Magnetar Magnetospheres

1. Magnetar Magnetospheres Yue Shen AST 541 Thompson, C., Lyutikov, M., Kulkarni, S. R. 2002, ApJ, 574, 332 Beloborodov, A. & Thompson, C. 2006, Ap&SS, 308, 631 (2007, ApJ, 657, 967) John Rowe Animations

2. Outline • What is a “Magnetar”? • Magnetar Candidates: SGRs and AXPs • A model for magnetar magnetosphere • Open questions

3. Magnetar: strongly magnetized neutron star (B >~ 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)

4. 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.

5. Spectral properties of SGRs and AXPs Persistent X-ray emission AXP 4U 0142+614 (Rea et al. 2007, MNRAS, 381, 293) AXP 1E 1841-045 (Kuiper et al. 2004, ApJ, 613, 1173)

6. Spectral properties of SGRs and AXPs Energy spectrum of SGR/AXP bursts Woods & Thompson (2004) Typical light curves of SGR bursts

7. 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

8. Spatial distribution Characteristics of SGRs/AXPs SGRs/AXPs

9. Magnetar Model • Magnetic field decay as the main energy source for persistent X-ray luminosity and bursts/flares • What happens when a B>~1e14-1e15 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

10. 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.

11. 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.

12. Thompson et al. 2002 NS surface The twist is initiallyconfined to the interiorof the star, sothat the current closesat the surface byflowing across the magneticfield. The resulting (1/c)J×B force causes the liquidnear the surface torotate, so as todistribute the twist moreuniformly along the magneticfield lines. The neteffect is to forcethe current to flowout of the star,into its "magnetosphere." Inthe case of amagnetar, this process maybe partly stabilized bythe rigidity of thecrust, so that theexternal field twists upintermittently (giving rise toSGR flares).

13. Thompson et al. 2002 • Construct a self-consistent twisted external field. • Force-free hydromagnetic equilibrium • Self-similar configurations: labeled by net twist angle

14. Thompson et al. 2002 Dipole magnetic field Twisted magnetic dipole

15. Thompson et al. 2002 • 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 Based on this magnetic field geometry, they carry out Consistent with typical persistent X-ray luminosity of AXPs

16. Thompson et al. 2002 • Decay of the external twist • The energy of a twisted magnetosphere exceeds the energy of a pure dipole

17. 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.

18. 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.

19. 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. Duncan’s Magnetar homepage (http://solomon.as.utexas.edu/~duncan/magnetar.html)