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

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.

X-flare on 21 April 2002

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.

RHESSI ContoursWhite: 6 – 12 keVBrown: 12 – 25 keVBlue: 25 – 50 keV

TRACE Image

1.95 nm

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.

Shutters Out

RHESSI Spectra

Thin Shutters In (A1)

Detector #4

1-minute accumulations

RHESSI Spectra

Thin Shutters (A1)

Thin + Thick (A3)

Detector #4

1-minute accumulations

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.

Mewe & Chianti Thermal Spectra

Peak Position of Iron-line ComplexE

nerg

y in

keV

Iron-line ComplexRHESSI Measurements of Peak Energy

A0 A1 A3

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.

Equivalent Width of Iron-Line Complex

Mewe_spec(coronal - Meyer 1985)

Chianti (coronal abundances)

Chianti (photospheric)

Iron-line ComplexRHESSI Measurements of Equivalent Width

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).

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.