Neutron Stars and Pulsars

Neutron Stars:

  • A neutron star (NS, for short) is the collapsed core of a massive star, which remains after a Type-II supernova explosion.
  • NS are extremely small (about 20 kilometers in diameter) objects, smaller than stars and planets, yet have a mass of 1  3 M. The result is an incredibly denseobject, as dense as an atomic nucleus, with an average density of 1014 grams per cubic centimeter.
  • NS are quasi-solid objects, held up by the pressure of degenerate neutrons, which resist the crushing gravity, and refuse to be packed together any tighter. The nature of matter at these densities is poorly understood.
  • Neutrons stars were first theorized in the 1930s, as possible evolutionary endpoints for massive stars.
  • NS are extremely hot, around 1 million K. Yet because of their extremely small size, they are very faint (a consequence of the luminosity – temperature – radius relation). They are more easily found at x-ray wavelengths, where most of their energy is emitted.
  • Neutron stars are born as fast rotaters, and gradually spin down with time. Old neutron stars spin slower and, after billions of years, rotation should all but end.
  • The Galaxy should contain large numbers of neutron stars, since almost all massive stars ever formed explode as supernovae, leaving a neutron star behind.

Pulsars:

  • Neutron stars were demonstrated to exist in 1967, identified with strange objects termed pulsars.
  • Pulsars are an observed phenomena: a radio source whose emission is periodic, or “pulsed” as seen by observers here on Earth.
  • Hundreds of pulsars have been cataloged within our galaxy. The first pulsars were astounding objects, initially thought to be signals from extraterrestrials.
  • Pulsars are found with a range of rotation period: from fractions of a second to many seconds.
  • While most pulsars have been discovered via radio observations, some pulsars are now known to pulse over the entire electromagnetic spectrum, from radio to gamma-rays.
  • Pulsars appear to be rapidly rotating, magnetized neutron stars, in which the rotation axis and magnetic axis are not aligned.
  • This rotation causes the emission of radiation from the pulsar to periodically “sweep” over the Earth, producing a pulsed signal from our perspective. This concept is usually termed the lighthouse model. Energy is emitted along the magnetic axis of the fast-spinning pulsar. If we are located “within the beam”, we see the pulsed radiation.
  • Pulsars have been discovered within supernova remnants, confirming the theory that neutron stars are “born" in these explosions. The Crab Nebula pulsar is the best example.
  • The Crab Nebula pulsar spins about 30 times per second, and is observed to pulse in the radio, visible, UV and X-ray. Its fast rotation rate is consistent with its young age.
  • Pulsars spin down with time, with the pulse period gradually getting longer. Eventually, these objects will slow down to the point that they can no longer produce pulsed radiation.

Neutron Star Binaries:

  • Neutron stars in binary systems may accrete matter from a companion star, just like white dwarfs, producing even more energetic fireworks.
  • In the 1970s, the initial surveys of the sky in X-rays detected many strong x-ray sources, the energy output of many of these objects was highly variable. The most powerful sources were labeled “x-ray bursters”.
  • The x-rays emitted by these sources indicate extremely hot gas. Within a neutron star binary, gas accretes onto the NS from a surrounding disk, occasionally igniting a fusion reaction, violently heating the gas, and producing a flood of high energy radiation.
  • If an accreting neutron star collects enough mass, it may collapse into a black hole, perhaps accompanied by a burst of gamma-rays.

Millisecond Pulsars:

  • Radio observations in the 1980s uncovered a new class of pulsars: those rotating at incredible rates. Termed millisecond pulsars, they rotate several hundreds of times per second.
  • For the fastest millisecond pulsars, matter on the star’s equator is moving at 10-20 % of the speed-of-light.
  • Many millisecond pulsars are found in globular clusters, old stellar systems 8-11 billion years old. No young or fast-rotating pulsars should be found there.
  • Millisecond pulsars appear to form when an ancient pulsar is “spun-up” by infalling matter, which strikes the NS surface in the direction of its rotation.
  • This process occurs in binary systems; another example of stars evolving differently when located within binaries.