Документ взят из кэша поисковой машины. Адрес оригинального документа : http://hea.iki.rssi.ru/conf/hea2010/Presentations/24/Shakura.pdf Дата изменения: Wed Jan 26 13:25:09 2011 Дата индексирования: Tue Oct 2 00:47:14 2012 Кодировка: Поисковые слова: arp 220

Accretion torques in windaccreting pulsars
Nikolay Shakura Sternberg Astronomical Institute Moscow, Russia K.Postnov (SAI), D. Klochkov, V. Doroshenko, V. Suleimanov (IAAT), L. Hjalmarsdotter (SAI)

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Outline
Introduction: disc and quasi-spherical accretion Settling accretion regime Spin-up/spin-down torques Specific pulsars Conclusions

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I. Disc and quasi-spherical accretion

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Disc accretion
Shakura N.I. Astron. Zh. 49, 921 (1972) Shakura N.I., Sunyaev R.A. Black holes in Binary systems. Observational Appearance. Astron. Astrophys. 24, 337 (1973)

Realized when the secondary overfills Roche lobe
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Quasi-spherical accretion
"Canonical" picture
Neutron star Normal star

Interchange instabilities

?
Shocks Accreting matter
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Can occur when matter with low angular momentum is captured from stellar wind Free-fall (Bondi) accretion onto magnetosphere with shock If carries angular momentum, only spinup is possible (unless counter-rotating)

(Arons & Lea 1976)
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Subsonic settling accretion without shock near magnetosphere
Stellar wind from the secondary Convective isomomentum shell Matter subsonically settles down inside the spherization radius R'B

(R)~1/R

2

RA is the Alfven radius of NS R
A

RB'~(1/2-1/3) R Spherization radius

B

RB~2GM/V2 (Bondi radius) R characterizes bow shock location in the wind


II. Observations: spin-up/spin-down of X-ray pulsars

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X-ray pulsars (BATSE)

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Bildsten et al 1997

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GX 1+4:

P~155 s, Porb=1161 d

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Gonzalez-Galan et al 2010

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Cross-correlation P-LS (Fermi-Swift)

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Correlation P-Fx (Fermi GBM)
500

P/P~ F/F

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4U 1626-67: P=7.66 s, Porb~42 min

Camero-Arranz et al.2010

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GX 301-2: P~680 s, Porb=41.5 d

Bildsten et al 1997
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Shakura et al. 2010
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Vela X-1: P~283 s, Porb=8.96 d

Bildsten et al 1997
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Shakura et al. 2010
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III. Accretion torques: theory
Torque due to magnetic force
over Alfven surface

Bp is (almost) dipole B t ~ Bp
~ (RC RA)-3/2
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Cf. disc accretion: ~ (Rc)
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Alfven radius:
From pressure balance:

K2=(Bp/B(dipole))2 ~ 7.56 (Arons & Lea 1976) f(s), f(u) < 1, f(u) = uR/uff, uff= (2GM/R)
1/2

Settling regime: (almost) adiabatic hydrostatic equilibrium, P~5/3

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Angular momentum equation

Taking into account that the matter falling onto NS surface brings angular momentum z Mdot RA2

(Spin-down is possible only for Z>z)
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Spin-up

4.4
Spin-down

GX 1+4 Correlations period-flux fluctuaitons (Fermi-Swift)

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Magnetic field determination
At any luminosity:

1 4.4
At the torque reversal point:

y

NB: Magnetic field estimate is independent of L
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x


Specific systems
Pulsar GX 1+4 4U 1626-67 GX 301-2 Vela X-1
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P, s

Porb

Vw, km/s

~140 1161 d 100 7.66 680 283

[K1f()]1/2 Z 1030G cm3 53 5.5 6 2.7 2
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42 min 2000 1 41.5 d 500 8.96 d 500
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Is disc accretion possible in these pulsars?
Equilibrium period for quasi-spherical accretion:

Spin evolution equation Quasi-spherical: disc accretion:

Unrealistically small for typical B~1012G
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Conclusions
A theory of quasi-spherical subsonic accretion onto rotating magnetized NS is constructed Accretion rate through the shell is determined by the ability of the plasma to enter the magnetosphere Magnetic fields of X-ray pulsar with quasispherical accretion can be determined without the knowledge of X-ray luminosity
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Spin-up/spin-down correlation with X-ray flux behave differently for specific windaccreting pulsars depending on accretion rate ( dot M_cr) Theory has three dimensionless parameters, which must be the same in all systems once has been determined in one system (the similarity principle). Dimensional parameters , dot M, * can be different.
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Vela X-1 GX 301-2 4U1626-67

1 4.4
GX 1+4

y

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