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.