Diagnostic Power in the “Low-Resolution” Spectroscopy of Non-Equilibrium Plasmas

Hiroya Yamaguchi (NASA/GSFC, UMD)

Apologies: - I have little experience of “high-resolution spectroscopy” (i.e., grating). - I hate to show lots of “fake data” of future missions (i.e., calorimeter).

So I’m going to ..., present real data from the X-ray CCD Suzaku/XIS with low moderate energy resolution (E/△E ~ 40).

In order to convince you that: - There are a number of powerful diagnostics even in the moderate-energy spectroscopy. - Knowledge of physics behind radiation processes helps get an unexpected, exciting results.

Today’s Talk

5%

35%

60%

Atomic physics(but pretty basic)

Astrophysics

How I love Boston ... sorry, DC/MD guys!

Supernova Remnants (SNRs, especially of Type Ia): to solve progenitor’s evolution and explosion mechanism.

Scientific Motivation

Cliche: “X-ray observation of SNRs is best suitable for studying SNe.”... Is it true?

In principle, TRUE: SNRs allow us to directly measure chemical composition and distribution.In practice, NOT TRUE: Plasma is in non-equilibrium ionization (NEI), making the abundance measurement very difficult.

Thanks to the NEI, - We can understand supernova physics in detail.- We can discover new interesting phenomena.

CIE? NEI? (“Textbook”-ish explanation)

Definition:

Fez+ Fe(z+1)+ + e-

CIE: Ionization rate = Recombination rate (for all ions)NEI: Ionization rate ≠ Recombination rate (for any ion)

Convention among plasma physicists:CIE: Electron temperature = Ionization temperatureNEI: Electron temperature ≠ Ionization temperature

What’s the major difference between CIE and NEI in terms of atomic processes and resulting spectral features?

Ionization & recombination rates depend on electron temperature. ➔ Ion fraction in a CIE plasma is uniquely determined by kTe.

0.01 0.1 1

Ar-like

Fe8+

Ne-like

Fe16+

2+

4+

12+

6+

10+

14+18+

20+ 22+

10

Electron temperature (keV)

1

0.1

K-shell

Neutral Ar-like Fe8+ Ne-like Fe16+ He-like Fe24+ Bare Fe26+

105 K 107 K 108 K106 K

L-shell

M&N-shells

K-shell

L-shell

K-shell

L-shell

K-shell

M-shell

M shellPotential Energy L shell K shell

Ion

fra

ctio

n He-like

Fe24+

Fully-ionized

Fe26+

25+

CIE? NEI? (Atomic-physics point of view)

Abundant in young SNRs

Typical kTe of young SNRs

CIE? NEI? (Atomic-physics point of view)

0.01 0.1 1

Ar-like

Fe8+

Ne-like

Fe16+

2+

4+

12+

6+

10+

14+18+

20+ 22+

10

Electron temperature (keV)

1

0.1

105 K 107 K 108 K106 K

Ion

fra

ctio

n He-like

Fe24+

Fully-ionized

Fe26+

25+

Fe26+

25+12+

10+14+

18+ 20+ 22+

1012

1011

1010

109

Ionization age: net (cm-3 s)

1

0.1

10-2

Ar-like

Fe8+

Ion

fra

ctio

n

He-like

Fe24+

Ne-like

Fe16+

Ionization speed depends on the electron densityTo reach CIE, ne t ~ 1012 cm-3 s ➔ 3×104 (ne / 1 cm-3)-1 yr

Ionization & recombination rates depend on electron temperature. ➔ Ion fraction in a CIE plasma uniquely determined by kTe.

Typical kTe of young SNRs

In an NEI plasma, (i) hot free electrons with the energy higher than the K-shell potential, and (ii) low-ionized ions with a lot of L-shell and M-shell electrons co-exist.

➔ Innershell processes take place. Fluorescence emission offers key to diagnosing the plasma condition. (e.g., Kaastra+1993; Palmeri+2003)

Q_ Q`

Electron impact Innershell ionization

Fluorescence

K-shellL-shell

M-shell

Characteristic Atomic Processes in NEI

9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0

7.0

7.5

log (net [cm-3 s])

log (T e

[K])

8.5

8.0

6395 6499 6603 6707 6811 6915

6400

6500

6600

6700

0 10 15 20Charge Number

Ener

gy [e

V]

5

Fe K_ Centroid

Neutral Ar-likeNe-like

8 9 10 11log(n t [cm-3 s])e

e.g., centroid energy which depends on the charge number (hence net).

