X-ray emission from the gamma

Report
X-ray emission
from the gamma-ray binary LS 5039
(Yamaguchi & Takahara 2012, ApJ, 761, 146 )
大阪大学 山口正輝
共同研究者:
高原文郎
第25回理論懇シンポジウム「計算機宇宙物理学の新展開」
@つくば国際会議場 2012/12/23
OUTLINE
I. ガンマ線連星について
II. LS 5039とその観測
III. X線放射モデル
IV. 結果
V. まとめと結論
ガンマ線連星
• ガンマ線が連星周期に同期して変動している連星
Objects
Period Scale
LS 5039
3.9d
5x1012cm O + ?? (BH or NS)
LS I +61° 303
26d
1013cm
Be + ??
PSR B1259-63
3.4yr
1014cm
Be + NS
HESS J0632+057
320d
1014cm
Be + ??
1013cm
O + ??
1FGL J1018.6-5856 16d
Consists of …
O star
Compact star
• AUスケールから10TeV以上のガンマ線が出ている!
こんな天体はガンマ線連星だけ!
高密度星の近くの物理状態を調べられる
(パルサー風やジェット)
…ただ、放射機構はよくわかっていない
Observations of LS 5039
superior
inferior
F. Aharonian, et al., 2006, A&A, 460, 743
A. A. Abdo, et al., 2009, ApJL, 706, 56
T. Takahashi, et al., 2009, ApJ, 697, 592
Fermi
HESS
INFC
SUPC
Suzaku
HESS
SUPC
Fermi
MS
INFC
Suzaku
Photon energy (eV)
Phase-averaged spectra
Orbital phase
Light curves
・TeV、GeVは反相関
・TeV、X線は相関
・光度はGeVで最大
先行研究
T. Takahashi, et al., 2009, ApJ, 697, 592

X線からTeVを説明しようと
する研究はいくつかある

いずれもスペクトルまた
は周期変動に問題あり
B. Cerutti, et al., 2010, A&A, 519, 81
磁場強
最も重要なのは…
シンクロトロン冷却に
よるTeVの抑制
TeV小
MSY & Takahara, 2010, ApJ, 717, 85

→X線をシンクロトロン起
源としているのが問題?
Observations of LS 5039
superior
inferior
A. A. Abdo, et al., 2009, ApJL, 706, 56
T. Takahashi, et al., 2009, ApJ, 697, 592
Fermi
Fermi
INFC
SUPC
Suzaku
Suzaku
SUPC
MS
INFC
Orbital phase
Light curves
Photon energy (eV)
Phase-averaged spectra
・X-rayとGeV がきれいに反相関している
・スペクトルが滑らかにつながりそう
・逆コンプトン(IC)で冷えた電子があるはず
X線はIC放射?(シンクロトロンでなく)
モデル


注入電子の分布:  min   e   1 べき2.5
定常の電子分布
→ IC冷却により分布変わる
n ( e )
(  c : t cool  t escで決まる)
-2



-3.5
電子は高密度星の位置にいる
電子の速度は等方として入れる
電子はO型星の光子をIC散乱する


星由来光子の異方性考慮
電子静止系ではトムソン散乱
c
 min
1
ICスペクトルを軌道位相ごとに計算
 min :フリーパラメータとする
 c  5 ,  1  3  10
注入率を与える → 冷却電子の数を決める
4
結果1(fixed γmin)
GeVの変動は
Fermiスペクトル
と合うように、軌
道傾斜角を調整
(→ i = 15°)
 γmin = 103

GeVとX線は同じように変動
 X線のべきが観測とよく合う(∵冷却電子の放射)

結果2( γmin を周期変動させfitting)


γmin ∝ FGeV のときにX線観測を再現できた
本質は注入率の変動で冷却電子の数が変動すること
まとめと結論

5039において、5   e  3  10
LS
の電子に対してICスペ
クトルと光度曲線を計算し、Suzakuの観測と比べた
4
結果
 スペクトルべき指数が観測と一致
 γmin ∝ FGeV なら光度曲線を再現
 X線はICで冷却した電子からの放射
 X線の変動は注入率の変動による


GeV, TeVはIC放射で説明できる(我々の先行研究)
→ X線からTeVまですべてIC放射で説明できる
star
star
展望
ガンマ線連星を本当に理解するには…

