Introduction Stellar flares Summary
X-RAYING THE STELLAR CORONAE
Lalitha SairamTata Institute of Fundamental Research
06 February 2014
1
Introduction Stellar flares Summary
X-rays across the HR diagram
-0.5 0.0 0.5 1.0 1.5 2.0B-V Colour
20
15
10
5
0
-5
-10
Ab
solu
te M
agn
itu
de
BSC/Gliese detections
2
Introduction Stellar flares Summary
Flaring star – Algol
• System – short period eclipsing binary
• B-type star – lack surface convective envelope
• Magnetically active secondary – major flares
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Introduction Stellar flares Summary
General picture
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Coronal loop
Acceleration
Coronal X−ray source
Precipitating electrons
Evaporation
Flare footpoint
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Introduction Stellar flares Summary
Temporal variation
Hudson et al. (1998)
FSXR ∝
∫FHXR(t)dt
or
d
dtFSXR(t) ∝ FHXR(t)
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Introduction Stellar flares Summary
Temporal variation
Proxima Centauri
6.1 6.2 6.3 6.4 6.5Time (h since MJD 54904)
-2
0
2
4
6
Sca
led
flu
x
EPIC LC derivativeEPIC LCUVES-B LC
AB Doradus A
2.6 2.8 3.0 3.2 3.4 3.6Time (h since MJD 55160)
-2
0
2
4
6
Sca
led
flu
x
EPIC LC derivativeEPIC LCUVES continuum LC
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Introduction Stellar flares Summary
Algol as observed with SXT
Quiescence
1Energy [keV]
0.0001
0.0010
0.0100
0.1000
1.0000
10.0000
Co
un
ts/s
/keV
Flare
1Energy [keV]
0.01
0.10
1.00
10.00
100.00
Co
un
ts/s
/keV
7
Introduction Stellar flares Summary
Abundance variation – Fe K complex
5.5 6.0 6.5 7.0 7.5Energy [keV]
0.001
0.010
0.100
1.000
10.000
Co
un
ts/s
/keV
8
Introduction Stellar flares Summary
Abundance variation – Fe K complex
5.5 6.0 6.5 7.0 7.5Energy [keV]
0.001
0.010
0.100
1.000
10.000
Co
un
ts/s
/keV
9
Introduction Stellar flares Summary
Abundance variation – Fe K complex
5.5 6.0 6.5 7.0 7.5Energy [keV]
0.001
0.010
0.100
1.000
10.000
Co
un
ts/s
/keV
10
Introduction Stellar flares Summary
Abundance variation – Fe K complex
5.5 6.0 6.5 7.0 7.5Energy [keV]
0.001
0.010
0.100
1.000
10.000
Co
un
ts/s
/keV
11
Introduction Stellar flares Summary
Abundance variation – Fe K complex
5.5 6.0 6.5 7.0 7.5Energy [keV]
0.001
0.010
0.100
1.000
10.000
Co
un
ts/s
/keV
12
Introduction Stellar flares Summary
Abundance pattern
• Sun, inactive or low activity level stars - a normal FIPeffect - elements with a low first ionization potential (FIP)are enhanced in the corona
• Active stars - inverse FIP effect - enhanced high-FIPelements and depleted low-FIP elements
5 10 15 20 25FIP[eV]
0
1
2
3
4
Ab
un
da
nc
e r
ela
tiv
e t
o s
ola
r
MgFeSi S C O N Ar Ne
AB Doradus A
Proxima Centauri
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Introduction Stellar flares Summary
Flare energy distribution
Parker model - nanoflare for the Sun – < 1027 erg
N(> E) =∫∞
E
dN
dEdE = KE−α+1
• In Solar context
→ α = 1.5− 1.6 for a normal flare (Crosby et al. 1993)
→ α = 2.3− 2.6 for small events in the quiescent corona
(Krucker et al. 1998)
• Solar analogs:
→ α = 2.2 for solar analogs EK Dra and 47 Cas
(Audard et al. 1999)
• M dwarfs:
→ α = 1.6 for X-ray flares using EXOSAT observations
(Collura et al. 1988)
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Introduction Stellar flares Summary
Flare energy distribution
1033 1034
E [erg]
10-36
Rat
e [e
rg-1 y
ear-1
]
N (> E) ~ E - 1.93 +/- 0.06
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Introduction Stellar flares Summary
Summary
• overall picture of the atmosphere from the chromosphere tothe corona - the X-ray and optical/UV light curves ofactive low mass stars during the flare onset
• abundances, emission measures, and temperatures of flaringplasma
• short term energy release and their role in heating of theatmosphere of active stars.
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Introduction Stellar flares Summary
Appendix
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Introduction Stellar flares Summary
X-raying stellar coronae with SXT
2T XSPEC model + APEC
• Case 1 – Bright coronal X-ray source
• Case 2 – RS CVn and dMe
• Case 3 – Sun-like stars
• Case 4 – Late-type stars
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Introduction Stellar flares Summary
Case 1 – Bright coronal X-ray source
1Energy [keV]
0.0001
0.0010
0.0100
0.1000
1.0000
10.0000
Co
un
ts/s
/keV
Fx=1×10−10 ergs cm−2 s−1, nH=6×1018 cm−2
T1=0.6 keV, T2=2.5 keV, EM2=2EM1
Exposure – 10 ks19
Introduction Stellar flares Summary
Case 2 – RS CVn and dMe
1Energy [keV]
10-5
10-4
10-3
10-2
10-1
100
Co
un
ts/s
/keV
Fx=1×10−11 ergs cm−2 s−1, nH=1×1019 cm−2
T1=0.5 keV, T2=2.0 keV, EM2=EM1
Exposure – 20 ks20
Introduction Stellar flares Summary
Case 3 – Sun-like stars
1Energy [keV]
0.0001
0.0010
0.0100
0.1000C
ou
nts
/s/k
eV
Fx=1×10−12 ergs cm−2 s−1, nH=1×1019 cm−2
T1=0.5 keV, T2=2.0 keV, EM2=EM1
Exposure – 50 ks21
Introduction Stellar flares Summary
Case 4 – Late-type stars
1Energy [keV]
10-5
10-4
10-3
10-2
Co
un
ts/s
/keV
Fx=1×10−13 ergs cm−2 s−1, nH=3×1019 cm−2
T1=0.5 keV, T2=2.0 keV, EM2=EM1
Exposure – 100 ks22
Introduction Stellar flares Summary
Temperature and emission measure variation
0 2 4 6 8 10 12 14Time bin
0
2
4
6
8
10
12
14
T [
keV
]
0 2 4 6 8 10 12 14Time bin
0
5
10
15
EM
[10
54 c
m-3]
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