Pulsars

1
Pulsars progress and puzzles

• Jocelyn Bell Burnell
• University of Oxford Astrophysics
• Mansfield College

2
Audio recording of a pulsar

• You are listening to a radio recording of a
  pulsar (the Vela pulsar) of mass 1027 tonnes
  spinning on its axis 11 times a second.

Vela SN remnant
3
Pulsars - a fast pulsar

• The fastest known pulsar used to be
• PSR193721, with a period of
• P 1.5578064688197945/-
  0.0000000000000004 ms
• The current fastest has a period 1.396 ms
• Question how fast can a pulsar spin?

4
Pulsar model

• Star spins like a lighthouse, sweeping radio beam
  around the sky.
• We see a pulse each time beam sweeps across us.

5
Pulsar timing

• Their accurate timekeeping means
• Pulsars can be used as clocks to test Einsteins
  Theory of Relativity.
• Binary orbits can be studied precisely
• Progress the 5 post-Keplerian parameters have
  been obtained precession due to relativistic
  spin-orbit coupling has been reported.
• Doppler shifts can reveal presence of planets
• Puzzle why one and only one system with
  planets?

6
Pulsar test of GR

• The orbit of the Binary Pulsar shrinks exactly as
  predicted by Einstein.
• It implies that gravitational radiation exists as
  predicted by Einstein.

7
Pulsars an amazing fact

• The roundest known thing in the universe is the
  orbit of a pulsar around its companion star. It
  is round to 1 micron in 100000km.
• Information obtained from Doppler shifts as
  pulsar orbits.

8
Magnetic field

• Assuming magnetic dipole radiation
• Magnetic field is 108 Tesla (1010 - 10 11Tesla in
  magnetars)
• Questions
• How is magnetic field created?
• How is it sustained?
• What shape is it?

9
Pulsars radio emission

• They are weak emitters of radio waves
• Question how and where is the radio emission
  produced? (Magnetic pole)
• Intermittent pulsars have recently been
  discovered.
• How do you make an intermittent pulsar?

10
Mass and density of pulsars

• Mass 1027 tonnes
• Radius 10 km
• Average density density of nucleus
• Squeeze the 6 billion people
• that inhabit Earth into a thimble
• and it has the same mass as a
• thimble full of neutron star material

11
Mass and density contd

• Question what is range of allowable masses?
• Not known, but hints of masses as large as
  1.8MSun - this could require different
  structure.
• Progress physics of very condensed matter now
  better understood. Structure of neutron star
  being studied.

12
Rigid crust of
Fe56 atoms
Neutron- rich
nuclei
Neutron
superfluid Core solid? liquid? Hyperons? Quarks?
Possible structure of a neutron star
of a neutron starStructure
13
Pulsars an amazing fact

• If you drop a coin on to the surface of a neutron
  star, it will hit the surface with a speed of
  0.6c

14
Pulsar H-R diagram

• Pulsar P dP/dt plot
• TG TeraGauss field derived assuming magnetic
  dipole radiation.
• Age estimated from current rate of evolution

dP/dt
Period (s)
15
Pulsar H-R diagram

• Pulsar formed in SN explosion (core collapse or
  electron capture).
• Appears upper-centre of this diagram.
• Evolves down and to right, and becomes
• radio quiet.
• Pulsar in binary spun-up again and
• reappears as ms pulsar.
• Puzzle
• Magnetars - top right B 1014 G

• Progress

dP/dt
Period (s)
16
Conclusions

• Field keeps re-inventing itself still full of
  drama!
• Current issues
• Magnetars
• Intermittent pulsars
• Testing GR and alternatives
• Pulsar timing array to detect Grav Radiation