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Pulsars

Pulsars. Basic Properties. part of angular momentum carried away by shell. field lines frozen into solar plasma. Supernova Explosion => Neutron Stars. (surface field). What is a Neutron Star ?. ” Normal“ Pulsars T > 20 ms. Millisecond Pulsars 1 ms < T < 20 ms.

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Pulsars

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  1. Pulsars Basic Properties

  2. part of angular momentum carried away by shell field lines frozen into solar plasma Supernova Explosion => Neutron Stars (surface field)

  3. What is a Neutron Star ?

  4. ”Normal“ Pulsars T > 20 ms Millisecond Pulsars 1 ms < T < 20 ms Pulsar Statistics => 2 basic groups number log( T / s ) Crab: T = 33 ms Vela: T = 89 ms

  5. Why T > 1 ms ? virial theorem centrifugal forces would disrupt neutron star if rotating faster!

  6. Discovery of Pulsars in Radioastronomy PSR 1919+21 T = 1.33 s Where is the radiation coming from?

  7. Pulsars as Magnetic Dipole Antennas misaligned magnetic rotator model magnetic braking by emission of polarized magnetic dipole radiation:

  8. The Braking Index magnetic braking predicts: braking index n measurable via: • Crab: n = 2.515 ± 0.005 • PSR 1509-58: n = 2.8 ± 0.2 • PSR 0540-69: n = 2.01 ± 0.02 not bad...but other braking mechanisms seem to be active in addition!

  9. R Crab Pulsar: Magnetic Field at Surface

  10. · log10 T 108 T 106 yr 106 T 1010 yr decay of BSwith τ = 107 yr T (s) The Age of a Pulsar

  11. explosion observed in 1054 => pulsar properties today: T = 33 ms prediction: Example: Crab Pulsar Not too bad !!

  12. Rotating Pulsar = Unipolar Inductor • Eind surface forces 1012 times stronger than gravity (Crab) • charge particles (electrons...) dragged off surface and accelerated to large energies => • pulsar wind (power source for plerions) • coherent radio emission from e+e–-cascades in B-field at poles

  13. · log10 T T (s) Limit for Coherent Radio Emission (M.A. Ruderman, P.G.Sutherland: Astrophys. J.196 (1975) 51.) theory for coherent emission from e+e– cascades along pole field lines => e+e– death line

  14. magnetosphere:plasma moves along rigid field lines The Pulsar Magnetosphere Crab in X rays

  15. The Pulsar Magnetosphere rigid body approximation breaks down at light cylinder:

  16. Magnetosphere Charge Density (P. Goldreich, W.H.Julian: Astrophys. J.157 (1969) 839.) ignore currents

  17. general case Magnetosphere Charge Density

  18. STAR Open Magnetic Field Lines toroidal field outside light cylinder dragged by outflowing plasma equatorial neutral current sheet for misaligned pulsar

  19. The Origin of X and γ Radiation • vacuum gap acceleration of e± at • polar cap ?? • outer gap ?? • other models (J.Kirk et al.) ?? • differentiate by observation • X/γ vs. radio pulse pattern • shape of high energy cut-off

  20. Pulse Patterns up to Egret Energies

  21. · log10 T T (s) • What are Millisecond Pulsars? mostly binary systems! very bright X-ray sources close to Eddington luminosity => accretion!!

  22. Alvén radius: Accretion in Strong B-Fields giant star feeds accretion disk around neutron star accretion disk dynamics: orbits with slowly decreasing Kepler radii until plasma magnetically dominated: rM

  23. · log ( -T / T ) M = M R = 10 km L = LE • log ( T (L/1030W)6/7 ) Spin-Up Phase angular momentum transfer from disk onto pulsar (friction at rM ): end of spin-up at Ω = ΩKepler(rM):

  24. · log10 T T (s) Spin-Up Limit spin-down due to magnetic dipole radiation limited by spin-up (Tmin): spin-up limit fulfilled for all ms-binaries but not at all for normal pulsars (as expected)!!

  25. Summary • models for pulsars and X-ray binaries pretty successful • open problem: gamma ray emission from pulsars ? • open problem: which processes contribute to gamma ray emission from surrounding SNRs ?

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