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ARE RELATIVISTIC JETS ALWAYS MAGNETIC?

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

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ARE RELATIVISTIC JETS ALWAYS MAGNETIC?

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  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

  7. Simulations by Guillochon & Ramirez-Ruiz 2013

  8. 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

  9. 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

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

  11. 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

  12. An alternate approach

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

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

  15. Reconnection converts energy to radiation Reconnection geff MRI

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

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

  18. 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

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

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

  21. 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

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

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

  24. 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?

  25. 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

  26. 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?

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

  28. 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

  29. 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

  30. 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

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