1. An MHD Model for the Formation of Episodic Jets
Feng Yuan
Shanghai Astronomical Observatory, CAS
with:
Jun Lin (YNAO/CfA); Kinwah Wu (UCL); Luis C. Ho (OCIW)

2. Outline Introduction to two types of jets:
Continuous jets
Episodic jets
CME in the Sun: observation and model
MHD model for episodic jets

3. Real pictures of a jet (in M87)
43GHz; Walker et al. 2007
M87:VLA+ HALCA
Resolution: 0.21mas

4. Evidence for episodic jet in Sgr A*
Radio light curve and cross correlation
X-ray flare
Conclusion: Ejection of radio blobs; associated with X-ray flare.
Note: there is no (normal) jet in Sgr A*!

5. Episodic jets in GROJ1655-40 Radio light curve
Hjellming & Rupen 1995, Nature
VLBA Images at 1.6 GHz

6. Episodic jets in GROJ1655-40 From Hjellming & Rupen (1995, Nature):
The source appears to be made up of a number of discrete, fast-moving components ejected from a central source.
The jet ejection, at 92% of the speed of light, appears episodic and asymmetric. The observed asymmetry in brightness is often much higher (than relativistic beaming prediction), and in any case flips from side to side;
These cannot be explained by relativistic beaming. The jets themselves must be intrinsically asymmetric, and the sense of that asymmetry must change from event to event.
The spectrum is initially optically thick, then optically thin.

7. Episodic jets associated with hard-soft state transition
Fender, Belloni & Gallo 2004

8. Summary: comparison between continuous and episodic jets
continuous jets episodic jets
steady episodic
optically thick spectrum optically-thin spectrum
low polarization (<5%) high polarization (~20%)
low velocity highly relativistic
associated with hard state associated with the
hard->soft transition
Fender & Belloni 2004, ARA&A

9. MHD model for continuous jetsB
grav. force BH r
Blandford & Payne 1982; From Spruit 1996
Question: How to understand the formation of episodic jets??

10. Two type of mass outflow in the Sun
Solar wind
Continuous
Coming from region of open magnetic field
Coronal mass ejection (CME)
Episodic
Coming from region of closed magnetic field
Speed up to 2000 km/s and beyond
Occurrence rate: from once a few weeks to several times per day

11. Internal structure of the Sun

12. Examples of observed CME events

13. Formation of flux rope

14. Lin & Forbes Model of CME
Lin & Forbes 2000, JGR

15. Lin & Forbes CME/Flare Model (Movies)
flare ribbons and loops (code for animation courtesy of Terry Forbes)
global evolution of the eruption

16. Structure of black hole accretion disk system
De Villers et al. 2003

17. Magnetic field configurations in black hole accretion disk system
1.Reconnection and flare
2.Formation of flux rope
From Blandford 2002

18. Numerical simulation: Episodic feature of jets
De Villers et al. 2003; Machida, Nakumura & Matsumoto 2004

19. MHD model for Episodic jets (I): formation of flux rope in disk corona
Yuan, Lin, Wu & Ho 2008 (arxiv: )

20. MHD model for episodic Jets (II): ejection of flux rope
Yuan, Lin, Wu & Ho 2008 (arxiv: )

21. Equations describing the dynamics of the ejecta
Bext: the whole magnetic field except that produced by the
current in the flux rope
Fg: gravitation force
The current and magnetic field are described by (force free
since it is magnetically dominated in the corona):

22. Velocities of ejecta vs. Time & Distance for the Case of Sgr A*
Yuan, Lin, Wu & Ho 2008 (arxiv: )

23. Explaining the main features of episodic jets (I)
require an energy-storage process
High velocity (larger Lorentz factor):
Magnetic energy is higher because of the storage stage and the energy is released quicker (impulsive release).
High polarization:
Radio emission comes from synchrotron emission of the shocked ISM in front of the blob
High polarization is the result of ordered magnetic field and the small optical depth

24. Explaining the main features of episodic jets (II)
Association with state transition from hard to soft states of black hole X-ray binary:
The state transition is associated with the collapse of the accretion flow;
2. During the collapse, magnetic field is strongly amplified due to the conservation of flux
3. Such a configuration is highly unstable, and substantial magnetic field is expelled out
4. These magnetic energy will drive CME and flares
Esin, McClintock & Narayan 1997

25. Thank you!