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Winds and Jets from accretion flows

Ramesh Narayan. Winds and Jets from accretion flows. Pre-ADAF History. Shapiro, Lightman & Eardley (1976): hot 2T solution – thermally unstable Ichimaru (1977): Hint that there are two hot 2T solutions

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Winds and Jets from accretion flows

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  1. Ramesh Narayan Winds and Jets from accretion flows

  2. Pre-ADAF History • Shapiro, Lightman & Eardley (1976): hot 2T solution – thermally unstable • Ichimaru (1977): Hint that there are two hot 2T solutions • Rees et al. (1982): Ion torus model – unclear which 2T solution (unstable?) • Narayan & Yi (1995), Abramowicz et al. (1995): ADAF, topology of solutions, stability, etc.

  3. ADAFs, Winds and Jets Narayan & Yi (1994, Abstract): … the Bernoulli parameter is positive, implying that advection-dominated flows are susceptible to producing outflows … We suggest that advection-dominated accretion may provide an explanation for … the widespread occurrence of outflows and jets in accreting systems Narayan & Yi (1995, Title): “Advection-Dominated Accretion: Self-Similarity and Bipolar Outflows” Strong outflows confirmed in numerical simulation ADAFs  WINDS, JETS

  4. Steady One-Dimensional Adiabatic Flow Be: Bernoulli parameter

  5. Bernoulli Parameter • Be is conserved in a steady adiabatic flow • For the self-similar ADAF solution, • Beis positive (for  < 5/3) which means that the gas is not bound to the BH – it can expand to infinity and flow out • Hence strong outflows/winds are expected in an ADAF • Outflow speed: v ~ (2Be)1/2 ~ 0.3vK • In contrast, gas in a thin disk is tightly bound: Be ~ -vK2/2

  6. Convection Narayan & Yi (1994, Abstract): … Convection is likely in many of these flows and, if present, will tend to enhance the above effects (winds, outflows)… Narayan & Yi (1995, Abstract): … In addition, all the solutions are convectively unstable, and the convection is particularly important along the rotation axis…we suggest that a bipolar flow will develop along the axis of these flows, fed by material from the surface layers of the equatorial inflow. ADAFs  WINDS, JETS

  7. Why is there Convection? • Accreting gas is steadily heated by viscous dissipation • But it is not radiating any of the energy • Entropy increases with decreasing R: P/ ~ R-(5-3)/2 ~ R-1/4 • Satisfies the classic Schwarzschild criterion for convective instability

  8. Outflows/Convection in Viscous Rotating Flows • Numerical simulations of viscous rotating radiatively inefficient hydro flows reveal considerable convective activity (Igumenshchev et al. 1996, 2001; Stone et al. 1999; Igumenshchev & Abramowicz 1999, 2000) • These flows are called convection-dominated accretion flows (CDAFs) Abramowicz et al. (2001)

  9. Computer Simulations of ADAFs 2D MHD: Stone & Pringle (2001) 3D MHD: Igumenshchev et al. (2003) 3D hydro: Igumenshchev et al. (2000)

  10. GRMHD Simulation of a Magnetized ADAF • The simulation spontaneously generates: • geometrically thick flow • strong wind • magnetized relativistic jet McKinney & Gammie (2004)

  11. Mass Loss in the Wind • If mass is injected at a rate Mdotinj at some outer radius Rinj, accretion rate decreases with decreasing R • Less mass reaches the center than is supplied on the outside

  12. How Much Mass Does the BH Actually Accrete? • Less than what is supplied • SMBH: Assuming Bondi flow on the outside, which circularizes at some radius rcirc rBondi, then MdotBH ~ MdotBondi/rcircs • BHXRB: Mdot is set by transition radius: MdotBH ~ Mdot(rtr)/rtrs • The value of s is highly uncertain…

  13. Cooling Flow External Medium ADAF ADAF Geometry of ADAF Model rcirc rBondi rtr

  14. Why are Quiescent BHs Extraordinarily Dim? • Why are quiescent XRBs and quiescent SMBHs like Sgr A* so dim? • Is it because they have • Low radiative efficiency? • Low mass accretion rate? • Both?

