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Relativistic Jets in the Universe

Relativistic Jets in the Universe

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Relativistic Jets in the Universe

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  1. Relativistic Jets in the Universe Yosuke Mizuno Institute of Astronomy National Tsing-Hua University

  2. Contents • Introduction: Relativistic Jets (observations) • Jet formation / acceleration mechanism • Magnetohydrodynamic (MHD) process • Jet collimation mechanism • Jet is self-collimated? • Dissipation of jets • How magnetic energy converts to jet kinetic energy • Summary

  3. Astrophysical Jets • Astrophysical jets are a tremendous, elongated and collimated outflows of plasma • Astrophysical jets can be observed in a huge spatial and energetic scale reaching from stellar size to galaxy size • There are many sources for jets • Jets (Outflows) are common feature in the universe • The astrophysical jets seen in AGNs, BHBs, and GRBs have a relativistic speed = relativistic jets M87 jets (AGNs, optical)

  4. Radio observation • M87 = Virgo A • Nearby: D ~ 16 Mpc (1 mas = 0.08 pc) • AGNs: FR I, Misaligned BL Lac (q ~ 14 deg) • SMBH mass: 6.6 x 109 Msun • VLBA resolution: 20 rs at 43 GHz Frequency: 43GHz

  5. Sources of Jets jet with relativistic speed

  6. Relativistic jets - properties • Highly relativistic jets are lunching from accreting compact objects (BHs/NSs, “central engine”) • Jets transport energy and angular momentum from central engine to remote locations and provide feedback to ISM/IGM • They are fast: Lorentz factors ~ a few to a few tens in AGNs and microquasars (BHBs), and >100 in GRBs • High Lorentz factors mean that special relativistic effects are important (general relativistic effects is also important in the vicinity of compact objects). • Jet phenomenon bridges many orders of magnitude in size: forming in size scale of rg and extending up to 109-10 rg

  7. Jet Similarity Similar Morphologies But … • Jets from a protostar • Size: Few-light across • Speed: few 100 km/s • Optical: atomic line emission • Jets from a quasars • Size: ~ million light-years across • Speed: ~ c (light speed) • Radio emission: synchrotron emission (non-thermal) Cygnus A

  8. Jet Speeds • Sub-relativistic: protostars, v/c ~ 10-3 • Mildly-relativistic: SS433, XRBs (v/c = 0.26) • Doppler-shifted emission lines • Highly-relativistic: X-ray binaries (microquasars), ~10% of AGNs (G~ 2-30) • Doppler beaming (One-sideness) • Illusion of superluminal motion • Gamma-ray flares (to avoid gg-pair production) • Hyper-relativistic: Gamma-Ray Bursts (G ~ 100 - 1000) • Gamma-ray variability • Ultra-relativistic: Pulsar jets/winds (G ~ 106) • Modeling of radiation and pulsar nebulae

  9. Superluminal Motions in Relativistic Jets Apparent superluminal motion (Rees 1967) towards observer q c v

  10. Relativistic Jets in AGNs • Jet launched from vicinity of a supper-massive BH (105-9 Msun) • ~ 10 % of AGN are radio-loud i.e., have prominent jets • AGN jets can extend from ~ 10-4 pc to several hundred kpc in size • Jet composition is unknown (normal or pair plasma) • Jet powers : 1043-47 erg/s • Jet power integrated over radio source lifetime: 1057-62 erg Urry & Padovani (1995)

  11. Blazars • AGN with relativistic jets seen almost pole-on • Two sub-category: Flat-spectrum radio quasars and BL Lac objects • Jet emission enhanced due to relativistic effects by a factor of thousands • Broad-band SED dominated by non-thermal emission from jets (Synchrotron + Inverse-Compton ) • Emission from radio up to TeV gamma-rays Average SED of blazars (Fossati et al. 98)

  12. Relativistic Jets in Microquasars Observed in radio Superluminal motion in microquasar GRS 1915+105; Vapp =1.5c Microquasar is a scaled down (by a factor of 106)version of active galactic nuclei

  13. Relativistic Jets in Universe Mirabel & Rodoriguez 1998

  14. Fundamental Problems in Relativistic jets • How are the jets formed near the compact objects (BHs or NSs)? (formation mechanism) • How are the jets accelerated up to relativistic speed? (acceleration mechanism) • How the collimated jet structure make? (collimation mechanism) • How the jets remain stable over large distance? (Jet stability mechanism) • How the jets influence ISM and IGM (feedback)? • How to emit the radiation in the relativistic jets? (Radiation mechanism and particle acceleration mechanism)

