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 Feature Composition

~8-keV Fe-Ni-line Feature Composition

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 Chianti Coronal 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/T 0 ) Equivalent width vs. α and T 0

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

Fe-line Equiv. Width vs. T 0 DEM = B exp (-T/T0) cm -3 K -1 Integral from 10 – 100 MK Equivalent Width in keV/B T 0 (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 as T-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 to Fe XXV resonance line. Thus, centroid energy of the Fe-line feature increases with T. RHESSIs 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 RHESSIs 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, its 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.) – (Fludra & Schmelzs 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 RHESSIs purposes. Plan to switch from Mewe to Chianti in SPEX.