line features in rhessi spectra kenneth j. h. phillips brian r. dennis gsfc rhessi workshop taos, nm...
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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– Fe XXVI lines at higher T (>80MK)
• ~8 keV– Fe XXV & Fe XXVI lines, more highly excited– Ni XXVII, Ni XXVIII lines
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.
Chianti Spectrum, T=20MK
Fe XXV +satellites
Fe XXV
Ni XXVII + sats
Ca XIX
Fe XXV
Fe edge
Chianti Spectrum, T=50MK
Fe XXV + sats
Fe XXVI
Ni XXVII + sats
Fe XXV, XXVI high-n lines
Ca XIX, XX lines
~6.7-keV Fe-line FeatureComposition
~8-keV Fe-Ni-line FeatureComposition
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.
Equivalent Width of Fe-line Feature
ChiantiCoronal Fe abundance
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
Fe-line Equivalent Width vs α
α
DEM = A T-α cm-3 K-1
Integral from 10 to 100 MK
Equ
ival
ent
Wid
th in
keV
/A
Fe-line Equiv. Width vs. T0
DEM = B exp (-T/T0) cm-3 K-1
Integral from 10 – 100 MK
Equ
ival
ent
Wid
th in
keV
/B
T0 (MK)
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.
Intensity ratio of the 2 features
Chianti
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.
Fe-line Feature Centroid Energy vs. T
Stars – SMM/BCS
Diamonds – Yohkoh/BCS
Curve - Chianti
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.
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.
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.).
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.
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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