  15. Jet formation/acceleration mechanism • Jet is formed near the central compact objects (BHs/NSs). • Some accreting matter is getting some force to make jet-like outflows. • Ingredients: rotation, accretion disk, magnetic fields • Jet base: rotating disk or compact objects (BHs/NSs) • The jet formation/acceleration mechanism is still under debate but … • The most promising mechanism is the acceleration/formation by rotating, twisting magnetic fields (magnetohydrodynamic (MHD) process) • Other possibility: gas pressure, radiation pressure, …

  16. Jet formation/acceleration mechanism (cont.) • Gas or radiation pressure (Blandford & Rees 1974, O’Dell 1984) • push accretion matter to make and accelerate outflows by pressure gradient • Expansion of magnetic tower (Lynden-Bell & Boily 1994) • Mainly toroidal field from start • Acceleration by magnetic pressure • Magnetocentrifugal acceleration (Blandford & Payne 1982) • Mainly poloidal field anchored to disk or rotating objects • Disk or ergosphere of BH acts like crank • Torque transmitted though poloidal field powers jet • Blandford-Znajek process (Blandford & Znajek 1977) • Directly extract the BH rotating energy and convert to outward Poynting flux • Consider force-free limit (MHD Penrose process is similar mechanism, MHD version )

  17. Magnetic field line outflow (jet) accretion Acceleration & Collimation in MHD model • Assume: in ideal MHD, plasma is attached with magnetic field • Acceleration • Magneto-centrifugal force • Magnetic pressure • Like expansion of spring • Collimation • Magnetic pinch force • Like shrink lubber band Centrifugal force Accretion disk Jets Magnetic field lines Magnetic field lines

  18. Jet formation/acceleration mechanism (cont.) • In ideal MHD limit (infinite conductivity), plasma flow (motion) is connected with magnetic field • The rotation of accretion disks or compact objects (BHs / NSs) twisted up the magnetic field into toroidal components Collapsing, magnetized supernova core (GRBs) Pulsar magnetosphere Magnetospheres of rotating black holes Magnetized accretion disks around neutron stars and black holes Courtesy to David Meier

  19. Relativistic Jets Formation from GRMHD Simulations • Many GRMHD simulations of jet formation (e.g., Hawley & Krolik 2006, McKinney 2006, Hardee et al. 2007) suggest that • a jet spine (Poynting-flux jet) driven by the magnetic fields threading the ergosphere via MHD process or Blandford-Znajek process • may be surrounded by a broad sheath wind driven by the magnetic fields anchored in the accretion disk (mildly-relativistic wind). • High magnetized flow accelerates G >>1, but most of energy remains in B field. Non-rotating BH Fast-rotating BH Spine Sheath Total velocity Disk Disk Jet/Wind BH Jet Disk Jet/Wind Density distribution (McKinney 2006) (Hardee, Mizuno & Nishikawa 2007)

  20. Jet Energetics Gravity, Rotational energy (BH or accretion disk) Poynting flux (magnetic energy) Jet kinetic energy Efficient conversion to EM energy Easy to get ~ equipartition, hard to get full conversion Magnetic field is a medium for a transmission not a source

  21. Magnetic hoop stress Jet Collimation • Jet is produced by MHD process near the central objects and magnetic field is tightly tied (toroidal field is dominated) • Lorentz force >> plasma pressure & inertia • Huge tension force of wound up magnetic field (hoop stress) compress the flow towards the axis (self-collimation)? • Answer: No! • In the current closure region, the force acts to de-collimation • Need external confinement

  22. External Confinement • In BH - accretion disk systems, the relativistic outflows from the black hole and the internal part of the accretion disk could be confined by the mildly-relativistic magnetized wind from the outer parts of the disk. • In GRBs, a relativistic jet from the collapsing core pushes its way through the stellar envelope (confinement).

