The Black Hole in the Galactic Center. Eliot Quataert (UC Berkeley)

Transcription

1 The Black Hole in the Galactic Center Eliot Quataert (UC Berkeley)

2 Why focus on the Galactic Center? The Best Evidence for a BH: M M (M = mass of sun) It s s close! only ~ Planck Lengths Away (~ 25,000 light-years) proximity unique observational probes of environment around a BH Largest BH on the sky (horizon rad 8 µ-arcsec) direct images of the horizon possible in the next decade Extreme low luminosity illuminates accretion physics

3 Outline Dynamical Evidence for a Massive BH in the GC Electromagnetic Evidence for a Massive BH Accretion Physics Inward Bound: Towards the Event Horizon

4 The Milky Way Galaxy Scale: Size of Solar System: 0.01 light-years Typical Distance btw. Stars: few light-years ~ 10 5 light-years M Black Hole Distance btw. Sun & GC light-year ( 8 kpc) Total mass (dark matter): ~ M Stellar mass (disk): ~ M Stellar mass (bulge): ~ M Central Black Hole mass: ~ M (also ~ 10s of millions of ~ 10 M BHs)

5 The Central Few Light Years VLT Infrared image Chandra X-ray Image Baganoff et al. Genzel et al.

6 Dynamical Evidence for a BH at the Center of our Galaxy 10 x size of solar system QuickTime and a YUV420 codec decompressor are needed to see this picture. Motion of stars at the center of the Milky Way over the past decade (full 3D velocities for ~ 6 stars) Genzel et al.; also Ghez et al.

7 Stellar Orbits Kepler s Laws 3.6 x 10 6 M in a region smaller than our solar system Closest star to BH (so far!) Ghez et al R min 500 R S V max km/s 0.04c

8 Alternatives to a BH? R < 500 R s or ρ > M pc -3 ~ 10-5 g cm -3 No Viable Astrophysical Alternative to BH Hypothesis Cluster of stars, planets, NSs, solar mass BHs, would destroy itself (by collisions or evaporation) in ~ 10 5 yrs <<< Hubble time Maoz 1998

9 Electromagnetic Signatures of Massive BHs Quasars & Active Galactic Nuclei Point sources of radiation with luminosities up to ~ solar ~ W (brighter than a galaxy!) Emission from radio to γ-rays Highly Variable (hours to days) small size < solar system Interpreted as M BHs accreting ~ M of gas per year Relativistic jets : BHs influence extends into surrounding galaxy

10 Many Varieties of Massive BHs Brightness of Central Black Hole Our Galaxy AGN Quasars BHs Spend Most of their Time Faint and Unobtrusive (Like Ours), But Occasionally Light Up as AGN/Quasars

11 Electromagnetic Evidence for a BH in the GC Galactic Center in X-rays X (Chandra( Chandra) X-ray Flare Baganoff et al X-ray Flux 3 hrs L X ~ L

12 Infrared Signature of a BH IR Flare Observed Flux Time (min) Genzel et al Light crossing time of Horizon: 0.5 min Orbital period of matter at 3 R S : 28 min

13 Total Power ~ ergs s -1 ~ W ~ 100 L ~ 10-9 L EDD (mostly in radio) Eddington Limit radio IR X-ray Force of Radiation On Inflowing Gas Balances Gravity X-ray Flares L EDD = 4πGMc κ es = M 7 ergs s -1

14 Fuel Supply Chandra X-ray Image of Galactic Center Hot gas surrounding BH (T = 1-2 kev; n = 100 cm -3 ) produced via shocked stellar winds Baganoff et al.

15 Spherical Model (Bondi( Accretion) BH surrounded by gas with density ρ and sound speed c s GM R >> A 2 cs R M Ý 4πR ρc captured A2 s S Estimates Give: (R A 10 5 R S ) M 10 5 R A R A determines the BHs gravitational sphere of influence M yr 1 ρ density c s sound speed (temperature)

16 Inflowing Gas with Angular Momentum Accretion Disk Angular Momentum Transport Balbus & Hawley 1991 Rotating magnetized disks are linearly unstable: magnetic stresses remove angular momentum and allow accretion to proceed Inflowing gas radiates its gravitational potential energy. Power produced up to L GM M Ý R η M Ý c 2 efficiency η ~ (depending on spin) up to 50 x more efficient than fusion in stars

17 Total Power ~ ergs s -1 ~ W ~ 100 L ~ 10-9 L EDD ~ 10 6 Ý M c 2 radio IR X-ray X-ray Flares Inferred efficiency <<<<< ~ 10% efficiency in luminous BHs

18 Radiatively Inefficient Accretion At low densities (low accretion rates), cooling is inefficient Grav.. Pot. Energy Heat; not radiated L << M Ý c 2 very hot plasma: kt ~ GMm p /3R s ~ 100 MeV near BH in T p ~ 100 MeV >> T e ~ 10 MeV our Galactic n e ~ 10 6 cm -3 B ~ 30 G Center e-p collision time ~ 10 8 x inflow time Note: Complexities in Accretion Theory Are Due to MHD & Plasma Physics. GR is easy.

19 Observed Emission Infrared X-ray Time (min) Light crossing time of Horizon: 0.5 min Orbital period of last stable orbit (a = 0): 28 min timescale emission arises close to BH ~ few R S

20 John Hawley QuickTime and a YUV420 codec decompressor are needed to see this picture. Accretion flow is time-dependent, with large fluctuations in density, temperature, magnetic field strength, etc. observed emission due to turbulent plasma close to BH (synchrotron + Inverse Compton radiation)

21 Analogy: Solar Corona QuickTime and a YUV420 codec decompressor are needed to see this picture. Solar Flare Active Regions as seen by the SOHO satellite

22 GC horizon: R S cm 4x10-13 rad 8 µ-arcsec Inward Bound M87 at 7 mm (R S 2 x smaller) GC is largest BH on the sky! can be directly imaged with Very Long Baseline Interferometry (VLBI) at mm λ s s in the next ~ 10 years Human Eye: resolution ~ 20 arcsec Hubble: resolution ~ 0.1 arcsec VLBI: resolution ~ 10-6 arcsec Biretta et al R S

23 Toy Models Predict a True Black Hole (light bending, grav. redshift,, photons captured by BH, suppression in observed flux from near the BH) 10 R S Theory & simulations predict emission strongly peaked near BH where GR effects important Emission from very small radii also implied by rapid variability Falcke et al. 2000; based on Bardeen 1973

24 Summary Our Galaxy Hosts a 3.6 x 10 6 solar mass BH stellar orbits mass contained w/in ~ 500 R S (~ dist. btw. Sun & Pluto) Electromagnetic counterpart (accreting gas) is very faint accretion is inefficient :: little mass is accreted and little radiation is produced true for most BHs,, most of the time Observations probing conditions near horizon of BH rapid variability consistent w/ turbulent accretion flow in inner r few R S direct images of horizon feasible in ~ 10 years; novel GR effects?