1. ARE RELATIVISTIC JETS ALWAYS MAGNETIC? Mitch Begelman & Eric Coughlin JILA, University of Colorado

2. Why we assume relativistic jets are propelled by large-scale B-fields: • Only way to tap BH spin (?) • OK, good point • Rel. electrons cool too rapidly so thermal pressure won’t work • Could use ion pressure if coupling to electrons weak • Radiation drag due to aberration limits acceleration by radiation pressure • Only applies at low optical depth (cf. fireball models of GRBs) • Need super-Eddington flux for radiative acceleration

3. The Magnetic Flux/Spin Paradigm- strict requirements here as well Angular velocity of engine Magnetic flux threading engine Jet power limited by amount of flux available

4. Tidal Disruption Event A star ventures inside the tidal radius of a black hole and is torn apart

5. Tidal forces … ... unbind ~half the debris … throw the other half into highly eccentric orbits Semi-major axis:

6. Energy pumped into the stellar debris by tides … Simulations by Guillochon & Ramirez-Ruiz 2013

8. Simulations by Guillochon & Ramirez-Ruiz 2013

10. Common wisdom says that matter falling back in excess of ṀE should be blown away: R> (Ṁ/Ṁ)Rg: thin Keplerian disk R> (Ṁ/Ṁ)Rg: regulates L~LE … but this may not always happen Shakura & Sunyaev 73

11. Super-Eddington TDE Swift J1644+57 l Tchekhovskoy et al. 2014 Bloom et al. 2011 Swift J2058+05 a second case? • Swift + Chandra light curves • L corrected for beaming • Radio “re-brightening” after ~ 4 months Cenko et al. 2012

12. Swift J1644+57 outburst suggestive of a beamed, relativistic flow = jet

13. Do TDEs have enough magnetic flux? Transient accretion events have access to a fixed amount of flux… Tidal Disruption Event candidate Swift J1644+57: Jet power: Lj > 1045 erg s-1 ~ 100 LE Flux needed:  > 1030 G-cm2 Flux available:  ~ 1025 B3 (R/R)2 G-cm2 Collapsar Gamma-Ray Burst: Jet power: Lj > 1050 erg s-1 ~ 1011 LE Flux needed:  > 1028 G-cm2 Flux available:  ~ 1025 B3 (R/R)2 G-cm2

14. An alternate approach

15. Powered by dissipation of turbulent B “Empty” funnel geff MRI

16. Powered by dissipation of turbulent B “Empty” funnel geff MRI

17. Reconnection converts energy to radiation Reconnection geff MRI

18. Mass-loading, collimation and acceleration Entrainment (by rad’n force) Reconnection geff MRI

19. Self-shielding (from drag) Entrainment (by rad’n force) Reconnection geff MRI Self-shielding from radiation drag

20. Max.  of a radiation-propelled jet: • Jet power Lj = l LE • “Terminal” Lorentz factor  = Lj/Ṁjc2 • based on available energy • Increase  by decreasing Ṁjc2 • but if Ṁ too small photons leak out before  is reached (for conical flow; 2/7 instead of 1/4 for paraboloidal) Rees & Meszaros 2005

21. Radiative self-shielding: Lj, j pressure p Drag important if STATIONARY

22. Radiative self-shielding: Lj, j pressure p Drag important if Boundary layer dragged by jet radiation, retarded by radiation from wall STATIONARY

23. Radiative self-shielding: Lj, j pressure p Drag important if Boundary layer dragged by jet radiation, retarded by radiation from wall BL1 for rays impinging on boundary layer  STATIONARY

24. Radiative self-shielding: Lj, j pressure p Drag important if BL can be dominated by kinetic energy: STATIONARY

25. Radiative self-shielding: Lj, j pressure p Drag important if Ratio of BL to jet energy: STATIONARY

26. Radiation-driven jet is pressure-confined • Spherical envelope with pressure pa~r- •  > 2  jet blows up envelope •  > 2  envelope crushes jet • Need evacuated funnel held open by rotation • … but not too wide a funnel (otherwise radiation can drive circulation or slow wind) WHERE MIGHT WE FIND SUCH FUNNELS?

27. Slim Disk Models of Hyperaccretion • Radial pressure force significant • Angular momentum below Keplerian • H/r ~ few tenths • Vertical and radial structure coupled • Can be modeled in 1D • 2D models more reliable Only possible if l/lKep large enough

28. disk opening angle 0.74 0.88 l/lKep A clue from self-similar slim disk models • Gyrentropes: s(l) • Quasi-Keplerian • Disk closes up at l close to lKep What is going on?

29. Case with mass loss … Assume  scaling for radial transport: Add radial pressure balance (ADIOS scaling)

30. Answer: l is too small to set up a flow with • Dynamical conditions don’t allow a bound disk-like flow • Flow “closes up” to axis as B  0  • Flow becomes “star-like” (with a rotational funnel) • Less disk “surface” to lose energy via wind • Flow reduces B instead by steepening density/pressure profiles

31. Less l steeper  higher accretion L Flow blows up or finds way to vent excess energy  equilibrium with B~0 (B<< GM/R) B=0

32. Summary: • Some low-l accretion flows unavoidably produce hyper-Eddington luminosities • TDEs, GRBs, maybe quasi-stars • Magnetic flux available might be too small to drive electromagnetic jets with adequate power • Radiation pressure is an alternative to driving relativistic jets under these conditions • Can drive the fastest jets: max~(L/LE)1/4 • Self-shield from drag: boundary layer can carry substantial energy flux • BLs slower (~j1/2) but wider beaming angle