  23. Collimation vs Acceleration • For jet collimation, external confinement is necessary • Without external confinement, the flow is near radial and acceleration stops at an early stage(Tomimatsu 1994; Beskin et al. 1998) • The gas pressure profile of external confinement medium is the important parameter • The spatial distribution of confining gas pressure determines the shape of the jet flow boundary, magnetic field configuration and acceleration rate (Tchekovskoy et al. 2009, 2010; Komissarov et al. 2009; Lyubarsky 2009,2010). • Optimal collimation  pressure decrease slowly along jets • Optimal acceleration  pressure decrease rapidly along jets => Collimation and acceleration of jet are related (poloidal) magnetic field configuration

  24. Effects of external confinement 2D RMHD simulations (Komissarov et al. 2009) Parabolic (z ∝ r2) Acceleration: slow, collimation: OK Conical (z ∝ r): Acceleration: fast, collimation: X Lorentz factor • Some part of jets can convert Poynting flux to Kinetic Energy but most can’t. • Energy conversion is too slow to become kinetic energy dominated, it is unreasonably long distance = inconsistent of observations. • We need to consider some sort of dissipation (rapid energy conversion)

  25. Global structure of M87 jet (Asada & Nakamura 2012) Observed Jet structure • The parabolic structure (z ∝ r1.7) maintains over 105 rs, external confinement is worked. • The transition of streamlines presumably occurs beyond the gravitational influence of the SMBH (= Bondi radius) • Stationary feature HST-1 is a consequence of the jet recollimation due to the pressure imbalance at the transition • In far region, jet stream line is conical (z ∝ r) Parabolic streamline (confined by ISM?) Conical streamline (unconfined, free expansion) Over-collimation at HST-1 stationary knot (recollimation shock?) HST-1 region

  26. Observed Jet structure (cont.) • In M87 jet, the asymptotic acceleration from non-relativistic (0.01c) to relativistic speed (0.99c) occurs over 102-5 rs • This is very slow acceleration = consistent with theoretical results? • The absence of bulk-Comptonization spectral signatures in blazars implies that Lorentz factors >10 must be attained at least ~1000 rg (Sikora et al. 05). • But according to spectral fitting, jets are already matter-dominated at ~1000 rg (Ghisellini et al 10). Transition of Sub- to super-luminal motion in M87 jet Asada & Nakamura (2014)

  27. Dissipation in the Jet • Time-dependent energy injection to jet => Internal shocks in jets • Sudden change of confined external medium spatial profile => Recollimation shock/ rarefaction acceleration • Magnetic field reversal or deformation of ordered magnetic field => Magnetic reconnection • MHD Instabilities in jets • Kelvin-Helmholtz instability at jet boundary • Current-Driven Kink instability at jet interior => Turbulence in the jets and/or magnetic reconnection?

  28. Dissipation in the Jet: Energetics • Tapping kinetic energy • Internal shock • Recollimation shock • Kelvin-Helmholtz instability • Tapping magnetic energy • Rarefaction acceleration • CD kink instability • Magnetic reconnection Prefer dissipation mechanism for Poynting-dominated jet (conversion from Poynting flux to Kinetic energy)

  29. Rarefaction Acceleration • If the external confined media is suddenly disappeared and jet becomes free expansion (parabolic => conical), the rarefaction wave is formed by overpressure of jet and propagates in the jets during the transition from jet boundary. • Rarefaction wave converts jet thermal & magnetic energies to jet kinetic energy efficiently (e.g., Aloy & Rezzolla 2006; Mizuno et al. 2008; Komissarov et al. 2010), so-called rarefaction acceleration • The mechanism is favor for GRBs and may be possible for AGNs. • stationary feature by recollimation shock? Komissarov et al. 2010 Rarefaction wave propagates from the jet boundary Change channel shape from parabolic to conical Lorentz factor

  30. CD Kink Instability • Well-known instability in laboratory plasma (TOKAMAK), astrophysical plasma (Sun, jet, pulsar etc). • In configurations with strong toroidal magnetic fields, current-driven (CD) kink mode (m=1) is unstable. • This instability excites large-scale helical motions that can be strongly distort or even disrupt the system • For static cylindrical force-free equilibria, well known Kruskal-Shafranov (KS) criterion • Unstable wavelengths: l > |Bp/Bf |2pR • However, rotation and shear motion could significant affect the instability criterion • Distorted magnetic field structure may trigger of magnetic reconnection. Schematic picture of CD kink instability 3D RMHD simulation of CD kink instability in PWNe (Mizuno et al. 2011)

  31. Mizuno et al. (2009) CD Kink Instability in Jets • Helical structure is developed by CD kink instability. • Growth rate of CD kink instability is small • Magnetic energy in the jets converts thermal and kinetic energies by development of instability (via turbulent structure) • Jet structure is strongly deformed but may be not disrupted entirely (depends on magnetic pitch, density, and flow profiles) (Mizuno et al. 09, 11, 12, 14). Density + B-field Mizuno et al. (2014) • Regular helical magnetic field is strongly deformed via CD kink instability => may be triggered magnetic reconnection jet Density + B-field

  32. Magnetic reconnection • Ideal MHD gives frozen in magnetic fields. • Resistive MHD (non-ideal MHD) allows diffusion of fields. • Magnetic reconnection occurs through diffusion in narrow current sheets. • Magnetic reconnection is the process of a rapid rearrangement of magnetic field topology. • Magnetic energy could be converted to thermal and kinetic energies • Problem: how differently oriented magnetic field lines could come close each other? • Global MHD instability • Alternating B-field formed at jet begging.

