Pulsars


Pulsars are stellar objects which emit extremely regular pulses of radiation many times per minute, hence the name "pulsars."  The pulses are generated by the neutron star remnant of a Type II supernovae explosion.  The progenitor star's iron core magnetic field is preserved and concentrated during the supernova core collapse.  This extremely strong magnetic field focuses and accelerates charged particles, like electrons, into "jets" which are centered around the axes of the magnetic field. 

During a supernova explosion, the angular momentum of the progenitor star's iron core is conserved.  The collaped core of neutrons spins-up in the same manner that a skater spins faster when they pull in their arms.  Neutron stars can spin-up to rotation rates of three thousand RPM when the progenitor star's core collapses, i.e., However, the neutron star's rotational axis seldom coincides with its magnetic axis. 

When the rotational axis of a neutron star does not coincide with its magnetic axis, the associated particle beam, will sweep out a circle in the sky above the neutron star.  If the beam happens to sweep by near the Earth's celestial location, in the neutron star's sky, we observe a pulse of radiation, the Lighthouse Effect (watch video below).  These pulses are extremely regular and their timing depends on the rotational speed of the neutron star. 

Pulsar Structure
Rotational Axis and Magnetic Field (blue)
Credit: Bill Saxton (YouTube)
M1 "Crab" Nebula (false color)
Synchrotron Radiation (blue)
Credit: NASA_ESA
M1 "Crab" Nebula (X-ray image)
Jet (red lower left) Synchrotron emission (blue)
Credit: NASA_Chandra Telescope
Pulsar Structure M1 Synchrotron M1 X-ray Image

Particle jets are fed by protons and electrons (decaying neutrons) on the surface of the neutron star.  These particles are routed toward the magnetic poles and accelerated to relativistic (approaching light speed) velocities by the neutron star's magnetic field.  The particles spiral outward along the star's magnetic lines-of-force with those closer to the axes forming "jets."  Some of these high speed particles collide with matter blown off by the supernova blast causing this matter to radiate x-rays, ultraviolet light, visible light and radio waves. 

The path of electrically charged particles (protons and electrons) is changed when they move at an angle relative to the magnetic field lines-of-force causing them to spiral around these lines-of-force.  When the path of a relativistic (approaching light speed) charged particle changes, polarized synchrotron radiation is emitted.  Polarized radiation data provides information about the strength and structure of a neutron star's magnetic field. 

Video Animation
Lighthouse Effect
Credit: Animations for Science and Astronmy
Video Animation
Jet (green) and Synchrotron Radiation (purple)
Credit: NASA (YouTube)
Video Animation
Spin-Up
Credit: NASA (YouTube)
Lighthouse Effect (described above) Synchrotron Radiation (described above) Pulsar Spin-Up (described below)

If the neutron star has a companion, usually an old red giant star, the neutron star's gravity can drag matter from the companion into an accretion disk around the neutron star.  Disk matter orbits the neutron star at relativistic velocities.  Frame draging causes orbital decay and disk matter falls onto the neutron star's surface.  The angular momentum of this in-falling matter is transferred to the neutron star, increasing its rotational speed.  This "spin-up" process can increase the neutron star's rotational speed to thousands of RPM.