1. Pulsars – progress and puzzles 1 Jocelyn Bell Burnell University of Oxford Astrophysics & Mansfield College

2. 2 Audio recording of a pulsar You are listening to a radio recording of a pulsar (the Vela pulsar) of mass 10 27 tonnes spinning on its axis 11 times a second. Vela SN remnant

3. 3 Pulsars - a fast pulsar The fastest known pulsar used to be PSR1937+21, 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. 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 Einstein’s 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. 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 10 8 Tesla (10 10 - 10 11 Tesla in magnetars) Questions – How is magnetic field created? How is it sustained? What shape is it?

9. 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. 10 Mass and density of pulsars Mass 10 27 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 1.8M Sun - this could require different structure. Progress – physics of very condensed matter now better understood. Structure of neutron star being studied.

12. 12 Rigid crust of Fe 56 atoms Neutron- rich nuclei superfluid Core: solid? liquid? Hyperons? Quarks? Neutron Possible structure of a neutron star of a neutron starStruc ture

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 Period (s) dP/dt

15. Pulsar ‘H-R’ diagram Progress Period (s) dP/dt 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 ~ 10 14 G

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