  33. Magnetic Reconnection (cont.) Dissipation of alternating magnetic fields in the jet (e.g., Giannios & Spruit 05,07; Giannios 06,08; McKinney & Uzdensky 2012) j B g multipolar field (local closed field) Ordered field broken by instability Alternating polarity by misaligned dipole field Mckinney & Uzdensky (2012)

  34. Ultra-Fast TeV Flare in Blazars • Ultra-Fast TeV flares are observed in some Blazars (AGN jets). • Vary on timescale as sort as • tv~3min << Rs/c ~ 3M9hour • For the TeV emission to escape pair creation Γem>50is required (Begelman, Fabian & Rees 2008) • But PKS 2155-304, Mrk 501 show “moderately” superluminal ejections (vapp ~several c) • Emitter must be compact and extremely fast • Model for the Fast TeV flaring • Magnetic Reconnection inside jet (Giannios et al. 2009) PKS2155-304 (Aharonian et al. 2007) See also Mrk501, PKS1222+21 Giannios et al.(2009)

  35. Advantage of Magnetic Reconnections • Magnetic reconnection easily provides large radiative efficiencies and strong variability inferred in AGNs and GRBs. • Reconnection at high s produces relativistic “jets in a jet”; this could account for the fast TeV variability of blazars (Giannios et al 2010) • Questions: why fast reconnection occur in jets? • If we consider anomalous resistivity in small dissipation region, we get fast reconnection. But we do not know what is anomalous resistivity. • This is still unsolved problem in physics.

  36. Mizuno 2013, ApJS Relativistic Magnetic Reconnection using RRMHD Code • To handle relativistic magnetic reconnection numerically, we need to perform resistive relativistic MHD simulations. • Initial condition • Consider Pestchek-type magnetic reconnection • anti-parallel magnetic field • Anomalous resistivity for triggering magnetic reconnection • Results • B-filed:typical X-type topology • Density:Plasmoid • Reconnection outflow: ~0.8c

  37. Five Regions in AGN Jets Modified Fast Point Alfven Point Slow MS Point Fast MS Point Collimation Shock Kinetic Energy Flux Dominated with Tangled (?) Field Modified from Graphic courtesy David Meier Jet Launching Region Jet Collimation Region (10 –100  Launching Region) Sheath High speed spine Poynting Flux Dominated CD Unstable Magnetic Helicity Driven Region Combined CD/KH Unstable Region KH Unstable Velocity Shear Driven Region

  38. Summary • Relativistic jet is formed and accelerated by the MHD process near the compact objects. • Flow is dominated by the Poynting flux (EM energy) • No self-collimation: need external confinement • External confinement is crucial for efficient jet collimation and acceleration of Poynting dominated outflows. • Relativistic jet accelerates gradually => need dissipation to convert EM energy of jets • Magnetic reconnection is a key for the dissipation

  39. Frontier of research • Numerical simulations: • GRMHD simulations are possible • Additional physics: Resistivity, radiation, microphysics, … • Effects of time-dependence, non-asymmetry (3D) • GR radiation transfer calculation is a key tool to connect MHD simulations and observations (Need correct radiation process including particle acceleration) • Observations: mm & submm-VLBI trying to observe BH shadow & jet launching region (EHT, GLT, Black Hole Cam projects)

  40. Takahashi et al. (2004) Pu et al. (2014) BH shadow image (no jet) a=0 a=0.5 a=0.999 Optically thick Geometrically thin i=10 deg i=85 deg Optically thin Geometrically thick i=85 deg

  41. High resolution VLBI observations • In radio VLBI observation, angular resolution ∝ l/D, l: wavelength, D: baseline • Resolution ~ 50 mas at l ~1mm (300GHz) & D=4000 km • In GLT project (with other sub-mm array), D > 9000 km => 20 mas at 345 GHz Submm-VLBI GLT SMA ALMA