DETECTING BLACK HOLES VIA
ACCRETION
• Accretion = swallowing of gas
– gas heated by compression/turbulence in strong gravity
field
X-rays (or UV)
• but need a source of gas
DETECTING BLACK HOLES VIA
ACCRETION
• Accretion = swallowing of gas
– gas heated by compression/turbulence in strong gravity
field
X-rays (or UV)
• but need a source of gas
– accretion from interstellar matter insignificant
– mass transfer in binaries to the rescue
X-RAY BINARY: A BLACK HOLE
SWALLOWS ITS COMPANION
(GRADUALLY)
X-ray binary
(artist’s
impression)
How does this
situation arise?
THE ALGOL PARADOX
• Two stars formed simultaneously
in binary
• Less massive one has used up
more hydrogen (opposite to
expectation)
• Resolution: less massive one used
to be more massive
• They exchanged mass
SOME FEATURES OF
MASS EXCHANGE
• Roche lobe = “sphere” of influence
– actually teardrop shaped
• Matter flows across Lagrange
point
• Too much angular momentum to
fall straight in
ACCRETION DISK
Simulation by
J. Blondin
ACCRETION
DISK
• Gas must give up angular
momentum to go down the drain
REQUIRES VISCOSITY
LIKE A
WHIRLPOOL…
(~FRICTION)
• Heats gas and makes it radiate
…ONLY FLAT
When the accreting body is a black hole…
• We can see radiation from gas just before
it crosses the horizon
• Heating extremely intense – 10x more
efficient per gram than nuclear reactions
• Radiation from an X-ray binary
– X-rays produced near the center
– 100,000 times more power than Sun
– Incredibly fast flickering (< 1/100 second)
ACCRETION DISKS ALLOW US to
PROBE the HORIZON
Energy flows from one form to another...
GRAVITY
matter swirling inward
MOTION
friction
HEAT
RADIATION (X-rays, UV…)
Do giant black holes in galactic
centers also accrete?
YES! They are called
ACTIVE GALACTIC NUCLEI
(a.k.a. quasars, …) but the
evidence is somewhat different
ACTIVE GALACTIC NUCLEI
• Pinpoint nuclei emitting intense light
– can be 100 x entire galaxy!
– Variable ⇨ incredibly compact: < size of Solar System
– “Nonstellar” (“nonthermal”) spectrum of radiation
• Spectral lines smeared out in wavelength
– interpret by Doppler shift
atoms move at ~10% c
– randomly moving gas clouds, not thermal motion
• High-speed jets of gas (will discuss Thursday)
– close to speed of light
STARLIGHT (thermal): PREFERRED WAVELENGTHS
AGN (nonthermal): NO PREFERRED WAVELENGTHS
... BUT: AGN accretion disks peak at UV rather
than X-ray wavelengths
ULTRAVIOLET SPECTRUM of a
SEYFERT GALAXY
Carbon
Hydrogen
Oxygen
NGC 4151:
Digital Sky Survey
UV spectrum of the Seyfert galaxy NGC 4151
observed with Hopkins UV Telescope (Kriss et al. 1992)
“BROAD” LINES FROM
“COLD” CLOUDS
Schematic of an AGN
C.M. Urry and P. Padovani
What you see
depends on
where you stand
NGC 1068: An Obscured Seyfert
This bright cloud has a direct view
of the intense nucleus
Radiation and (possibly) a wind escapes
through a cone-shaped funnel
Nucleus is hidden in here
46,420 QUASAR SPECTRA! (X. Fan, Sloan Digital Sky Survey)
QUASARS: VERY
DISTANT AND
LUMINOUS AGN
M87 - 3 billion solar masses
MASERS IN NGC 4258
NGC 4258: 36 million solar masses
CAN WE IMAGE ACCRETING
BLACK HOLES?
CAN WE IMAGE BLACK HOLES?
…NOT QUITE, BUT SOMEDAY…?
• HUBBLE: read newspaper @ 1 mile
– Optical/UV telescope in space
– Falls short by 100,000
• VLBA: read newspaper in Colo. Springs
– Transcontinental radio telescope
– Falls short by 1,000 for most BHs
– Getting close for two: Galactic Center & M87!
• MAXIM: Read newspaper on moon
– X-ray interferometer in space
– Can do it! Ready for launch (?) 2030+
VLBA, MAXIM use the technique
of “interferometry”
• You don’t need a “filled aperture” to
make see fine detail (but it’s hard to do)
1.22
Diffraction Limit 
d
VERY LONG BASELINE ARRAY
(VLBA) RADIO TELESCOPE
Predicted detectability: “shadow” of the Galactic Center
BH with VLBI/different wavelengths (Falcke et al. 2000)
200
M
MAXIM =
Microarcsecond
X-ray
Imaging
Mission
CONSTELLATION
BORESIGHT
Hub Spacecraft
10
KM
COLLECTOR
SPACECRAFT
(32 PLACES
EVENLY SPACED)
CONVERGER
SPACECRAFT
5000
KM
DELAY LINE
SPACECRAFT
DETECTOR
SPACECRAFT
X-rays from AGN disks: a tool for
mapping the disks indirectly
Direct
“Reflected”
X-ray spectroscopy to map accretion disk
structure
SUMMARY
• Black holes “shine” via energy released during
accretion
– Stellar-mass BHs ⇨ X-ray binaries
– Supermassive BHs ⇨ Active Galactic Nuclei
• Gas spirals inward via a viscous accretion disk
• Acccretion disk structure can be probed via
spectroscopy and, someday, direct imaging