Neutron Stars and Black Holes

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Neutron Stars and Black Holes
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Why do we expect neutron stars exist? How do we
know neutron stars exist? What theoretical
arguments predict the existence of black
holes? What evidence is there that black holes
indeed exist?
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Neutron stars how do they form?
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Neutron Stars
If we pack electrons close enough together ?
white dwarf (electron degenerate)
If we pack neutrons close enough together ?
neutron star (neutron degenerate)
Q Recall the Chandrasekhar limit (1.4 solar
masses), what happens if the collapsing core is
greater than this?
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Properties of neutron stars
10 km in radius Density 1014g/cm3 Between 1.4
and 3 Msun
Q What happens when a NS becomes more massive
than 3 Msun?
Spin rapidly Hot Strong magnetic field
Q Why would we expect neutron stars to spin
rapidly, be hot, and have strong magnetic fields?
Pressure becomes so high that electrons and
protons combine to form stable neutrons
throughout the object.
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Internal structure of a neutron star
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Pulsars
1967 Jocelyn Bell noticed pulses which repeated
regularly in the sight line of a distant galaxy ?
first pulsar that was detected.
Periods range from 0.030 to 3.75
seconds Gradually slow down
Suppose it was a white dwarf of 12,000 km
diameter emitting the pulse Since the near side
is 12,000 km closer than the far side, the light
from the near side would arrive 0.04 s sooner
than the light from the far side ? The pulse
would be smeared out over a longer interval.
Pulses last 0.001 s
This places an upper limit on the size of the
object emitting the pulse
An object cannot change its brightness in an
interval shorter than the time it takes light to
travel its diameter.
? For a 0.001 s pulse interval, the diameter must
be smaller than 300 km.
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The link between neutron stars and pulsars
In 1968, astronomers discovered a pulsar in the
Crab nebula.
The Crab Pulsar is roughly 25 km (16 mi.) in
diameter and rotates 30 times/second!
Its slowing in its rotation by 38
nanoseconds/day due to energy loss by the pulsar
wind.
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Theoretical model of a pulsar
Pulsars do not pulse, but rather emit beams of
radiation that sweep around the sky as the
neutron star rotates Strong magnetic and electric
fields are likely the cause of the intense beams
of radiation
Note that we only can see the pulsars whose beams
sweep over Earth.
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The evolution of pulsars
Q the Crab pulsar is slowing down in its
rotation by 38 nanoseconds/day why? Pulsars lose
energy as they emit beams of radiation and the
pulsar wind (high-speed atomic particles) Q
Where, ultimately, does this energy come from? ?
The energy of rotation! (Thats why they slow
down)
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Roaming pulsars Some pulsars appear to be moving
at a high speed through space
pulsar B150855 path 1000km/s
Q What could explain these strange motions of
pulsars that are observed?
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Compact Objects with Disks and Jets x-rays
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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.
Binary pulsars allow us to measure the mass and
all the other good things we get from binaries ?
Looking for x-ray sources is one way to detect
neutron stars (and black holes).
Heats up to a few million K.
gt Strong X-ray source!
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Binary pulsars
In 1974, Taylor and Hulse detected the first
binary pulsar (PSR191316) The pulses were
changing, growing longer, and then shorter over a
period of 7.75 hours From Doppler shifts, the
orbital velocities and masses were
calculated and it turned out that this system
was two neutron stars orbiting each other with a
separation of roughly the radius of our sun!
PSR191316 held another surprise In 1916
Einstein predicted that a rapid change in a
gravitational field should spread out like waves
(gravitational radiation) Taylor and Hulse were
able to show that the orbital period was
decreasing because the stars were spiraling
toward each other. They won the Nobel prize in
1993.
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Neutron Stars in Binary Systems X-ray binaries
Her X-1
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Star eclipses the neutron star and accretion disk
every 1.7 days hiding the x-ray pulses for a few
hours
Her X-1
Pulses every 1.2 seconds
2 Msun (F-type) star
Orbital period 1.7 days
Accretion disk material heats to several million
K gt X-ray emission
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Masses of pulsars
From Doppler shifts, astronomers have estimated
the masses of dozens of binary pulsars. Typical
masses are 1.35 solar masses. Q If the core
must be at least 1.4 solar masses to form a NS,
then how could the typical mass of a NS be
1.35? A A NS of slightly less than 1.4 solar
masses can exist if the NS loses mass. Also, a
1.4 solar mass WD produces a 1.2 solar mass NS.
Some of the mass is converted into binding
energy.
The gravitational fields near neutron stars are
so strong, that a marshmallow dropped onto a
neutron star from a distance of 1AU would release
the equivalent energy of a 3 Mt nuclear bomb!
(231 Hiroshima-sized bombs!)
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X-ray bursters

• Matter flows onto the NS where it accumulates
  until it becomes hot and dense enough to ignite
• The result is a burst of x-rays
• x-ray burster
• Notice the similarity between this and the
  mechanism which generates novae.

