Massive Objects at the Centers of Galaxies Roger Blandford KIPAC Stanford.
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Transcript of Massive Objects at the Centers of Galaxies Roger Blandford KIPAC Stanford.
An History
• …• 1961-2 Hoyle, Fowler - radio sources are
powered by explosions involving superstars• 1963 Hazard, Schmidt - quasars• 1963 Kerr metric• 1964 Zel’dovich & Novikov, Salpeter et al -
black holes• 1965 Dent - variability
More history
• 1966 Rees - superluminal expansion
• 1968 Wheeler - Black Hole
• 1969 - Whitney….. - SLE measured
• 1969 Lynden-Bell - dead quasars, disks
• 1974 Balick & Brown, Lynden Bell & Rees
• 1975 Kellermann Cygnus A - pc scale collimation => black hole
Observational Evidence
• Accretion disks– NGC 4258 masers => Keplerian– Molecular disks
• Stellar Orbits – Velocity dispersion and rotation– Individual, disruption?
• X-rays from inner disks– MCG 6-30-15 Fe =>maximal rotation?– Comptonized, synchrotron, inverse Compton
• Variability– Blazar jets– Disks?
• Winds– BALQ– ?
Black Holes
• Kerr Metric (not Kerr-Newman)– Mass m=M8AU=500M8s[=5Gm=17s]– Spin = a / 2mr+
• Ergosphere• Reducible mass• Shrink smallest stable circular orbit
– GR untested
• Black hole is strongly curved space(time) outside horizon - not just the horizon– Use infalling coordinate systems not just Boyer-Lindquist
Spin energy of a black hole
71.0 Or
;2 mra O
Irreducible RadiusIrreducible Mass
Specific Angular Momentum
Rotational Speed
Gravitational mass mmm
m OO 71.0;
1 2
OO mA
r 24
21
Kerr Spacetime• Dragging of inertial frames
– Physics of ergosphere very important– Need numerical simulation - MHD
• Thin disk efficiency probably irrelevant to real disks; binding energy curve very shallow– Accretion Gap– Proper distance between horizon and
marginally stable orbit 7m - 2m as a -> m
Modes of Accretion and Sgr A*
• LE ~1046M8 erg s-1 [~3 x 1044 erg s-1]
• M’E ~1025M8 g s-1[~3 x 1023 g s-1]
• Mass supply– M’ < 0.1 M’E : Thick, ion-supported disks [~1021 g s-1]
• Mass accretion << Mass supply [~1018g s-1]
– 0.1 M’E < M’ < 10 M’E : Thin, radiative disks
– 10M’E < M’ : Thick, radiation-dominated disks
Ion-Supported Thick Disks
• Low mass supply and efficient angular momentum transport, low radiative efficiency– Adiabatic/altruistic/demand-limited accretion (ADIOS)– Most mass escapes in a wind carrying off the energy
liberated by the accreting gas– Wind may be matter-dominated or magnetically-
dominated [~ 1039 erg s-1]
Transition radius
Self-similar disk models
•Gas dynamical model•Convective Disk•Gyrentropic structure
•S(L), B(L)•Meridional circulation•Thermal Front
•Mass, momentum, energy conserved
•Outflow carries off energy•Centrifugal funnel
Relativistic Ion-supported Torus
•Gyrentropic - S(L)•Asymptotes to self-similar non-relativistic disk•Similar discussion for transition to thin disk
Magnetic Field
• Magnetorotational Instability
• Disk-Hole Connection
• Magnetized Outflows
• Extraction from Hole
BMW
Emission from Ion Torus
• Trans-sonic, Alfvenic, relativistic differentially-rotating flow– =>particle acceleration easy!– =>Nonthermal emission
• X-rays not thermal bremsstrahlung
• cm emission from outer disk (jet?)
• Radio/mm polarization
Jets and Radio Sources
• Energy (+ mass, angular momentum) exhausts – Fluid
• Ions
– Hydromagnetic – Relativistic MHD / Electromagnetic
• Disordered• Ordered
– Jets highlight the current flow– Sgr A* jet ?
• Evolution of mass, momentum, energy along jet– Entrainment, dissipation and radiation
3-D, adiabatic MHD model
Hawley, Balbus & Stone 01
DENSITY PRESSURE
p, Contours similar: BARYTROPIC
Rotation on cylinders:Von Zeipel
(azimuthally averaged)
3-D, adiabatic MHD model
Hawley & Balbus 02 NRMHD wind plus RMHD/EM jetCentrifugal force important
n~108cm-3 P ~ 1 Pa
Pictor A
Magnetically-pinched current?Magnetic reservoirOhmic dissipation . B constant
Sgr A* Jet?B~100G, ~3PVI~300TALEM~1030W
• Powerful compact radio sources
• Superluminal jets V ~ 0.99 c
• Variable GeV-ray source – eg 3C 279 - L ~ 1049 f erg/s >> Lrad
• MKN 421 - 30 min variability at 1 TeV!
• Intraday variability => V ~ 0.999(9) c– Refractive scintillation– Coherent emission?
• Gyrocyclotron by mildly relativistic electrons?
• Sgr A*may be a TeV source
Ultrarelativistic Jets
Why is Sgr A* interesting?• Very dark energy!• Why is the sun interesting?• Extreme accretion mode• Quantitative?!• Stellar dynamics
– Cradle to grave– Things unseen
• Complexity– Molecular gas, orientation, IRS13, SNR, magnetic environment…..
• Black holes - strong field test of GR– (Sub)mmVLBI for black hole shadow– Periodicities?
Summary
• Sgr A* paradigm for slow accretion• Detailed MHz - TeV observation• Possibly best (and cheapest) laboratory for strong
field GR– Radio astronomers have produced almost all the good,
quantitative affirmations of weak field relativity. Why stop now?
• Complexity of circum-nuclear gas flow, stellar dynamics