星風とパルサー風orジェットの
(磁気)流体シミュレーションが必要!
流体計算を取り入れた放射の計算はいくつか
ある(Takata et al. 2012; Zabalza et al. 2012)
 これからもっと発展させていくべき!
ガンマ線連星系を用いて
高密度星近傍の物理に迫れる

議論


γminを固定した時の変数を添え字0で表わす
FX: X線フラックス、γX: X線を出す電子のγ
F X  F X,0
n ( X )
n 0 ( X )
電子分布より、
( F  n )
  min
n (  X )  n (  1 ) 
 1




 ( p 1)
n 0 ( 1 )  n 0 ( X )
 n (  X )  n 0 (  X )  min
p  2 . 5 なら
F X  F GeV
-1/2
F X  F X,0  min
, F X,0  FGeV より
1 p
1 .5
 min  F GeV
 X


 min




2
Microquasar model (= accretion + jet)
 コンパクト星はBH
 星風をaccretion → jet
 jet内の衝撃波で粒子加速→非熱的放射
Pulsar model (= pulsar wind + stellar wind)
 コンパクト星はNS
 二つのwindの衝突により衝撃波
 そこで粒子加速→非熱的放射
star
対立する二つの放射モデル
star
Orbital parameters of LS5039
supc
CS
periastron
CS
MS
apastron
CS
infc
observer
CS
Orbit of LS 5039(head on)
Compact star (CS) +
Massive star (MS, O6.5)
 Period : 3.9 days
 Separation
at periastron… ~2Rstar
at apastron…~4Rstar
(Rstar~ 10 12 cm)

Observations of LS 5039
superior
inferior
F. Aharonian, et al., 2006, A&A, 460, 743
A. A. Abdo, et al., 2009, ApJL, 706, 56
T. Takahashi, et al., 2009, ApJ, 697, 592
Fermi
HESS
INFC
SUPC
Suzaku
HESS
SUPC
Fermi
MS
INFC
Suzaku
Photon energy (eV)
Phase-averaged spectra
・X-ray & GeV anticorrelate
Orbital phase
Light curves
Model (Yamaguchi & Takahara 2010)




Constant and isotropic injection of electrons at CS (power-law
distribution)
observer
Cooling only by IC process → cascade
Electrons emit photons at
×
×
the injection or creation sites
×
The uniform magnetic field
MS
CS
×
We calculate spectra and light curves by
① the cascade process with Monte Carlo
method (GeV to TeV)
②the synchrotron emission using the e±
distribution for B = 0.1 G (X-ray)
(parameters: the inclination angle &
the power-law index of injected electrons)
×: annihilation
position
→:IC photon path
→:MS photon path
Electron distribution and anisotropic IC pectra
Electron energy distribution
in steady state (index: 2.5)
apastron
Anisotropic IC spectra
without γγ absorption
Head-on
Rear-end
periastron
・KN effect flattens the electron ・Anisotropic IC emission of headdistribution
on collision is more intense since
collision rate is higher
・The electron number is larger
at apastron due to suppression ・Anisotropy is suppressed by
of IC cooling
KN effect at higher energy
Comparison with observations (spectra)
・variation in GeV band
・ratio of TeV to GeV flux
is fitted
_
INFC

_
synchrotron

Qualitative fit to
observations
No fit to X-ray
observations when
B = 0.1G

When 3G, the best fit
SUPC
3G
0.1G
Inclination angle: 30°
Power-law index: 2.5
IC cascade
Photon energy (eV)
Under this,
synchrotron cooling is
dominant
Comparison with observations (light curves)
Inclination angle: 30°
power-law index: 2.5
TeV
X
Orbital phase
GeV
Orbital phase
TeV: roughly reproduced
GeV: well reproduced
X-ray: a phase difference
Orbital phase
(numerical results are normalized with maxima of observation)
Modulation mechanism in TeV, GeV and X-ray



TeV: absorption is dominant
TeV
At supc, flux is smaller than
infc by the large density of
stellar radiation field
GeV: IC anisotropy is dominant CS(superior) MS
At supc, flux is larger than
GeV
supc by head-on collision of IC
scattering
X-ray: e± number variation by
IC cooling
At periastron, the e± number CS(superior) MS
in steady state is smaller than
X-ray
apastron by IC cooling in the
large density of stellar
radiation field, so emissivity
by synchrotron is smaller,
CS(pariastron) MS
therefore flux is smaller
Binary axis
CS(inferior)
CS(inferior)
CS(apastron)
spectral break at ~1 GeV

