1 / 24

Relativistic Reconnection Driven Giant Flares of SGRs

Cong Yu ( 余聪 ) Yunnan Observatories. Relativistic Reconnection Driven Giant Flares of SGRs. Collaborators : Lei Huang Zhoujian Cao. Outline. Relevant Background Catastrophic Behavior of Flux rope Reconnection Driven Giant Flares Conclusion. Flux Rope ---

otisr
Download Presentation

Relativistic Reconnection Driven Giant Flares of SGRs

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Cong Yu ( 余聪) Yunnan Observatories Relativistic Reconnection Driven Giant Flares of SGRs Collaborators : Lei Huang Zhoujian Cao

  2. Outline • Relevant Background • Catastrophic Behavior of Flux rope • Reconnection Driven Giant Flares • Conclusion Flux Rope --- Helically Twisted Magnetic Structure Flux Rope --- magnetic structure Double Helix DNA

  3. What is Magnetar ? Artists’ Imagination Magnetic+Star => Magnetar Energy source : magnetic field persistent emission & sporadic burst Ultra-strongly magnetic field due to the dynamo amplification Duncan & Thompson, 1992, ApJ Thompson & Duncan, 1995, MNRAS

  4. Basic Facts about Magnetars Slow Rotation : typically 5 – 12 s Fast spin-down rate :10-11 s s-1 (cf. 10-15 s s-1 ) Young objects:1000 – 10000 yrs Typical spin-down power: 1032-33 ergs/s Persistent emission:1035-36ergs/s, 0.5-10 Kev Short Bursts:0.01-1s, 1041-42ergs/s, 20-40 Kev Giant Flares:minutes, 1044-46ergs/s, 250-500 Kev

  5. Main manifestations of Neutron Stars: • (Radio) Pulsars - Powered by rotational energy • Accreting X-ray binaries - Powered by gravitational energy Magnetars do not fit in these two categories !

  6. How Strong Magnetic Field is in Magnetars ? Magnetars B = 1014 – 1015 G Typically, NS B = 109 - 1012Gpowered by accretion/rotational energy cf. the strongest man-made B-fields:~ 5 x 105 G (steady) ~ 107 G (for a few microsecond)

  7. Giant Flares 1979 March 5 – SGR 0526-66 L peak ~ 4 x 1044 erg/s ETOT ~ 5 x 1044 erg 1998 August 27 – SGR 1900+14 L peak > 8 x 1044 erg/s ETOT > 3 x 1044 erg 2004 December 27 – SGR 1806-20 L peak ~ 2-5 x 1047 erg/s ETOT ~ 2-5 x 1046 erg

  8. Giant Flares • Initial spike:t ~ 0.3 s , Eiso ~ a few1044-46 erg • hard spectrum • ~ ms rise time • Pulsating tail • Lasts a few mins • Modulated at the NS rotation period • Softer spectrum The 2004 Dec. 27 giant flare from SGR 1806-20

  9. Energy Release Mechanisms Physical processes by which the energy is released remains one of the great puzzles in magnetars. Two different models : I. Crust Model II. Magnetosphere Model

  10. Crust Model Sudden Crust Brittle Fracture, similar to Earthquake Strong magnetic fields induce stress on the crust. Abrupt untwisting of the interior magnetic field leads to sudden crust fracture Timescale ~ Shear Alfven wave timescale ~ 0.2 second Crust Model Thompson & Duncan (2000)

  11. Magnetosphere Model Note:Sudden magnetosphere rearrangement due to gradual changes occur at the surface. NOT sudden crust quakes ! Alfven crossing timescale ~ ~ R/Va ~ R/c Given the ~ ms rise time of giant flares, this model seems to be more consistent with observations. Lyutikov, 2006, MNRAS However, no quantitative constraint has been placed on the relativistic reconnection rate based on real observations.

  12. Part II Flux Rope Eruption Model

  13. Cartoon for Giant Flares (Flux Rope Model) Crust motion leads to the formation of helically twisted flux ropes. Similar to prominences in the solar corona.

  14. Views in another Perspective Yu, 2012, ApJ

  15. Little Complication: Current Sheet Central Object : Neutron Star Electric current inside flux rope : I Surface Magnetic Field : Major Radius : h Minor Radius : r0 Current sheet : r1 , r2 Yu & Huang, 2013, ApJ Letter Flux Rope : Local Twist cf. Beloborodov (2009) Global twist cf. Contopoulos, Kazanas, Fendt (1999) Current Sheet beyond Light Cylinder

  16. Inhomogeneous Grad-Shafranov (GS) Eqn Flux function : ↓ Inhomogeneous term : contribution from currents in the helically twisted flux rope We numerically solve this GS equation.

  17. Equilibrium Constraints Force Equilibrium Condition : Total Force = 0 (on the flux rope) Flux Frozen Condition : h : major radius of the flux rope r0 : minor radius of the flux rope

  18. Catastrophic Transition Huang & Yu, 2014a, ApJ Huang & Yu, 2014b, ApJ Catastrophic behaviors naturally explain a puzzle associated with magnetospheric model： gradual variations (quasi-static timescale)  sudden energy release (dynamical timescales) Three equilibrium branches appear alternatively : stable, unstable, and stable cf. S curves in accretion disc in CVs

  19. However … Take energy dissipation, i.e., magnetic reconnection into account ! We need a time-dependent model ! Part III

  20. Time-Dependent Relativistic Models Key parameter : Yu & Huang, In prep.

  21. Some Results Yu & Huang, In prep.

  22. Effects of Reconnection Reconnection Rate Spin-down rate during Giant Flare Yu & Huang, In prep.

  23. Conclusion We developed a self-consistent model which can constrain the relativistic reconnection according to the actual observations about magnetars. Reconnection may not be that rapid, Mach number ~ 0.001 is already enough to explain the observations. Spin-down behavior is consistent with magetar’s observations Important implication for solar CMEs, such as flareless CMEs.

  24. Thank you !

More Related