  15. Radiatively Inefficient vs Mass Outflow • Sgr A* is extremely underluminous because of 3 (roughly equal) factors (Yuan et al. 2003): • Low mass supply:MdotBondi ~ 10-4 MdotEdd • Mass Outflow: MdotBH ~ 10-2.5MdotBondi • Low Rad. eff.: Lacc ~ 10-2 (0.1 MdotBH c2) • All part of the ADAF paradigm (e.g., if radiatively efficient, MdotBH=MdotBondi)

  16. Nuclear SMBHs and Feedback • Bright AGN have thin disks, LLAGN have ADAFs • SMBHs produce most of their luminosity in the thin disk phase (quasars, bright AGN) • SMBHs spend most of their time (90-99%) in the ADAF phase (quiescence) • SMBHs accrete most of their mass in the thin disk phase (Hopkins et al. 2005) • SMBHs probably produce a lot of their outflow energyin theADAFphase – 100% coupled to the external medium

  17. Energy Output in the Wind • The wind will carry substantial kinetic energy which might have an important effect on the surroundings • Energy is of order a few percent of the outflow mass energy • AGN could modify mass supply from external medium (AGN feedback) • Disk outflow during core collapse may drive SNe(Kohri et al. 2005)

  18. ADAFs and Feedback • Mechanical feedback from SMBH during super-Eddington accretion phase • Radiative feedback from AGN during bright quasar phase • Mechanical feedback through winds (and jets) during ADAF phase • Causes reduced accretion – important for understanding AGN evolution • Strongly affects galaxy formation • “Radio mode” is related to ADAFphysics

  19. ADAFs and Jets Narayan & Yi (1994, Abstract): … the Bernoulli parameter is positive, implying that advection-dominated flows are susceptible to producing outflows … We suggest that advection-dominated accretion may provide an explanation for … the widespread occurrence of outflows and jets in accreting systems The connection to outflows/winds was obvious The connection to jets was a wild guess!!

  20. Relativistic Jets • The power in an accretion flow is ~ 0.1 Mdot c2 • If a substantial fraction of this energy goes into a substantial fraction of the mass, expect only subrelativistic outflow • To get a relativistic jet, we have to concentrate the accretion energy in a small fraction of the mass • Even better: extra source of energy

  21. Relativistic Jets

  22. “Superluminal” Motion 3C273 GRS 1915+105

  23. Two Kinds of Jets • BH XRBs have two kinds of jets: • Steady low-power jet in the hard state • Impulsive high-power jet ejections • Radio-loud quasars come in two types • FRI sources: steady low-power • FRII sources: blobby(?) high-power • Perhaps the physics is the same for both classes of objects • ADAF connection for Hard State/FRI

  24. BH Accretion Paradigm: Thin Disk + ADAF + Jet Narayan 1996; Esin et al. (1997) Fender, Belloni & Gallo (2003) BH XRBs: strong connection between ADAFs and jets Hysteresis in low-high-low state transitions not fully understood

  25. ADAFs/Jets in LLAGN • Enhanced Radio emission/Jet activity seen in low-luminosity AGN (LLAGN)  •  = L/LEdd • R’ = 6 cm /B band • Radio-quiet AGN probably have no ADAFs, only thin disks Ho (2002)

  26. ADAF vs Jet • ADAFs are clearly associated with Jets • Observed radiation is a combination of emission from ADAF and Jet • Radiation from thermal electrons likely to be from the ADAF • Radiation from power-law electrons likely to be from the Jet

  27. Radiation: ADAF vs Jet • Radio emission is almost always from PL relativistic electrons in the jet • X-rays in the hard state look very thermal, and must be from the ADAF • But, at lower accretion rates, the jet may dominate even in X-rays • IR/optical could be from outer thin disk, or from ADAF, or from jet…

  28. Ingredients Needed for Relativistic Jets • Impressive observational evidence for a connection between ADAFs and relativistic jets • At the same time there is considerable evidence that thin disks are not conducive to producing jets • Therefore, the accretion mode is clearly one major factor behind jet activity • What about BH spin?

