Download Part B

Who are the usual suspects?
Type I Supernovae
All very similar light curves (maximum light
and shape) suggesting common process.
Favoured model is the detonation of a
white dwarf star in a binary system.
No fusion in white dwarf, star is supported only by electron degeneracy
pressure. This sets max mass for a white dwarf, Chandrasekhar limit, at
approximately 1.4 solar masses. Beyond this it cannot be supported by
degeneracy pressure alone.
Type I supernova occurs when white dwarf acquires additional mass from a
companion star, usually a red giant. At critical mass (> 1.4 times mass of
Sun) the heat and pressure in the centre of the star are sufficient to initiate
runaway nuclear fusion, and the white dwarf explodes.
Who are the usual suspects?
X-rays and gamma rays are
produced during
nucleosynthesis and also
during radioactive decay of
the various products. They
also originate from the
shockwave which heats
interstellar gas creating
thermal X-ray emission.
Type I supernovae has led to their use
as standard candles in extragalactic
astronomy.
Who are the usual suspects?
Neutron star : a supernova remnant.
Mass about 1 solar mass, with radius of about 15 to 30 km. Neutron
stars have overall densities of 1017 kg/m3.
During neutron star formation angular momentum and magnetic flux are
conserved. Both are inversely proportional to the square of the radius,
giving rotation periods between about 1.4 ms to 30 seconds and
magnetic fields of up to 108 T.
Energy is lost as neutrinos emitted
carry away so much energy that the
temperature falls within a few years
to around 106 kelvin.
Who are the usual suspects?
X-ray pulsars
Strong magnetic field of neutron star forces
charged particles to follow magnetic field
lines.
http://www.youtube.com/watch?v=33Ldqkd0Fa4&feature=related
They emit synchrotron radiation in a beam
along the N-S magnetic field axis.
Just as on Earth, the magnetic poles don't
coincide with the rotational poles. Thus
beams of radiation are at an angle to the
rotational axis of the neutron star; as the
neutron star rotates, the beams swing
around in a cone.
Mistake in notes
Who are the usual suspects?
X-ray pulsars
Crab Nebula formed in 1054 A.D. supernova with neutron star (X-ray pulsar
at its centre)
Who are the usual suspects?
X-ray bursters
Neutron stars in close proximity to
another star may be able to capture
material, mostly hydrogen.
Accretion of hydrogen on the neutron star’s surface produces a layer
where hydrogen fusion takes place.
Normally this would raise temperature and pressure and cause the
material to expand and cool (-ve feedback).
However ideal gas laws don’t apply on neutron stars and temperature
increase just makes fusion go faster leading to a thermonuclear flash.
X-rays are emitted via black body radiation in a process lasting a minute.
This is an irregular process.
Who are the usual suspects?
X-ray bursters
Who are the usual suspects?
Gamma-ray bursters
• Flashes of gamma rays associated with extremely energetic processes in
distant galaxies ( 1044 joules).
• Extremely rare (a few per galaxy per million years). They are the most
luminous electromagnetic events known to occur in the Universe.
• Short GRB lasts few seconds.
• Long GRB lasts several minutes.
• Unlike X-ray bursters GRB sources
only emit once in their history.
• Distributed isotropically across night
sky suggesting extragalactic origin.
• New GRBs discovered at rate of about
one per day.
Who are the usual suspects?
Gamma-ray bursters
• Flashes of gamma rays associated with extremely energetic processes in
distant galaxies ( 1044 joules).
• Extremely rare (a few per galaxy per million years). They are the most
luminous electromagnetic events known to occur in the Universe.
• First detected in 1967 by the Vela satellites,
• Redshifts clarified their distance and combined
with their luminosity connected them to the
deaths of massive stars.
• GRBs are now thought to be highly focused
explosions, with most of the energy collimated
into a narrow jet from between 2 and 20
degrees in angular spread.
Who are the usual suspects?
Gamma-ray bursters
Short and long GRBs
recorded by Vela.
Who are the usual suspects?
Long Gamma-ray bursters - origin
• Hypernovae - collapse of stars of greater than 30 solar
masses which are spinning rapidly.
• The black hole forms before star outer layers contract very
much.
• An accretion disc from the surrounding stellar material
forms.
• Some of the infalling material does not fall into the black
hole but is ejected in powerful back to back jets along the
axis of rotation of the accretion disc.
• If one of these jets is directed towards Earth we would see
a powerful GRB formed as the relativistic particles slow
down and convert their kinetic energy to gamma rays.
• GRBs have a short existence as it doesn’t take long for
the black hole to eat the accretion disc!
http://www.youtube.com/watch?v=Xb1mkE4WCtQ&feature=related
Who are the usual suspects?
Short Gamma-ray bursters - origin
• Merger of a binary system consisting of two neutron stars.
• They rip each other apart and collapse into a single black hole. The infall of
matter into the new black hole in an accretion disk then powers the GRB.
http://www.youtube.com/watch?v
=g8s81MzzJ5c
Because GRBs are visible to
massive distances
encompassing billions of
galaxies, they must be
exceedingly rare events per
galaxy.
Expect rate of long GRBs in
Milky Way to be about one
burst every 1,000,000 years.
Who are the usual suspects?
Quasars
Discovered in mid 1960s Doppler
shift indicating massive source.
1 quasar emits over 100 times as
much energy as our own galaxy with
its 200 billion stars!!!!!
Ultra luminous objects located at
centre of very distance galaxies.
At the centre of every quasar is a
supermassive black hole.
As matter is pulled into the black hole
it releases vast amounts of energy.
Strong magnetic field in the accretion
disc, produces beams of electrons and
plasma in the jets which generate
http://www.youtube.com/watch?v=RkBak1aETYE&feature=related
radio emission.
Who are the usual suspects?
Active Galactic Nuclei
Active galaxies are galaxies which have a small core of emission embedded
in an otherwise typical galaxy. This core is typically a quasar.
The total energy of normal galaxies like the Milky Way is just the sum of the
emission from each of the stars.
For the "active" galaxies, this is not true
and the core dominates light emission.
This is a photo of Circinus.
Who are the usual suspects?
Other X-ray sources
Lots of energetic interactions of cosmic bodies emit black body radiation
peaked in X-rays.
e.g. Galactic clusters formed by the merger of individual galaxies (infalling
material on collision heated to 108 K).
Bullet cluster, the colour representing X-ray emission.
Who are the usual suspects?
Other X-ray sources
All main sequence stars are likely to have hot enough coronae to emit Xrays (right) and SN remnants (left) also emit in the X-ray band.
Planetary nebula around a white dwarf is also a weak source of X-rays.
Problems associated with
Earth based observation
Attenuation of light by dust and gas in the interstellar medium, and by the
Earth’s atmosphere. Only the visual, infrared, and radio regions of the
electromagnetic spectrum are directly observable from the Earth’s surface.