Neutron Stars and Black Holes

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Neutron Stars and Black Holes
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• Chapter 14

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Outline
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I. Neutron Stars A. Theoretical Prediction of
Neutron Stars B. The Discovery of Pulsars C. A
Model Pulsar D. The Evolution of Pulsars E.
Binary Pulsars F. The Fastest Pulsars G. Pulsar
Planets II. Black Holes A. Escape Velocity B.
Schwarzschild Black Holes C. Black Holes Have No
Hair D. A Leap into a Black Hole E. The Search
for Black Holes
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Outline (continued)
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III. Compact Objects with Disks and Jets A.
X-Ray Bursters B. Accretion Disk
Observations C. Jets of Energy from Compact
Objects D. Gamma-Ray Bursts
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Formation of Neutron Stars
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A supernova explosion of a M gt 8 Msun star blows
away its outer layers.
Compact objects more massive than the
Chandrasekhar Limit (1.4 Msun) collapse beyond
the formation of a white dwarf.
The central core will collapse into a compact
object of a few Msun.
? Pressure becomes so high that electrons and
protons combine to form stable neutrons
throughout the object
p e- ? n ne
? Neutron Star
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Formation of Neutron Stars (2)
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Properties of Neutron Stars
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? a piece of neutron star matter of the size of a
sugar cube has a mass of 100 million tons!!!
Typical size R 10 km
Mass M 1.4 3 Msun
Density r 1014 g/cm3
A neutron star (more than the mass of the sun)
would comfortably fit within the Capital Beltway!
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Discovery of Pulsars
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Angular momentum conservation
gt Collapsing stellar core spins up to periods of
a few milliseconds.
Magnetic fields are amplified up to B 109
1015 G.
(up to 1012 times the average magnetic field of
the sun)
gt Rapidly pulsed (optical and radio) emission
from some objects interpreted as spin period of
neutron stars
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Pulsars / Neutron Stars
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Neutron star surface has a temperature of 1
million K.
Cas A in X-rays
Wiens displacement law, lmax 3,000,000 nm
/ TK gives a maximum wavelength of lmax 3
nm, which corresponds to X-rays.
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Pulsar Periods
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Over time, pulsars lose energy and angular
momentum
gt Pulsar rotation is gradually slowing down.
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Pulsar Winds
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Pulsars are emitting winds and jets of highly
energetic particles.
These winds carry away about 99.9 of the energy
released from the slowing-down of the pulsars
rotation.
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Lighthouse Model of Pulsars
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A Pulsars magnetic field has a dipole structure,
just like Earth.
Radiation is emitted mostly along the magnetic
poles.
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Images of Pulsars and Other Neutron Stars
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The vela Pulsar moving through interstellar space
The Crab nebula and pulsar
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The Crab Pulsar
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Pulsar wind jets
Remnant of a supernova observed in A.D. 1054
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The Crab Pulsar (2)
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Visual image
X-ray image
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Light Curves of the Crab Pulsar
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Proper Motion of Neutron Stars
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Some neutron stars are moving rapidly through
interstellar space.
This might be a result of anisotropies during the
supernova explosion forming the neutron star
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Binary Pulsars
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Some pulsars form binaries with other neutron
stars (or black holes).
Radial velocities resulting from the orbital
motion lengthen the pulsar period when the pulsar
is moving away from Earth...
and shorten the pulsar period when it is
approaching Earth.
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Neutron Stars in Binary Systems X-ray Binaries
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Example Her X-1
Star eclipses neutron star and accretion disk
periodically
2 Msun (F-type) star
Neutron star
Orbital period 1.7 days
Accretion disk material heats to several million
K gt X-ray emission
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Pulsar Planets
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Some pulsars have planets orbiting around them.
Just like in binary pulsars, this can be
discovered through variations of the pulsar
period.
As the planets orbit around the pulsar, they
cause it to wobble around, resulting in slight
changes of the observed pulsar period.
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Black Holes
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Just like white dwarfs (Chandrasekhar limit 1.4
Msun), there is a mass limit for neutron stars
Neutron stars can not exist with masses gt 3 Msun
We know of no mechanism to halt the collapse of a
compact object with gt 3 Msun.
It will collapse into a single point a
singularity
gt A Black Hole!
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Escape Velocity
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Velocity needed to escape Earths gravity from
the surface vesc 11.6 km/s.
vesc
Now, gravitational force decreases with distance
( 1/d2) gt Starting out high above the surface
gt lower escape velocity.
vesc
vesc
If you could compress Earth to a smaller radius
gt higher escape velocity from the surface.
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The Schwarzschild Radius
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gt There is a limiting radius where the escape
velocity reaches the speed of light, c
2GM
____
Rs
Vesc c
c2
G Universal const. of gravity
M Mass
Rs is called the Schwarzschild Radius.
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Schwarzschild Radius and Event Horizon
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No object can travel faster than the speed of
light
gt nothing (not even light) can escape from
inside the Schwarzschild radius