(for kTe = 5 keV)

0

0.05

0.1

0.15

Flux

Rat

io

0 10 15 20Charge Number

5

Fe K`/K_�Flux Ratio

NeutralAr-like Ne-like

(no 3p e-)

8 9 10 11log(n t [cm-3 s])e

Innershell processes offer important diagnostics for NEI plasma.E/△E > 100 is not necessary for the simple diagnostics (centroid, ratio)

0.0 0.028 0.056 0.084 0.112 0.140

9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0

7.0

7.5

log (net [cm-3 s])

log (T e

[K])

8.5

8.0

Kβ/Kα flux ratio would be even more interesting (see also Adam’s poster).

(for kTe = 5 keV) CIE

K-shellL-shell

M-shell

K-shellL-shell

Low-netregime

Low-ionization regime: - Drops with ionization. (Fe atoms lose 3p electrons.) - Doesn’t depend on temperature. (ionization dominant)

Characteristic Atomic Processes in NEI

K-shell

High-netregime

High-ionization regime: - Goes up with ionization. (Fe atoms lose 2p electrons too.) - Strongly depends on temperature. (excitation dominant)

Ar-like Ne-like He-like

Fe Kα Emission Diagnostics on SNRs

Yamaguchi+2014a, ApJL, 785, L27

Centroid energies discriminate the SNR progenitor type.

12 16 17 18 19 20 21 22 23 240

N103B

0519-69.0

0509-67.5

G337.2-0.7

W49BCas A

N132D

Sgr A East

N63A

G349.7+0.2

SN 1987AN49

G292.0+1.8

G350.1-0.3

IC 443

Kepler

TychoG344.7-0.1

G352.7-0.1

RCW86

SN 1006

3C 397

G0.61+0.01

L K (1

040 p

hoto

ns s

-1)

6400 6500 6600 6700

110

100

1000

Energy (eV)

Charge number

Galactic Ia SNRsLMC Ia SNRsGalactic CC SNRsLMC CC SNRs

G1.9+0.3

DEM L71

Fe Kα Emission Diagnostics on SNRs

Yamaguchi+2014a, ApJL, 785, L27

Centroid energies discriminate the SNR progenitor type.

Density effect: - Ionization age ∝ ne t - Luminosity ∝ ne NFe

12 16 17 18 19 20 21 22 23 240

N103B

0519-69.0

0509-67.5

G337.2-0.7

W49BCas A

N132D

Sgr A East

N63A

G349.7+0.2

SN 1987AN49

G292.0+1.8

G350.1-0.3

IC 443

Kepler

TychoG344.7-0.1

G352.7-0.1

RCW86

SN 1006

3C 397

G0.61+0.01

L K (1

040 p

hoto

ns s

-1)

6400 6500 6600 6700

110

100

1000

Energy (eV)

Charge number

Galactic Ia SNRsLMC Ia SNRsGalactic CC SNRsLMC CC SNRs

G1.9+0.3

DEM L71

Fe Kα Emission Diagnostics on SNRs

Yamaguchi+2014a, ApJL, 785, L27(Patnaude+2015; Follow-up studies for CC models)

Comparison with hydro models: - Type Ia = uniform ambient - CC = dense CSM

Centroid energies discriminate the SNR progenitor type.

Density effect: - Ionization age ∝ ne t - Luminosity ∝ ne NFe

Ionization state (Fe-K centroid) constrains SNR’s environment.

12 16 17 18 19 20 21 22 23 240

N103B

0519-69.0

0509-67.5

G337.2-0.7

W49BCas A

N132D

Sgr A East

N63A

G349.7+0.2

SN 1987AN49

G292.0+1.8

G350.1-0.3

IC 443

Kepler

TychoG344.7-0.1

G352.7-0.1

RCW86

SN 1006

3C 397

G0.61+0.01

L K (1

040 p

hoto

ns s

-1)

6400 6500 6600 6700

110

100

1000

Energy (eV)

Charge number

Galactic Ia SNRsLMC Ia SNRsGalactic CC SNRsLMC CC SNRs

G1.9+0.3

DEM L71

SNR 3C 397

Fe Kα Emission Diagnostics on SNRs

Yamaguchi+2014a, ApJL, 785, L27(Patnaude+2015; Follow-up studies for CC models)

Comparison with hydro models: - Type Ia = uniform ambient - CC = dense CSM

Centroid energies discriminate the SNR progenitor type.

Density effect: - Ionization age ∝ ne t - Luminosity ∝ ne NFe

Ionization state (Fe-K centroid) constrains SNR’s environment.

Most evolved

SNR 3C 397

Reverse Shock

Forward Shock

Explosion Center

Young SNR Evolved SNR

X-rays from entire ejecta

cool ejecta

0 0.5 1

0.0

10.1

1

Ni

Fe

Mn

Mn

Cr

Ar

SiO

Lagrangian Mass (M )

Mass F

raction

0.01

0.1

1

Energy [keV]5 6 7 8 9

Cou

nts

s-1 k

eV-1

10-4

10-3

Cr Mn

Fe

Ni

Suzaku XIS

Yamaguchi+2015, ApJL, 801, L31

The Fe Kα diagnostic suggests 3C 397 is the most evolved Type Ia SNR.