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The X-Ray Burster 4U 1820-30
This is possibly the result of a collision of a
neutron star and a giant the NS then went into
orbit inside the giant!
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This is a neutron star orbiting a white dwarf The
period is only 11 minutes! ? The separation is
only about a third of the Earth/moon distance!
Optical
Ultraviolet
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The fastest pulsars
The fastest pulsars go by the name millisecond
pulsars. Why are they so fast? What happens to
them when they rotate so fast? Since the pulse
period of the pulsar is the rotation period,
these fast pulsars are probably flattened like
pancakes! Take PSR193721 Assume it is 10 km in
radius Spinning at 642 times a second, the
period is 0.0016 seconds and the equatorial
velocity is about 40,000 km/s!
Q Would you expect a pulsar that pulses rapidly
to be young or old? Due to the gradual slowing of
the rotation, one would expect young pulsars to
blink rapidly and old pulsars to blink slowly,
but A few that blink the fastest may be quite
old. One of the fastest (PSR193721) pulses 642
times a second! The energy contained in the
rotation of this pulsar is comparable to the
total energy of a supernovae explosion! Q How
could this be? To explain this, it appears that
this pulsar was sped up by accreting matter from
a binary companion.
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Pulsar Planets
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Small Doppler shifts were observed in the spectra
of PSR125712 Analysis revealed that this pulsar
was orbited by at least two planets with masses
roughly 4.3 and 3.9 Earth masses! Further
analysis revealed a third planet with a mass of
about that of our moon! And there is evidence
that a fourth planet about 100 Earth masses
orbits this pulsar with a much larger separation.
Q How can a NS have planets?!? (Recall that NS
are created by supernovae, and a giant star about
to explode would envelop any planets within an AU
or two)
As a planet orbits around a pulsar, the planet
causes it to wobble around, resulting in slight
changes of the observed pulsar period.
These planets are probably the remains of a
stellar companion that was devoured by the NS.
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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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• Escape velocity depends on two things
• Mass
• Distance from CoM

vesc
Velocity needed to escape Earths gravity from
the surface 11.6 km/s (25,000 mph).
vesc

• Gravitational force decreases with distance (
  1/r2)
• Starting out high above the surface ?lower escape
  velocity.

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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There is a limiting radius where the escape
velocity reaches the speed of light, c
2GM
____
Rs
c2
G gravitational constant
Vesc c
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 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
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An astronaut descending down towards the event
horizon of the black hole will be stretched
vertically (tidal effects) and squeezed laterally.
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General Relativity Effects Near Black Holes (II)
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Time dilation
In SR
In GR
Clocks starting at 1200 at each point. After 3
hours (for an observer far away from the black
hole)
Clocks closer to the black hole run more slowly.
Time dilation becomes infinite at the event
horizon.
Event horizon
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General Relativity Effects Near Black Holes (III)
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gravitational redshift
All wavelengths of emissions from near the event
horizon are stretched (redshifted). ? 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.
But 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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A compact object with gt 3 Msun must be black
hole!
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Jets of Energy from Compact Objects
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Your impression of a black hole might suggest
that its impossible to get energy out of such an
object.
Some X-ray binaries show jets perpendicular to
the accretion disk.
These bipolar flows are formed the same way as
they do for protostars. (Bipolar flow - angular
momentum ? hot accretion disk ? high-energy
photons emitted ? shot out via thermal magnetic
processes.
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Opposing jets of gas are streaming away from a
supermassive black hole at Centaurus As galactic
nucleus - remnants of a giant explosion.
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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 to near ¼ c (its receding and
approaching!)
Lines shift back and forth across each other
every 164 days due to jet precession
SS 433 is most likely a black hole!
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In 1963, a nuclear test ban treaty was signed
nuclear weapons tests were off limits In 1968,
the U.S. had satellites designed to detect gamma
rays signs of a nuclear detonation Those
satellites started detecting bursts of gamma rays
at a rate of about one burst a day That data
became declassified in 1973. The bursts usually
lasted only a matter of seconds They came from
all directions of the sky and not from any
particular region They occur without
warning And they have more power than the most
violent supernovae explosions.
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Gamma-Ray Bursts (GRBs)
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GRB of May 10, 1999 1 day after the GRB
2 days after the GRB
Some of these GRBs repeat known as soft
gamma-ray repeaters, soft low energy gamma
rays. We suspect that these originate from
neutron stars with really strong magnetic fields
(magnetars). When shifts in the magnetic field
breaks through the crust of a magnetar, bursts of
gamma rays are emitted. On August 27, 1998, one
of these ionized Earths atmosphere and disrupted
radio communications worldwide.
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Gamma-Ray Bursts (GRBs) II
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Possible origins Could be the result of the
merger of two neutron stars (recall the binary
pulsar PSR191316 detected by Taylor and
Hulse.) and/or from the collapse of really
massive stars (gt25 solar masses) - hypernovae

• March 29, 2003 GRB in Leo
• Left behind a spectrum which resembled that of a
  SN
• Hypernovae are indeed responsible for some GRBs
• But the NS merger is not ruled out.

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GRBs III
If a GRB occurred only 1,600 ly from Earth, we
would be showered with the radiation equivalent
to a 10,000 Mt nuclear blast! Possibly every few
hundred million years one could occur near enough
to Earth for us to be affected. Possibly one of
these caused one of the mass extinctions that
show up in the fossil record Q How could
something which seems so rare as a neutron star
merger, be so common that we detect at least one
of these GRBs every day?
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Over 800 GRBs detected by the BATSE instrument
onboard the CGRO
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