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– ?

M87

Halca

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

-4 -2 0 2-8

0

-4

-6

-2

M’S / M’E

L / LE

Sgr A*

Brightest quasarsLuminosity vs Supply Rate

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