Allows us to investigate if electron captures indeed take place in the Type Ia SN core.

Electron capture reactions (p + e- ➔ n + νe) increase abundances of n-rich elements.

Detected strong emission from Mn and Ni, also due to fluorescence; New AtomDB!

SN Ia nucleosynthesis model

(radius)

Tycho’s SNRConclusions: - Identified physical condition and location of immediate postshock plasma - (Actually,) this was robust evidence for collisionless electron heating

Tycho Brahe (1546−1601)

12 16 17 18 19 20 21 22 23 240

N103B

0519-69.0

0509-67.5

G337.2-0.7

W49BCas A

N132D

Sgr A East

N63A

G349.7+0.2

SN 1987AN49

G292.0+1.8

G350.1-0.3

IC 443

Kepler

TychoG344.7-0.1

G352.7-0.1

RCW86

SN 1006

3C 397

G0.61+0.01

L K (1

040 p

hoto

ns s

-1)

6400 6500 6600 6700

110

100

1000

Energy (eV)

Charge number

Galactic Ia SNRsLMC Ia SNRsGalactic CC SNRsLMC CC SNRs

G1.9+0.3

DEM L71

Ne-likeYamaguchi+2014b, ApJ, 780, 136

An early Suzaku observation achieved the first detection of Cr and Mn from a Type Ia SNR (Tamagawa+2009). ➔ Weak-line search became popular.

1s2 2s2 2p6

3s2 3p6 3d6 4s2

Kα↑

↓Kβ

K-shellL-shell

M&N-shells

Neutral Fe

1s2 2s2 2p6

Kα↑

Ne-like Fe16+

K-shellL-shell

No bound 3p electronresponsible for Kβ

10ï�

0.01

0.1

Cou

nts

s-1 k

eV-1

Energy [keV]

Cr Mn

Fe K_

Fe K`Ni

95 6 7 8

0

0.05

0.1

0.15

Flux

Rat

io

0 10 15 20Charge Number

5

Fe K`/K_�Flux Ratio

NeutralAr-like Ne-like

(no 3p e-)

8 9 10 11log(n t [cm-3 s])e

Kα centroid = 6435 (± 1) eVKβ centroid = 7104 (± 10) eVKβ/Kα flux ratio = 5.5 (± 0.5) %

Anomalously Strong Fe Kβ Emission

Detection of Fe Kβ is more surprising.

10ï�

0.01

0.1

Cou

nts

s-1 k

eV-1

Energy [keV]

Fe K_

Fe K`

9

Red: Fe (K`/K_ ~ 0.01)Green: Fe (K`/K_ ~ 0.15)

~16+

~8+

5 6 7 8

7000

7400

7800

0

0.05

0.1

0.15

Ener

gy [e

V]Fl

ux R

atio

0 10 15 20Charge Number

5

Fe K` Centroid

Fe K`/K_�Flux Ratio

Neutral

Neutral

Ar-like

Ar-like

Ne-like

Ne-like(no 3p e-)

8 9 10 11log(n t [cm-3 s])e1s2 2s2 2p6

3s2 3p6 3d6 4s2

Kα↑

↓Kβ

K-shellL-shell

M&N-shells

Neutral Fe

1s2 2s2 2p6

Kα↑

Ne-like Fe16+

K-shellL-shell

No bound 3p electronresponsible for Kβ

Anomalously Strong Fe Kβ Emission

Detection of Fe Kβ is more surprising.

An early Suzaku observation achieved the first detection of Cr and Mn from a Type Ia SNR (Tamagawa+2009). ➔ Weak-line search became popular.

3 arcmin

Contour: Kα, Color map: Kβ

0.5

01.

52.

01.

0

Radius [arcmin]30 1 2

K_ × 0.1

K`

[10-6

pho

tons

cm

-2 s

-1 a

rcm

in-2]

Surfa

ce B

right

ness

RS radius

- Fe Kβ emission reveals the location of the reverse shock front.- Furthermore, it actually indicates efficient electron heating at the RS.

Reverse Shock

Unshocked FeLow-ionized Fe

Highly-ionized Fe

Reverse Shock

Forward Shock

Explosion Center

Low-ionized plasma at the immediate post-reverse-shock region.

Origin of the Strong Kβ Emission

Evidence for Collisionless Electron Heating

151050Mean Charge of Fe ejecta

101

0.1

Elec

tron

Tem

pera

ture

[keV

]

` = `min ~ 10-5

` = 0.003` = 0.01

` = 0.03

RS front

7.47.2 7.6 7.8 8.0SNR Radius [1018 cm]

( = me /mFe)Temperature increasesdue to Coulomb collisions

β = Te /TFe at RS front

Predicted line luminosity for each charge number

151050Charge Number

200.

