!UNTING FOR BLACK HOLES
"N OUR BACKYARD #I.E. OU$
OWN GALAXY%
Smoking gun: “dark star” with Mass M>3MSun
How do we look for it?
• Effects on matter/light outside the horizon
Can we “see” a black hole in the sky?
[Images from http://antwrp.gsfc.nasa.gov/htmltest/rjn_bht.html]
… it would have to be VERY CLOSE to us
At a distance D=10 Rs it would appear as in the right panel of the figure;
at a distance of D=1000 Rs it would appear as in the left panel of the fig.
GRAVITATIONAL LENSING EFFECTS
(microlensing for BHs of a few solar masses)
Detections are possible, but mass of lensing object difficult to
estimate from these observations.
MORE CLUES FROM X-RAY EMISSION
FROM GAS FALLING INTO THE BH
• Accretion (swallowing) of gas
–gas heated by compression/turbulence
in strong gravity field
X-rays
–but need a source of gas
• accretion from interstellar matter
insignificant
• mass transfer in binaries to the rescue
Example of an evolutionary
path from a binary to an
X-RAY BINARY
Start with a system of two stars in a
Binary. The most massive star is the
quickest to evolve, burn its Hydrogen
supply and become a red giant. It
transfers mass to the other star and
eventually goes off as a supernova.
For a period of time the system will
consist of a compact object (NS, BH)
and a massive companion with a
strong mass outflow (stellar wind).
[Image from “Gravity’s Fatal Attraction
by M. Begelman & M. Rees]
Gas almost never falls directly into a BH or NS
too much swirl (i.e. angular
momentum)….
…makes it more
like a whirlpool
[Image from “Gravity’s Fatal Attraction” by Begelman & Rees]
[Drawing by A. Rackham’]
ACCRETION
DISK
Like a flattened whirlpool
Gas must give up angular momentum
to go down the drain
VISCOSITY (~FRICTION)
HEAT
Drawing by A. Rackham’]
RADIATION
[Image from “Gravity’s Fatal Attraction” by Begelman & Rees]
&r's()* concep'on of an X-ray Binar+
[Image credit: NASA]
ENERGY FLOW IN ACCRETION DISK
Energy flows from one form to another...
GRAVITATIONAL POTENTIAL ENERGY
falling matter
MOTION
friction
HEAT
particle collisions, etc.
RADIATION (X-rays, UV…)
[Slide courtesy of M. Begelman]
NEUTRON STAR VS. BLACK HOLE
IN AN X-RAY BINARY …how to tell
• NS if:
– Pulsing (X-ray pulsar)
– Evidence of nuclear explosions on surface (X-ray
burster)
– Extra emission from its “hard” surface
• BH if:
– None of the above is seen
– Mass M> 3Msun (the “smoking gun”)
X-ray pulsar
Accretion from the
disk heats the surface
of the NS, resulting in
extra X-ray emission
X-ray burster
Thermonuclear bursts
triggered on the surface of
the NS by the accreting
material
COMPARISON BETWEEN LUMINOSITIES OF
NEUTRON STAR AND BLACK HOLE CANDIDATES
IN X-RAY BINARIES
NSs
BHs have no
“surface”
emission, and
therefore they
are much
dimmer than
NSs (whose
“hard” surface
is hot and emits
radiation)
BHs
[Garcia et al. 2001]
The orbital motion of the companion star helps to
measure the mass of the remnant object (NS, BH)
The orbital speed of the companion star be measured
from the maximum and minimum Doppler shift of
the star spectral lines
Measured masses of compact objects believed to
be either neutron stars or black holes
[Image from “Gravity’s
Fatal Attraction”
by Begelman & Rees]
1
1
10
Mass (solar units)
001
0.1
A very famous BH candidate:
CYGNUS X-1
X-ray image
[Image from http://www.astro.msfc.nasa.gov/research/hero/flight2001_xdata/]
Cygnus X-1: properties of source
Optical image
One of the first X-ray
sources discovered
Optical counterpart is a
massive star (visible in the
image).
Star unable to account for
observed X-ray radiation
Evidence for binarity from
Doppler shift of lines in star
Companion is dark in optical
but bright in X-rays; mass
between 3-7 MSun
[Image from http://archive.ncsa.uiuc.edu/Cyberia/NumRel/Images]
Thorne-Hawking bet on whether Cyg X-1
contains a Black Hole
“DEATH SPIRALS” in CYG X-1:
are we probing the event horizon?
Observations of pulses of radiation might signal the final plunge
of blobs of material into the black hole.
[Image credit: NASA/STScI]
THE BLACK HOLE CANDIDATE GRO J1655-40
Emission of jets
with gas velocity
close to the speed
of light
Brightness
modulations from
gas at the last stable
orbit suggest that
GRO J1655-40 is
probably a
SPINNING BLACK
HOLE!
CAN WE IMAGE BLACK HOLES?
[Image from “Gravity’s fatal attraction” by M. Begelman and M. Rees]
CAN WE IMAGE BLACK HOLES?
…NOT YET, BUT SOMEDAY…?
• HUBBLE: read newspaper @ 1 mile
– Optical/UV telescope in space
– Falls short by 100,000
• VLBA: read newspaper in Philly
– Transcontinental radio telescope
– Falls short by 1,000
• MAXIM: Read newspaper on moon
– X-ray interferometer in space
– Can do it! Ready for launch (?) 2020-2030
Stay tuned for lecture by Prof. Web Cash on Nov.11
[Slide courtesy of M. Begelman]