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Detailed energy release in a microflare

and the quality index for X-ray image reconstruction

Zhentong Li , Yang Su, Astrid Veronig, Wenhui Yu, Weiqun Gan, Wei Chen

SPHERE 2022 workshop

2022.07.15 online

ztli@pmo.ac.cn

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Contents

  1. Background & motivation
  2. Detailed energy release in a microflare
      • Evidence of nonthermal component
      • The first imaging evidence of low-energy cutoff
  3. Summary
  4. Quality index for X-ray image reconstruction

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Contents

  1. Background & motivation
  2. Detailed energy release in a microflare
      • Evidence of nonthermal component
      • The first imaging evidence of low-energy cutoff
  3. Summary
  4. Quality index for X-ray image reconstruction

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Background

  • Discussions of nth e-

— many

  • Energy of nth e-

— varies with calculation method

  • Evidences of nth e-

— less (e.g., Hannah et al. 2008;

Glesener et al. 2020;

Sharma et al. 2020;

Battaglia et al. 2021)

Nonthermal electrons in microflares

:

Thermal and nonthermal energy

(Warmuth & Mann, 2020)

  • highly sensitive to low-energy cutoff
  • model dependent for spectral fitting

in microflares:

  • require: higher spectral resolution
    • FOXSI, NuSTAR, MinXSS, Chandrayaan-2 XSM ...

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Background

NuSTAR

:

The Nuclear Spectroscopic Telescope Array (NuSTAR)

  • Higher spectral resolution for low energy X-ray

Detect X-ray：2.5~78 keV

Focal length: 10m

Grazing incidence imaging

Direct-focusing

2013.06

NASA

For high energy astrophy.

Occasionally observe the sun

Glesener et al. 2020: nonthermal electrons of <7 keV

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Background

  • Spectral fitting: model dependent
    • isothermal, multi-thermal, nonthermal …
      • temperature distribution of multi-thermal plasma ?
      • possibility of super hot components ?

Spectral fitting

:

vth+bpow

T1=15 MK

T2=30 MK

T3=88 MK

large flare

microflare

energy

photon spectrum

(Krucker & Lin, 2008)

large flare: easy to find out the nonthermal comp.

microflare: steeper spectrum

    • multi-thermal or thermal + nonthermal ?

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Motivation

  • Spectral fitting: model dependent
    • isothermal, multi-thermal, nonthermal …
      • temperature distribution of multi-thermal plasma ?
      • possibility of super hot components ?
  • This study:
    1. Constrained the thermal comp.: sparse DEM code with new settings
    2. Confirmed the nonthermal comp. & energy
    3. Reported the first imaging evidence of low-energy cutoff of nonthermal electrons

(Glesener et al. 2020)

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  1. Background & motivation
  2. Detailed energy release in a microflare
      • Evidence of nonthermal component
      • The first imaging evidence of low-energy cutoff
  3. Summary
  4. Quality index for X-ray image reconstruction

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  • AR12671
  • center: [365”, 45”]
  • GOES class：A5.7

(Glesener et al. 2020: A5.7, Duncan et al. 2021: A7.7)

Observation

Overview

:

Animation is available online

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  • >10 MK：
    • firstly seen apart from the footpoints
    • shorter duration
  • AIA image decovolution
  • DEM: spatial distribution
    • tend to converge in the middle of the loop
  • DEM calculation：
    • rebin 2×2 pixels
    • counts > 0.1 DN s-1 pixel-1

Observation

Data processing

:

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  • modified based on the sparse DEM code (Cheung et al., 2015)
  • Su et al. 2018, ApJL
    • Accurate in the flare temperature ranges
    • Better for flaring and active regions
  • Li, Su et al. 2022, ApJ (this study)
    • Applicable for all types of coronal structures (except for filaments), and quiet regions
    • Latest version released by this paper

Scan this QR code to download the DEM code

Improved DEM code

http://paperdata.china-vo.org/yang.su/DEM/sparse_em_v1.001_ys.zip

Or click the link below:

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  • DEM →
  • spectral fitting: thick target model δ≈6.8
  • nonthermal e- flux: (1.52±0.28)×1035 e-/s
  • low-energy cutoff: (7.1±0.5) keV
  • energy:
    • deposition rate：~2.1×1027 erg/s
    • total nonthermal energy in 3min：

~3.8×1029 erg

  • difference：nonthermal comp.

