Neutron Stars and Black Holes

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Neutron Stars and Black Holes
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Formation of a Neutron Star
In the collapsing core, gamma rays break up the
atomic nuclei into their constituent protons and
neutrons.
Neutrons, like electrons, are fermions (particles
with a spin of 1/2) so they obey the
Pauli exclusion principle. Under the conditions
of the dead high mass star, the neutrons are as
tightly packed as possible. Just as white dwarfs
are supported by electron degeneracy pressure,
these dead stars are supported by neutron
degeneracy pressure theyre called neutron stars.
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Summary of Neutron Star Properties

• Radius 10 km (about 600 times smaller than
  Earth).
• Mass 1.4 to 3 times the mass of the Sun.
• Density 1017 kg per cubic meter. A ½ inch cube
  of this material with would weigh more than 100
  million tons.
• Neutron degeneracy prevents it from collapsing
  further
• Spins very rapidly.
• Has a powerful magnetic field.
• Spin rate and magnetic field strength normally
  decrease slowly with time.

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Properties of Neutron Stars - Density and Size
How dense is a neutron star?
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Rotation Rate and Magnetic Field
As the star collapses, its magnetic field is
concentrated in a smaller area, becoming as much
as a trillion times as strong as that of the Sun.
The stellar core was already hot, but the
collapse raises the temperature further, the
surface temperature of a young neutron star being
on the order of 10 million K.
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Internal Structure of a Neutron Star
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Pulsar Properties

• Periods from a few milliseconds to several
  seconds.
• Pulses last for as little as 1 millisecond.
• Pulses occur with great regularity. For the first
  pulsar discovered, P 1.33730119 s.
• Period decreases at the rate of a few billionths
  of a second per day.
• Glitches (sudden drops in pulsar period) occur.

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What is the source of the pulsar phenomena?

• Pulsating main sequence star or white dwarf? No -
  pulsations too fast.
• Rotating main sequence star or white dwarf with a
  hot spot? No - Either of these would disintegrate
  if it were to rotate this fast.
• Pulsating neutron star? No - these would pulsate
  too fast.
• Rotating neutron star. Yes, the pulsar periods
  are consistent with this.

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Lighthouse Model

• Neutron star can rotate rapidly without
  disintegration.
• Has a bipolar (having a north pole and a south
  pole) magnetic field.
• Strong, moving magnetic field lines create a
  powerful electric field.
• Strong electric field creates and accelerates
  charged particles.
• Electrons are trapped by the magnetic field and
  forced to travel along magnetic field lines away
  from the magnetic poles at speeds near the speed
  of light.
• Accelerated electrons emit synchrotron
  radiation, resulting in twin beams of
  electromagnetic radiation from the north and
  south magnet poles of the neutron star.
• A pulsar pulse is detected at Earthwhenever a
  beam sweeps across Earth.
• Rotational energy is converted into
  electromagnetic energy, including synchrotron
  radiation, so the neutron star slows down.
• Are glitches due to starquakes? Some probably
  are,but these are not frequent enough to
  accountfor all glitches.
• Most are probably due to vortex
  events,triggered by slowing of rotation.
  Angularmomentum of a large number of
  vortexestransfers angular momentum to the crust.

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Additional Pulsar-Related Discoveries

• Millisecond pulsars
• Binary pulsars
• Pulsar planets
• Magnetars
• Hercules X-1

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Black Holes
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Escape Velocity and Black Holes
No physical object can travel faster than light.
The speed of light, according to special
relativity, is an absolute upper limit.
What is the radius of an object of given mass
that has an escape velocity equal to the speed of
light?
M in solar masses and Rs in km
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The Event Horizon of a Non-rotating
(Schwartzschile) Black Hole
According to general relativity, the singularity
is enclosed by a spherical surface called the
event horizon, which has a radius denote by Rs,
the Schwartzschild radius.
RS
Nothing can cross the eventhorizon in the
outward direction. Since this includes light, we
cant observe anything inside the event horizon.
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Summary of Schwartzschild Black Hole Properties

• Nothing that enters the event horizon can escape
  from the black hole.
• No force can stop collapse to zero volume.
• The radius of the event horizon is called the
  Schwartzschild radius.
• Time slows down and light is red-shifted as the
  event horizon is approached.
• If it has a main sequence star or red giant
  companion, an accretion disk forms and friction
  gradually causes much of the disk material to
  eventually spiral into the black hole.
• Tidal forces squeeze, stretch, tear apart, and
  ionize material before it reaches the event
  horizon.