010.

1

` = 0.01

Collisionless heating caseK`

K_

Line

Lum

inos

ity [1

040 p

hoto

ns s

-1] 10

1151050

Charge Number20

0.01

0.1

Line

Lum

inos

ity [1

040 p

hoto

ns s

-1] 10

1

` = `min ~ 10-5

Coulomb-only case

K`

K_No emission from low-ionized Fe

Comparison with hydrodynamical simulations indicates instantaneous electron heating to a temperature ~1000 times higher.

Strong K-shell emission from very low ionized ejecta is NOT easy to produce, because these Fe ions must coexist with free electrons energetic enough to ionize K-shell electrons.

Efficient collisionless heating at the SNR RS! See Yamaguchi+2014b, ApJ, 780, 136 for details.

Relationship between shock velocity (Vs ) and downstream temperature (Ti ):

kTi = (3/16) mi Vs2 Tycho: Vs = 5000 km/s ➔ kTe ≈ 0.03 keV

12 16 17 18 19 20 21 22 23 240

N103B

0519-69.0

0509-67.5

G337.2-0.7

W49BCas A

N132D

Sgr A East

N63A

G349.7+0.2

SN 1987AN49

G292.0+1.8

G350.1-0.3

IC 443

Kepler

TychoG344.7-0.1

G352.7-0.1

RCW86

SN 1006

3C 397

G0.61+0.01

L K (1

040 p

hoto

ns s

-1)

6400 6500 6600 6700

110

100

1000

Energy (eV)

Charge number

Galactic Ia SNRsLMC Ia SNRsGalactic CC SNRsLMC CC SNRs

G1.9+0.3

DEM L71

He-like

IC 443Conclusions: - Detected a signature of “recombining plasma” (opposite of typical NEI) - Initiated many discoveries of such SNRs

Yamaguchi+2009, ApJL, 705, L6Ohnishi+2014, ApJ, 784, 74

Energy (keV)4 6 8 10

0.1

0.01

10-3

10-4

Cou

nts

s-1 k

eV-1

Fe XXV

K-shell

Recombination into the

excited level and cascade

L-shell

M-shell

K-shell

L-shell

M-shell

K-shell RRC

(hi = IK + Ee)

Free electron (Ee)

He-like line

(hi = IK - IL)

(L-shell RRC)

Direct recombination

into the ground level ➔ Direct recombination into ground level should also be observed.

IC 443 is an evolved SNR with a low electron temperature (~ 0.6 keV)

105 K

0.01 0.1 1

Ar-like

Fe8+

Ne-like

Fe16+

2+

4+

12+

6+

10+

14+18+

20+ 22+

10

Electron temperature (keV)

1

0.1

105 K 107 K 108 K106 K

Ion

fra

ctio

n He-like

Fe24+

Fully-ionized

Fe26+

25+

Why Surprising?

0.6

Anyway, K-shell electrons cannot be excited by a sub-keV free electron.

X-ray energy: hν = IK + Ee

Spectrum ∝ exp(− )hν − IK

kTe

CIE plasma

Emission originates from H-like ions?

Energy (keV)6.56.4 6.6 6.7 6.8 6.9

zw

Energy (keV)4 6 8 10

0.1

0.01

10-3

10-4

Cou

nts

s-1 k

eV-1

Fe XXV

Fe RRC

IK = 8.83 keV

Interpretation: the plasma was much hotter in the past and cooled rapidly.kTe(past) must be > 10 keV, not ~ 2 keV.

Another Type of NEI (Recombining)Radiative recombination continuum (RRC) was detected, confirming the H-like-ion origin of the Fe XXV line. cf. CIE: Fe XXV mainly from He-like ions.

Another kind of “co-existence”: (i) Low-energy electrons that can barely excite L-shell electrons.(ii) Highly-charged ions with a K-shell vacancy.

ASTRO-H should confirm enhanced forbidden emission (high G-ratio).

Conclusions and Prospects- SNRs are indeed best suitable for studying supernova physics, owing to the NEI (innershell process, recombination, etc.).- SNRs offer a good “laboratory” for fundamental atomic physics.- There are a number of simple, but powerful diagnostics (e.g., centroid, Kβ/Kα ratio, RRC/line) where the grating/calorimeter resolution (E/△E ~ 1000) is not necessarily required.

- There MUST be still new diagnostics in future high-resolution spectral data, not only famous ones, like G-ratio, R-ratio, etc. - Don’t rely on (only) performance of hardware and software. - Understand the physics behind radiation processes. - “Press-‘fit’ analysis” might overlook a key spectral feature.

- Ambitious for ground experiments of innershell processes.