Find Nonthermal Component

constrain the thermal spectrum

:

thermal emission photon spectrum

count spectrum

→

  • NuSTAR count spectrum

fitted th. in G+20

fitted nth.

DEM calc. th

remaining nth.

(thank Lindsay for providing this)

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Note：Restrictions on quantitative X-ray analysis by DEM:

  1. EUV images are not heavily saturated;
  2. Flare temperatures are lower than superhot;
  3. …… (see details in Su et al. 2018)

isothermal

multi-thermal

vs

Find Nonthermal Component

constrain the thermal spectrum

:

  • dependently derived the almost same results
  • the high accuracy of DEM calc.

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  1. Local high density thick targets
  2. Enough column density and low-energy cutoff

Why are the >10 MK sources away from the chromospheric footpoints?

 

 

Imaging Evidence of Low-energy Cutoff

The positions of hot sources: thermalization or heating

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initial energy of nonthermal electrons Es

column density N(p)

stopping (thermalization) position

the positions of >10 MK plasma

Imaging Evidence of Low-energy Cutoff

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  • density increasing → plasma filling
    • faster near the loop center
  • if Es=5 keV, conflict with the the positions of >10 MK plasma
  • if Es=Ec=7.1 keV, they fit well

hot and dense thermal sources are from thermalization of nonthermal electrons

the positions of thermalization of nonthermal electrons

Imaging Evidence of Low-energy Cutoff

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  • steep spectrum
  • low-energy cutoff EC

energy of the most majority electrons: EC

  • the hot sources
  • local density increasing

thermalize

Imaging Evidence of Low-energy Cutoff

scenario

:

 

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  • steep spectrum
  • low-energy cutoff EC
  • the hot sources
  • local density increasing

DEM constrain the thermal comp.

fit to the nonthermal comp.

AIA observations

NuSTAR observations

stopping positions of nonthermal electrons

self-consistent evidence

Imaging Evidence of Low-energy Cutoff

self-consistent picture

:

DEM

calc. density

energy of the most majority electrons: EC

 

thermalize

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  • Newly improved sparse DEM code
  • Evidence of nonthermal emissions in a microflare
  • Multi-thermal nature of the microflare plasma
  • First imaging evidence of low-energy cutoff of nonthermal electrons

Summary

What’s important :

    • Help to determine the low-energy cutoff
    • The research methods in this study provide valuable approach to discover or confirm the nonthermal emissions in microflares

Detailed thermal and nonthermal processes in an A-class microflare, The Astrophysical Journal, 2022, 930(147)

See more details in published paper:

ztli@pmo.ac.cn

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  1. Background & motivation
  2. Detailed energy release in a microflare
      • Evidence of nonthermal component
      • The first imaging evidence of low-energy cutoff
  3. Summary
  4. Quality index for X-ray image reconstruction

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Background & Motivation

RHESSI

STIX

HXI

(Su et al., 2019)

Very Large Array（VLA）, © Ground Antenna Arraying, NASA

MUSER, ©NOAC

  • Fourier transform imaging
  • u-v space sampling

How to evaluate the quality of reconstructed image?

 

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New index

 

 

: percentage proximity degree (PPD)

describe the area of side-lobes in different level

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Screening indices

response correctly

response wrongly

basically right, but not important

response wrongly, but don’t affect the evaluating

the technique for order preference by similarity to ideal solution (TOPSIS)

establish the synthetic index: QuIX

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The Quality Index for X-ray image reconstruction (QuIX) :

Test

Use QuIX to test PSF under different u-v configurations

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suggestions are welcome!

Applications

  • help to evaluate different imaging algorithms
  • help to determine parameters of imaging algorithms

this work on detailed imaging algorithm testing is also ongoing by Wenhui Yu et al.

The Quality Index for X-ray image reconstruction (QuIX) :

谢

谢

Thank you