Magnetar Magnetospheres

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Magnetar Magnetospheres

• Yue Shen AST 541
• Thompson, C., Lyutikov, M., Kulkarni, S. R. 2002,
  ApJ, 574, 332
• Beloborodov, A. Thompson, C. 2006, ApSS, 308,
  631 (2007, ApJ, 657, 967)

John Rowe Animations
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Outline

• What is a Magnetar?
• Magnetar Candidates SGRs and AXPs
• A model for magnetar magnetosphere
• Open questions

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Magnetar strongly magnetized neutron star (B gt
1e14-1e15 G)

• Theoretical motivations (Duncan Thompson 1992)
• Rapid dynamo during the formation of neutron
  stars (Burrows Lattimer 1988)
• Observational motivations gamma-ray bursts
  (GRBs) soft gamma repeaters (SGRs Paczynski
  1992) anomalous X-ray pulsars (AXPs)

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Magnetar Candidates

• SGRs (soft gamma repeaters)
• Repetitive soft gamma-ray bursts (flares) peak
  luminosity up to 1e41 erg/s
• 5-8 seconds
• 1000 yr
• 4

AXPs (anomalous X-ray pulsars) Persistent X-ray
luminosity 5e34-1e36 erg/s 6-12 seconds
3000-400,000 yr 6 (7?)
Obs. Definition
Spin period
Age
Number confirmed
Similarities in SGRs and AXPs indicate they
belong to the same class of objects.
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Spectral properties of SGRs and AXPs
Persistent X-ray emission
AXP 4U 0142614 (Rea et al. 2007, MNRAS, 381, 293)
AXP 1E 1841-045 (Kuiper et al. 2004, ApJ, 613,
1173)
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Spectral properties of SGRs and AXPs
Energy spectrum of SGR/AXP bursts
Woods Thompson (2004)
Typical light curves of SGR bursts
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Magnetar Candidates

• Evidence that SGRs and AXPs are neutron stars
• Association with SNRs (3 AXPs and 1/2 SGRs)
• Timing properties X-ray pulses, glitches, etc.
• But they are different from rotation-powered
  pulsars
• They are younger, sec spin period and rapid
  spin down ( )
• No radio pulses (might be a selection effect) Not
  true anymore!
• Extraordinarily strong magnetic fields (both
  internal and external)
• Much higher energy output (persistent X-ray
  luminosity and bursts/flares), powered by
  magnetic fields decay

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Characteristics of SGRs/AXPs
SGRs/AXPs
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Magnetar Model

• Magnetic field decay as the main energy source
  for persistent X-ray luminosity and bursts/flares
• What happens when a Bgt1e14-1e15 G magnetic field
  decays?
• The internal field is strong enough to push
  material around in the star's interior and crust,
  leading to the dissipation of a significant
  amount of magnetic energy it heats up the deep
  crust and core of the NS
• It transports magnetic helicity outward from the
  interior and twists the external poloidal field
  lines (especially important following periods of
  burst activity) it drives currents along arched
  magnetic field lines, which gives rise to
  streaming charged particles ? scatter X-ray
  photons off them slam against the star when they
  reach the footpoints of magnetic field lines,
  heating patches on the surface
• Increases the braking torque and spin-down rate
  after the burst

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Magnetar Model

• As the tremendous magnetic field drifts through
  the solid crust of the magnetar, it stresses the
  crust with magnetic forces which get stronger
  than the solid can bear. This causes sudden
  deformations (starquake) in the crust structure,
  leading to bright outbursts.

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A quantitative model for magnetar magnetosphere
(Thompson et al. 2002)

• Here we go

• Eq(1)Eq(2)...Eq(49)Eq(A1)
  Eq(B15)

• Internal magnetic field transports helicity
  outward and twists the external field, and
  diverts an electrical current from the interior
  to the exterior.

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Thompson et al. 2002
NS surface
The twist is initially confined to the interior
of the star, so that the current closes at the
surface by flowing across the magnetic field. The
resulting (1/c)JB force causes the liquid near
the surface to rotate, so as to distribute the
twist more uniformly along the magnetic field
lines. The net effect is to force the current to
flow out of the star, into its "magnetosphere."
In the case of a magnetar, this process may be
partly stabilized by the rigidity of the crust,
so that the external field twists up
intermittently (giving rise to SGR flares).
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Thompson et al. 2002

• Construct a self-consistent twisted external
  field.
• Force-free hydromagnetic equilibrium
• Self-similar configurations labeled by net twist
  angle

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Thompson et al. 2002
Dipole magnetic field
Twisted magnetic dipole
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Thompson et al. 2002
Based on this magnetic field geometry, they carry
out

• Calculations of resonant cyclotron scattering
  opacity
• Surface heating of a magnetar.
• Impact of the current-carrying charges on the
  stellar surface
• Resonant Comptonization of surface X-ray flux by
  the magnetospheric currents

Consistent with typical persistent X-ray
luminosity of AXPs
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Thompson et al. 2002

• Decay of the external twist
• The energy of a twisted magnetosphere exceeds the
  energy of a pure dipole

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Beloborodov Thompson (2007)

• Where comes the plasma needed to conduct the
  current?
• A dense thermalized plasma is present in the
  magnetosphere following the X-ray outburst caused
  by a starquake it remains suspended for some
  time during the thermal afterglow phase.
• When the plasma density decreases, the current
  decays and generates a sufficient self-induction
  voltage that helps the magnetosphere to
  re-generate the plasma that carries the current.

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Open questions

• Birth rate of AXPs and SGRs
• Do SGRs, AXPs and high B-field radio pulsars form
  a continuum of magnetic activity?
• How does the Magnetar model work? More
  quantitative calculations magnetar physics, etc.
  
• This is a rapidly evolving field, so nothing is
  conclusive at this moment.

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Additional References

• Manchester, R. N. et al. 2004 Science , 304,  542
  
• Woods, P. Thompson, C. 2006, in "Compact
  Stellar X-ray Sources", eds. W.H.G. Lewin and M.
  van der Klis
• Duncan, R. C. Thompson, C. 1992, ApJL, 392, 9
• And most importantly Robert C. Duncans Magnetar
  homepage (http//solomon.as.utexas.edu/duncan/mag
  netar.html)