Animation http//oposite.stsci.edu/pubinfo/pr/200
1/03/content/CygnusXR-1.mpg
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Kerr (Rotating) Black Holes

• A black hole can have only three properties
  mass, angular momentum, and charge.
• Stellar black holes are electrically neutral.
• A neutral rotating black hole is called a Kerr
  black hole.
• Outside its event horizon, a Kerr black hole has
  a region, called the ergosphere, in which
  spacetime is dragged along with the rotating
  black hole. In principle, energy can be extracted
  from the ergosphere.
• An object dropped into the ergosphere can break
  into two parts. One of them drops through the
  event horizon. The other leaves the ergosphere
  with more energy than the original object had,
  and the mass of the black hole decreases.

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Searching for Black Holes

• Isolated black holes are virtually impossible for
  us to see from Earth, because theyre small and
  emit no light.
• A black hole is more likely to be recognized if
  it has a visible companion that isnt a black
  hole.
• A black hole with a visible companion will be a
  source of x-rays. The x-ray emission intensity
  should exhibit rapid fluctuations because of the
  chaotic nature of the processes that cause the
  x-ray emission.
• So, we search for binary systems in which (a) one
  of the objects is visible, (b) the other is
  invisible and (c) there are x-ray sources that
  have short time scale fluctuations
• We deduce the mass of the visible companion from
  its spectrum.
• Having the mass of the visible companion and some
  information about the orbit, we can find a lower
  limit to the mass of the invisible companion. If
  its greater than about 3 times the mass of the
  Sun, its probably a black hole. Otherwise, its
  likely to be a neutron star.

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Behavior of a Blob of Matter Falling Toward a
Black Hole Event Horizon or Onto a Neutron Star
Neutron star Blob of matter spirals inward,
hits the hard surface, and explodes, producing a
powerful burst of high energy radiation.
Black hole Blob of matter spirals inward,
reddens, and gradually disappears. Very little
radiation escapes from the blob.
http//science.nasa.gov/headlines/y2001/ast12jan_1
.htm
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Some Stellar Black Hole Candidates
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X-Ray Bursters, Gamma Ray Bursters, QPOs, and
SS433
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X-Ray Bursters

• Powerful bursts of energy at irregular intervals.
• The longer the period between bursts, the
  stronger the burst.
• Explanation Neutron star with a normal star
  companion.
• Close enough for normal star material to pass
  through the inner Lagrangian point, form a disk
  around the neutron star, and accrete onto it.
• As the mixture of hydrogen and helium accumulates
  on the surface of the neutron star, the hydrogen
  fuses steadily and a layer of helium builds up.
• When the layer of helium becames dense enough and
  hot enough, it fuses to form carbon and emits a
  burst of x-rays. The burst lasts just a few
  seconds, but emits 1037 Joules of energy.
• The helium layer can then build up until another
  burst occurs.

How long would it take the Sun to produce the
amount of energy in a typical x-ray burst?
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Quasi-periodic Oscillations

• Observation X-ray pulses from accretion disks
  around neutron stars and black holes. Pulses have
  very short periods as short as 7.5?10-4 s.
  Pulse periods decrease rapidly before the pulse
  vanishes completely. Because of the changing
  pulse period, these are called QPOs
  (quasi-periodic oscillations).
• Explanation Blobs of material near the surface
  of a neutron star or black hole emit x-rays while
  orbiting in the accretion disk. The period
  decreases because the blob moves faster as it
  spirals into the compact object.

http//science.nasa.gov/headlines/images/blackhole
/cygxr1w.jpg
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Calculate the orbital period for a blob of
material 20 km from the center of a neutron star
of mass 2.0 times the mass of the Sun.
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SS433

• One set of spectral lines is blue-shifted and
  another is red-shifted.
• Model neutron star or black hole with a normal
  star companion.
• Accretion disk and bipolar jets.
• Disk and jet precess with a 164-day period.
• Jet velocity ¼ the speed of light.

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Gamma Ray Bursters

• Short (seconds or minutes) bursts of high energy
  gamma rays.
• Seen in all directions ? originate outside our
  galaxy.
• Measured red shifts indicate that they are
  billions of light years away.
• What are they?
• Binary neutron star systems? They emit energy in
  the form of gravitational waves and eventually
  merge. This results in a black hole a short
  burst of high energy gamma rays.
• Hypernovae (collapsars)? High mass star
  collapses, but supernova is suppressed by
  infalling mass from the stars envelope.? Star
  collapses to form a black hole. ? Bursts of high
  energy gamma rays along the polar axes.