The LowEnergy Cutoff to the Nonthermal Electron Spectrum Determined From RHESSI Observations

1
The Low-Energy Cutoff to the Nonthermal Electron
Spectrum Determined From RHESSI Observations

• Linhui Sui (CUA/NASA GSFC), Gordon D. Holman,
  Brian R. Dennis (NASA GSFC)

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What is the Low-Energy Cutoff?
Expected Photon Spectrum
Nonthermal Electron Spectrum
Flat cutoff
Flattening
Cutoff Energy
Ecutoff 25 keV
Sharp cutoff
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Why Study the Low-Energy Cutoff?

• Identify thermal and nonthermal contribution to
  the total flare spectra
• Calculate the total energy of nonthermal
  electrons
• Constrain models for particle acceleration (e.g.,
  Holman 1985)

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What is the Difficulty?
Preheating
RHESSI Flare Spectrum

• High threshold of spectrometers
• Insufficient spectral resolution
• Strong thermal bremsstrahlung
• at low energies

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An Early Impulsive Flare
M1.0 flare on 06/02/2002, Heliocentric Angle 21o

• Hard X-ray flux (gt 25 keV) increases before the
  soft X-ray flux rises significantly.

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Spectral Fitting

• Thermal spectra is produced by
• isothermal plasma
• Nonthermal spectra is produced by
• thick-target bremsstrahlung from
• nonthermal electrons
• Two spectral lines Fe and Fe/Ni
• Cutoff energy 28 keV

Flattening
EM 6e46 cm-3 T 15 MK Index 5.0
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Spectral Fitting

• Thermal spectra is produced by
• isothermal plasma
• Nonthermal spectra is produced by
• thick-target bremsstrahlung from
• nonthermal electrons
• Two spectral lines Fe and Fe/Ni
• Cutoff energy 33 keV

Flattening
EM 1.6e47 cm-3 T 15 MK Index 5.0
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Spectral Fitting

• Thermal spectra is produced by
• isothermal plasma
• Nonthermal spectra is produced by
• thick-target bremsstrahlung from
• nonthermal electrons
• Two spectral lines Fe and Fe/Ni
• Cutoff energy 38 keV

Flattening
EM 1.5e47 cm-3 T 17 MK Index 4.8
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Spectral Fitting

• Thermal spectra is produced by
• isothermal plasma
• Nonthermal spectra is produced by
• thick-target bremsstrahlung from
• nonthermal electrons
• Two spectral lines Fe and Fe/Ni
• Cutoff energy 23 keV

Flattening
EM 8e47 cm-3 T 14 MK Index 4.2
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Spectral Fitting

• Thermal spectra is produced by
• isothermal plasma
• Nonthermal spectra is produced by
• thick-target bremsstrahlung from
• nonthermal electrons
• Two spectral lines Fe and Fe/Ni
• Cutoff energy 32 keV

Flattening
EM 6e47 cm-3 T 17 MK Index 4.3
11
Spectral Fitting

• Thermal spectra is produced by
• isothermal plasma
• Nonthermal spectra is produced by
• thick-target bremsstrahlung from
• nonthermal electrons
• Two spectral lines Fe and Fe/Ni
• Upper limit of Cutoff energy 24 keV

EM 9e46 cm-3 T 17 MK Index 3.7
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Time History

• In general, the low-energy cutoff correlates with
  HXR flux
• The cutoff energy suddenly decreased at the
  second peak
• The nonthermal energy input correlates with HXR
  flux
• ENonthermal lt EThermal
• 1029 ergs 4 1030 ergs

25-50 keV
50-100 keV
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Photospheric Albedo

• Hard X-rays incident upon the photosphere with
  energies gt 10 keV are backscattered due to
  Compton collisions with electrons.(Tomblin 1972,
  Bai Ramaty 1978, Alexander Brown 2002)
• Observed photon flux
• Iobs Isource Ialbedo
• Albedo effect depends on power-law index,
  location, source height.

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Albedo Correction
up
down
No Albedo correction
Albedo correction (isotropic source)
Cutoff Energy 38 keV
Cutoff Energy 40 keV
Albedo effect may contribute the spectral
flattening to some extent.
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Preliminary Results of Albedo Correction

• In many cases, the low-energy cutoff decreases by
  1 to 3 keV with isotropic albedo correction.
• Spectral flattening is unlikely to be fully
  caused by the albedo effect.

25-50 keV
50-100 keV
Albedo
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Conclusions

• Flattening in the X-ray spectrum around 20 keV
  has been seen in an early impulsive flare.
• Existence of a low-energy cutoff to the
  nonthermal electron spectrum gives a natural
  explanation.
• The cutoff energy generally correlates with the
  HXR flux.
• Albedo effect could contribute to the flattening,
  but is unlikely to be the only cause.
• Many more similar flares have been seen with
  RHESSI.

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Flat Cutoff
Sharp Cutoff
Flat cutoff
Sharp Cutoff Energy 38 keV
Flat Cutoff Energy 51 keV
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Albedo Correction
up
down
Albedo correction (anisotropic source)
No Albedo correction
Cutoff Energy 38 keV
Cutoff Energy 52 keV
Albedo effect may contribute the spectral
flattening to some extent.
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Albedo Correction
up
down
Albedo correction (anisotropic source)
No Albedo correction
Cutoff Energy 38 keV
Cutoff Energy 56 keV
Albedo effect may contribute the spectral
flattening to some extent.