Episodic magnetic jets as the central engine of GRBs Feng Yuan With: Bing Zhang

OUTLINE Two key requirements to GRB model: Two key requirements to GRB model: Magnetically dominated Magnetically dominated Episodic ejection Episodic ejection Two types of observed jets: Two types of observed jets: Continuous jets Continuous jets Episodic jets Episodic jets Magnetic episodic model for GRB Magnetic episodic model for GRB

Challenge to the “ fireball-shock ” model from Fermi observations Sigma: ratio between Poynting flux and baryonic flux:  = L P /L b : at least ~ 20, 15 for GRB C Confirmed by Fan (2010) with a wider parameter space study. See also Gao et al. (2009) Fireball model prediction Zhang & Pe ’ er 2009

Debate on GRB composition: fireball or magnetic? Raleigh, NC, USA, Mar. 6, 2011

thermal component in most GRB implies: The absence of thermal component in most GRB implies: GRBs are magnetically dominated, the traditional “ Fireball-shock ” model does not work!

Episodic nature of ejection Observation: Rapid variability in the lightcurve Observation: Rapid variability in the lightcurve  episodic nature  episodic nature In almost any models: they require the collisions between shells or blobs In almost any models: they require the collisions between shells or blobs But in current models, it is unclear whether they can be produced: But in current models, it is unclear whether they can be produced: E.g.: BZ model: E.g.: BZ model: Steady model  Continuous jet/outflow Steady model  Continuous jet/outflow Kink instability or rapid variability of accretion rate? Kink instability or rapid variability of accretion rate?

Other problems with current models Low efficiency Low efficiency Fast cooling problem Fast cooling problem Electron number excess problem Electron number excess problem Ep – Eiso (Liso) correlation inconsistency Ep – Eiso (Liso) correlation inconsistency Magnetic shock scenario is inefficient (Narayan et al. 2011) Magnetic shock scenario is inefficient (Narayan et al. 2011) B configuration is helical: not easy for B reconnection (Zhang & Yan 2010) B configuration is helical: not easy for B reconnection (Zhang & Yan 2010) Zhang & Yan 2010 Fireball: BZ model:

Evidence for episodic jet in Sgr A* Radio light curves and cross correlation X-ray flare Ejection of radio blobs; associated with X-ray & IR flares

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

Continuous and episodic jets in M87 M87:VLA+ 43GHz; Walker et al. 2007

Two types of mass outflow in the Sun Solar wind Solar wind Continuous Continuous Coming from region of open magnetic field Coming from region of open magnetic field Coronal mass ejection (CME) Coronal mass ejection (CME) Episodic Episodic Coming from region of closed magnetic field Coming from region of closed magnetic field Speed up to 2000 km/s and beyond Speed up to 2000 km/s and beyond

Comparison between two types of jets (for GRS ) 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 (and also hard state!) Fender & Belloni 2004, ARA&A

Our idea: Magnetic episodic jet blobs collision  GRB

Episodic magnetic jet model of GRB: General picture 100 Rs Magnetic blobs Blobs collision  GRBs NDAF ADAF

Formation of flux rope in corona Blandford 2002 * Reconnection and flare * Formation of flux rope

MHD model for episodic jets (I): Formation of flux rope in disk corona The direction of B of two collision blobs has some angle, thus easy for reconnection Yuan et al. 2009

MHD model for episodic Jets (II): Ejection of flux rope MHD model for episodic Jets (II): Ejection of flux rope Yuan, Lin, Wu & Ho 2009

Two timescales of the model Emergence of B field line by Parker Inst. Emergence of B field line by Parker Inst. Energy transfer by Alfven wave Energy transfer by Alfven wave

Blob energy Blob energy Power output Power output Energetics  Energy & power: both consistent with observations  Power output from the ADAF dominates!

Two-Component Variability slow variability component related to central engine fast variability component related to turbulence Gao et al. 2011

How to explain them? Duration of prompt pulse (slow component): angular spreading time (Piran 1999) Duration of prompt pulse (slow component): angular spreading time (Piran 1999) consistent with observations consistent with observations Rapid spikes (fast component): Rapid spikes (fast component): Relativistic turbulence (Narayan& Kumar 2009) Relativistic turbulence (Narayan& Kumar 2009)

Summary: appealing features of the model Existence of episodic jet: Existence of episodic jet: strong observational evidence in BH systems; strong observational evidence in BH systems; more powerful than continuous jets more powerful than continuous jets Strongly magnetized: consistent with Fermi result Strongly magnetized: consistent with Fermi result Intrinsically individual blobs, whose collision is important to explain variability (Zhang & Yan 2010) Intrinsically individual blobs, whose collision is important to explain variability (Zhang & Yan 2010) Easy for B reconnection; thus high efficiency possible Easy for B reconnection; thus high efficiency possible