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)

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

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

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*!

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

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.

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

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

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

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

Internal structure of the Sun

Examples of observed CME events

Formation of flux rope

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

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

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

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

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

MHD model for Episodic jets (I): formation of flux rope in disk corona Yuan, Lin, Wu & Ho 2008 (arxiv:0811.2893)

MHD model for episodic Jets (II): ejection of flux rope Yuan, Lin, Wu & Ho 2008 (arxiv:0811.2893)

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):

Velocities of ejecta vs. Time & Distance for the Case of Sgr A* Yuan, Lin, Wu & Ho 2008 (arxiv:0811.2893)

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

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

Thank you!