If electrons scatter off stellar photons, the break is
not reproduced
 Assume that the break is due to γγ absorption
 Typical energy of absorbed photons: tens of GeV
E abs ~ ( E e, rest
mass
)
2
E  ~ 30 GeV
Yamaguchi & Takahara, 2010, ApJ, 717, 85
3G
If 10 times of this
E abs ~ 3 GeV
Therefore,

We assume that electrons
scatter off 100 eV photons
0.1G
2-area model (without 100eV photon)

e± are accelerated up to 1TeV and radiate in the area
(1) where B=3G ( rgyro ~ 10 9 cm )
e± are accelerated from 1 to 30TeV and radiate in the
area (2) where B=0.1G ( rgyro ~ L system )
Calculation method

B=0.1G
We inject e± with energy,
1 GeV  E e  50 TeV(index
: 2.5)
B=3G
9
e± with E e  1 TeV
10 cm
CS
 are injected in area(1) and IC
photons cascade in 100eV radiation field
e± with E e  1 TeV
12
10 cm ( L system )
 are injected in area(2) and IC photons
cascade in stellar radiation field
we count the escaped photons
O star

Results of 2-area model without 100eV
Inclination angle: 30°
INFC ー
SUPC ー
30TeV photons are
emitted and X-ray
flux match obs
Problem
 10GeV spectra do
not match obs
 As well, 10TeV
(SUPC)

Model with 100eV photons
Requirement for 100eV source
No influence on Suzaku data
L100eV  10

34
erg s
Optical depth τ > 1
-1
B=3G
9
10 cm
CS
O star

B=0.1G
R100eV  10 cm
8
12
Electron injection
10 cm ( L system )
e± are accelerated up to 1TeV and emit near 100
eV source where B=3G ( rgyro, max ~ 10 9 cm )
 e± are accelerated from 1 to 50TeV and emit far
from 100 eV source where B=0.1G ( rgyro, max ~ L system )
we calculate cascade with 100eV photons near
the source, and with stellar photons far from it

Results

i  30 , 1 GeV  E e , inj  50 TeV(index
: 2.5)
GeV break is
reproduced
 But…
 X-ray spectra terribly
underestimate
 No orbital variation
in GeV & X-ray band

Discussion
Underestimation at X-ray
 Energy density of 100 eV photons is larger than that of
2
3
stellar photons. U
U
~
L
L
(
R
R
)
~
10
100eV
Ostar
100eV
Ostar
Ostar
100eV
→ IC cooling time shorter
Superior conjunction
→ the number of e± smaller
No variation in GeV & X-ray band
 e± scatter off photons near CS
→ direction to CS independent of phase
O star
CS
→ No modulation in GeV band
Inferior conjunction
 The number of electrons does not
change by the orbital motion
→ No modulation in X-ray band
O star
CS
DISCUSSION 2
Underestimation at TeV
 TeV flux is underestimated
 GeV flux is overestimated
 We assume that 100eV photons are isotropic
 The flux by IC scattering is large compared with
anisotropic photon field
Anisotropic photon field
O star
HEe± source
Isotropic photon field
Photons through headon collision are seen
from any direction
Spectra only with inverse Compton
Actually, flux of IC in the 100eV field exceed that in the stellar field
2-area model
2-area model & 1-area model
SUPC
SUPC
INFC
INFC
Flux in the 2-area model is larger than the other
→the anisotropy of target photons is important
 Independent of photon density and target photon

Summary
For LS 5039, the break in calculated GeV spectrum is
different from that in observed one.
 So we introduce 100eV photon source
→ spectral break is reproduced but…
 X-ray flux is underestimated (by large photon density)
 X-ray & GeV have no variation (by isotropy of 100eV)
 it is difficult to explain the high energy emission by
the model with 100 eV photons
 With 100eV source, we introduce orbital variation of
injection (as in Owocki et al. 2010, proceeding)
 Without 100eV source, we regard GeV cutoff as high
energy cutoff of injected e±


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