inaccurate. • Chianti problems at the 20 – 30% level. • APEC similar to Chianti for RHESSI’s purposes. • Plan to switch from Mewe to Chianti in SPEX.

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