  29. Horizon shrinks: e.g., RH=GM/c2 for a*=1 • Singularity becomes ring-like • Particle orbits are modified • Frame-dragging --- Ergosphere • Energy can be extracted from BH

  30. Free Energy • Area Theorem: The surface area of a BH can never decrease • A BH of mass M and spin a* has less area than a non-spinning BH of the same mass • Therefore, by reversible processes, this BH can be converted to a non-spinning BH of lower mass, thereby releasing energy

  31. How Much Energy?

  32. Spinning Black Hole as an Energy Source • A spinning BH has free energy that can in principle be extracted (Penrose 1969) • Can be done with specially designed particles (Penrose 1969),but this is unlikely to happen in a real system • Is there a natural way to “grip” a BH to extract the free energy? • Magnetic fields are promising • Magnetic Penrose Process (Meier 2000)

  33. MHD Jet Simulations Numerical MHD simulations of ADAFs around rotating BHs produce impressive jets/outflows(Koide et al. 2002; de Villiers et al. 2003; McKinney & Gammie 2004; Komissarov 2004; Semenov et al. 2004; McKinney 2006; …) JET OUTFLOW

  34. 40M 400M a*=0.94 McKinney & Gammie (2004), McKinney (2006)

  35. Semenov et al. (2004) Other papers: De Villiers et al. (2003); McKinney & Gammie (2004); Komissarov et al. (2004), Tchekhovskoy et al. (2008)…

  36. Jets from Spinning Black Holes Semenov et al. (2004)

  37. Role of the Black Hole • The accretion disk produces a mass-loaded outflow with only mildly relativistic speed even from inner edge • Field lines from the ergosphere region inside the disk inner edge are much cleaner and are magnetically dominated (Poynting-dominated) • Rotation of these field lines is favorable for producing a relativistic jet

  38. Magnetic Hoop Stress and Jet Collimation • A popular picture of jet collimation is that the hoop stress of a helical magnetic field provides the inward collimating force • But this does not really work for relativistic jets, especially in the force-free regime

  39. Force-Free Magnetodynamics • Force-Free: An approximation in which we have charges, currents and strong magnetic fields, but no mass density/inertia • That is, we assume that the chargedparticles are massless • This is a reasonable first approximation for studying ultra-relativistic jets

  40. Spinning Split Monopole  Michel (1973) derived an exact solution for a spinning split monopole with a force-free magnetosphere Strong acceleration But no collimation! Field lines are swept back, but they do not collimate in the poloidal plane

  41. How are Jets Collimated? • Self-collimation is apparently not feasible with relativistic jets • We need some external medium to collimate the spinning magnetic fields • In the case of a Gamma-Ray Burst, the envelope of the star provides collimation • For other accreting BHs, the accretion disk has to do it  Strong Outflow

  42. Cartoon of a Jet System Gamma-Ray Burst XRB or AGN

  43. Necessary Ingredients: A Proposal • Powerful jet requires • Spinning BH/Star • Magnetic field • Currents (conducting) • Low inertia • Confining medium •  ADAF (disk wind) (Tchekhovskoy et al. 2008)

  44. Axisymmetric force-freejet from a spinning magnetized star surrounded by a magnetized disk(Tchekhovskoy et al. 2008) Toy Model: Numerical simulation of a force-free jet surrounded by a stellar envelope or a disk wind

  45. Near Zone: ~102rBH 80 5 Lorentz factor increases steadily as jet moves out: jet ~ z1/2 Rotation hardly affects the poloidal structure of the field even tho’ B Bz 4 g 3 2 1 -40 0 40

  46. Far Zone: ~106rBH 2x106 3 Lorentz factor continues to increase and reaches ~103by a distance of 106rBH Jet is naturally collimated: jet~ few degrees 2 log10 g 106 1 0 -5x104 0 5x104

  47. Main Results • Acceleration and collimation of a force-free jet depend on the radial profile of the confining external pressure • A profile P ~ r-5/2, as expected for a stellar envelope or an ADAF wind, seems to be favorable • Terminal Lorentz factor depends on how far out the confinement operates: max ~ (rmax)1/2

  48. ADAF vs Thin Disk • Nearly all simulation results to date are for non-radiative flows: ADAFs • Produce strong outflows and jets • What kind of jets/winds do thin disks produce? • Preliminary indication is that the jet is absent and the wind is relatively weak (e.g., Shafee et al. 2008) • Consistent with observations…

  49. Unresolved Issues • How different are mass-loaded jets compared to force-free jets? • Are their terminal Lorentz factors and collimation angles very different? • Given that BBz, why are jets stableover such enormous distances (e.g., Kruskal-Shafranovcriterion)?

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