• We have no way of finding out whats happening
  inside the Schwarzschild radius.

• Event horizon

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Black Holes in Supernova Remnants
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Some supernova remnants with no pulsar / neutron
star in the center may contain black holes.
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Schwarzschild Radii
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Black Holes Have No Hair
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Matter forming a black hole is losing almost all
of its properties.
Black Holes are completely determined by 3
quantities
Mass
Angular Momentum
(Electric Charge)
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General Relativity Effects Near Black Holes (1)
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At a distance, the gravitational fields of a
black hole and a star of the same mass are
virtually identical.
At small distances, the much deeper gravitational
potential will become noticeable.
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General Relativity Effects Near Black Holes (2)
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An astronaut descending down towards the event
horizon of the BH will be stretched vertically
(tidal effects) and squeezed laterally.
This effect is called spaghettification
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General Relativity Effects Near Black Holes (3)
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Time dilation
Clocks starting at 1200 at each point. After 3
hours (for an observer far away from the BH)
Clocks closer to the BH run more slowly.
Time dilation becomes infinite at the event
horizon.
Event Horizon
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General Relativity Effects Near Black Holes (4)
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Gravitational Red Shift
All wavelengths of emissions from near the event
horizon are stretched (red shifted). ?
Frequencies are lowered.
Event Horizon
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Observing Black Holes
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No light can escape a black hole
gt Black holes can not be observed directly.
If an invisible compact object is part of a
binary, we can estimate its mass from the orbital
period and radial velocity.
Mass gt 3 Msun gt Black hole!
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Candidates for Black Hole
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Compact object with gt 3 Msun must be a black hole!
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Compact Objects with Disks and Jets
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Black holes and neutron stars can be part of a
binary system.
Matter gets pulled off from the companion star,
forming an accretion disk.
gt Strong X-ray source!
Heats up to a few million K.
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X-Ray Bursters
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Several bursting X-ray sources have been observed
Rapid outburst followed by gradual decay
Repeated outbursts The longer the interval, the
stronger the burst
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The X-Ray Burster 4U 1820-30
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In the cluster NGC 6624
Ultraviolet
Optical
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Black-Hole vs. Neutron-Star Binaries
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Black Holes Accreted matter disappears beyond
the event horizon without a trace.
Neutron Stars Accreted matter produces an X-ray
flash as it impacts on the neutron star surface.
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Black Hole X-Ray Binaries
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Accretion disks around black holes
Strong X-ray sources
Rapidly, erratically variable (with flickering on
time scales of less than a second)
Sometimes Quasi-periodic oscillations (QPOs)
Sometimes Radio-emitting jets
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Model of the X-Ray Binary SS 433
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Optical spectrum shows spectral lines from
material in the jet.
Two sets of lines one blue-shifted, one
red-shifted
Line systems shift back and forth across each
other due to jet precession.
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Gamma-Ray Bursts (GRBs)
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Short ( a few s), bright bursts of gamma-rays
GRB a few hours after the GRB
Same field, 13 years earlier
Later discovered with X-ray and optical
afterglows lasting several hours a few days
Many have now been associated with host galaxies
at large (cosmological) distances.
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A model for Gamma-Ray Bursts
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At least some GRBs are probably related to the
deaths of very massive (gt 25 Msun) stars.
In a supernova-like explosion of stars this
massive, the core might collapse not to a neutron
star, but directly to a black hole.
Such stellar explosions are termed hypernovae