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Pulsars

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Astronomical Objects Series
  1. What are Asteroids, Meteors and Comets?
  2. Binary Stars and Double Stars
  3. Variable Stars
  4. Supernova and Supernovae
  5. Types of Nebula and Nebulae
  6. Black Holes
  7. Quasars
  8. Pulsars
  9. Gamma Ray Bursts
  10. Kuiper Belt
  11. Galaxy Types and Galaxy Formation
  12. The Messier Catalogue
  13. The Caldwell Catalogue
  14. 25 Stunning Sights Every Astronomer Should See

Pulsars are highly magnetised, rotating neutron stars that emit a beam of electromagnetic radiation. The radiation can only be observed when the beam of emission is pointing towards the Earth. This is called the lighthouse effect and gives rise to the pulsed nature that gives pulsars their name.

When a massive star dies in a cataclysmic explosion called a supernova, its core, weighing more than our Sun, collapsed under gravity. The intense pressure causes the protons and electrons to fuse to become neutrons. Finally, when the star is about 20km across, the pressure produced by the tightly packed neutrons halts the collapse resulting in a neutron star. As the rotating core collapses, it begins to spin more and more rapidly and its intense magnetic field produces two beams of radio waves that sweep around the sky. Should one or both of these beams sweep across the Earths position in space our radio telescopes will observe regular pulses of energy, much like flashes from a lighthouse, and the object is then called a pulsar. Around 1800 are known to exist.

The first pulsar was given the name LGM-1 – The LGM was short for Little Green Men, because nobody could rule out alien intelligence as an explanation.

Because neutron stars are very dense objects, the rotation period and thus the interval between observed pulses is very regular. For some pulsars, the regularity of pulsation is as precise as an atomic clock. The observed periods of their pulses range from 1.4 milliseconds to 8.5 seconds. A few pulsars are known to have planets orbiting them, such as PSR B1257+12.

Cycle of pulsed gamma rays from the Vela pulsar.
Cycle of pulsed gamma rays from the Vela pulsar.

Formation of Pulsars

The events leading to the formation of a pulsar begin when the core of a massive star is compressed during a supernova, which collapses into a neutron star. The neutron star retains most of its angular momentum, and since it has only a tiny fraction of its progenitor's radius (and therefore its moment of inertia is sharply reduced), it is formed with very high rotation speed. A beam of radiation is emitted along the magnetic axis of the pulsar, which spins along with the rotation of the neutron star. The magnetic axis of the pulsar determines the direction of the electromagnetic beam, with the magnetic axis not necessarily being the same as its rotational axis. This misalignment causes the beam to be seen once for every rotation of the neutron star, which leads to the "pulsed" nature of its appearance.

Schematic view of a pulsar. The sphere in the middle represents the neutron star, the curves indicate the magnetic field lines and the protruding cones represent the emission zones.
Schematic view of a pulsar. The sphere in the middle represents the neutron star, the curves indicate the magnetic field lines and the protruding cones represent the emission zones.

The beam originates from the rotational energy of the neutron star, which generates an electrical field from the movement of the very strong magnetic field, resulting in the acceleration of protons and electrons on the star surface and the creation of an electromagnetic beam emanating from the poles of the magnetic field.

The Arecibo Pulsar Survey

The Arecibo Observatory
The Arecibo Observatory

Arecibo observatory operates the giant 305m Arecibo telescope, and it is currently scanning the entire night sky searching for new pulsars. In 2004 it acquired a multi-beam radio receiver. This observes seven different points on the sky simultaneously, each covering an area about 3.6 arcseconds across. Its cut the time required to make a survey of the entire sky by seven. The sensitivity of the system allied with new highly sensitive data acquisition hardware makes the Arecibo telescope uniquely placed to search for distant pulsars, particularly those that are spinning quickly and in multiple star systems – just like the mysterious J1903+0327 which rotates in milliseconds, the whole star rotates 465 times per second!

Pulsar Naming Convention

Initially, pulsars were named with letters of the discovering observatory followed by their right ascension (e.g. CP 1919). As more pulsars were discovered, the letter code became unwieldy and so the convention was then superseded by the letters PSR (Pulsating Source of Radio) followed by the pulsar's right ascension and degrees of declination (e.g. PSR 0531+21) and sometimes declination to a tenth of a degree (e.g. PSR 1913+167). Pulsars that are very close together sometimes have letters appended (e.g. PSR 0021-72C and PSR 0021-72D).

The modern convention is to prefix the older numbers with a B (e.g. PSR B1919+21) with the B meaning the coordinates are for the 1950.0 epoch. All new pulsars have a J indicating 2000.0 coordinates and also have declination including minutes (e.g. PSR J1921+2153). Pulsars that were discovered before 1993 tend to retain their B names rather than use their J names (e.g. PSR J1921+2153 is more commonly known as PSR B1919+21). Recently discovered pulsars only have a J name (e.g. PSR J0437-4715). All pulsars have a J name that provides more precise coordinates of its location in the sky.

Last updated on: Sunday 12th August 2018

 

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