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What is a Pulsar ?

An entirely novel kind of star came to light on Aug. 6 last year and was referred to, by astronomers, as LGM (Little Green Men). Now it is thought to be a novel type between a white dwarf and a neutron. The name Pulsar is likely to be given to it. Dr. A. Hewish told me yesterday: "… I am sure that today every radio telescope is looking at the Pulsars."

This was the first appearance of the word "Pulsar" in print in 1968.

        A pulsar is a neutron star that emits beams of radiation that sweep through Earth's line-of-sight. Like a black hole, it is an endpoint to stellar evolution. The "pulses" of high-energy radiation we see from a pulsar are due to a misalignment of the neutron star's rotation axis and its magnetic axis. Pulsars seem to pulse from our perspective because the rotation of the neutron star causes the beam of radiation generated within the magnetic field to sweep in and out of our line of sight with a regular period, somewhat like the beam of light from a lighthouse. The stream of light is, in reality, continuous, but to a distant observer, it seems to wink on and off at regular intervals. The first pulsar was observed on November 28, 1967, by Jocelyn Bell Burnell and Antony Hewish. They observed pulses separated by 1.33 seconds that originated from the same location on the sky.

pulsar

               Neutron stars have very intense magnetic fields, about a trillion times stronger than Earth's own field. However, the axis of the magnetic field is not aligned with the neutron star's rotation axis. The combination of this strong magnetic field and the rapid rotation of the neutron star produces extremely powerful electric fields, with electric potential in excess of 1 trillion volts. To put this power into perspective: A single cubic meter of the magnetic field in the Crab pulsar contains more energy than humans have been able to generate, to date.

                 Electrons are accelerated to high velocities by these strong electric fields. These high-energy electrons produce radiation (light) in two ways. In the first, they act as a coherent plasma, and the electrons work together to produce radio emission by a process whose details are still being researched; and secondly, the electrons interact individually with photons or the magnetic field to produce high-energy emissions in optical, X-ray and gamma-ray wavelengths. The exact locations where the radiation is produced are uncertain, and they may be different for different types of radiation, but they must occur somewhere above the magnetic poles. External viewers see pulses of radiation whenever this region above the the magnetic pole is visible. Because of the rotation of the pulsar, the pulses thus appear much as a distant observer sees a the light from a lighthouse, which seems to blink as its beam rotates. The pulses come at the same rate as the rotation of the neutron star, and, thus, appear periodic.

              Though the general picture of pulsars as rapidly rotating neutron stars is widely accepted, Werner Becker of the Max Planck Institute for Extraterrestrial Physics said in 2006, "The theory of how pulsars emit their radiation is still in its infancy, even after nearly forty years of work."

         There are three different classes of pulsars known to exist and categorized on the basis of the source of the power of the electromagnetic radiation:

  • Rotation-powered pulsars, where the loss of rotational energy of the star provides the power.
  • Accretion-powered pulsars (accounting for most but not all X-ray pulsars), where the gravitational potential energy of accreted matter is the power source (producing X-rays that are observable from the Earth).
  • Magnetars, where the decay of an extremely strong magnetic field provides the electromagnetic power.

             The discovery of pulsars allowed astronomers to study the nature of the neutron star. This kind of object is the only place where the behavior of matter at nuclear density can be observed (though not directly). Also, millisecond pulsars have allowed a test of general relativity in conditions of an intense gravitational field.

             The various applications of pulsar stars are listed below:

Maps

             Pulsar maps have been included on the two Pioneer Plaques as well as the Voyager Golden Record. They show the position of the Sun, relative to 14 pulsars, which are identified by the unique timing of their electromagnetic pulses, so that our position both in space and in time can be calculated by potential extraterrestrial intelligences.Because pulsars are emitting very regular pulses of radio waves, its radio transmissions do not require daily corrections. Moreover, pulsar positioning could create a spacecraft navigation system independently, or be an auxiliary device to GPS instruments.

Precise clocks

            For some millisecond pulsars, the regularity of pulsation is more precise than an atomic clock.This stability allows millisecond pulsars to be used in establishing ephemeris time or building pulsar clocks.

       Timing noise is the name for rotational irregularities observed in all pulsars. This timing noise is observable as random wandering in the pulse frequency or phase. It is unknown whether timing noise is related to pulsar glitches.

Probes of the interstellar medium

          The radiation from pulsars passes through the interstellar medium (ISM) before reaching Earth. Free electrons in the warm (8000 K), ionized component of the ISM and H II regions affect the radiation in two primary ways. The resulting changes to the pulsar's radiation provide an important probe of the ISM itself.

      Because of the dispersive nature of the interstellar plasma, lower-frequency radio waves travel through the medium slower than higher-frequency radio waves. The resulting delay in the arrival of pulses at a range of frequencies is directly measurable as the dispersion measure of the pulsar. The dispersion measure is used to construct models of the free electron distribution in the Milky Way Galaxy.

    Additionally, turbulence in the interstellar gas causes density inhomogeneities in the ISM which cause scattering of the radio waves from the pulsar. The resulting scintillation of the radio waves—the same effect as the twinkling of a star in visible light due to density variations in the Earth's atmosphere—can be used to reconstruct information about the small scale variations in the ISM. Due to the high velocity (up to several hundred km/s) of many pulsars, a single pulsar scans the ISM rapidly, which results in changing scintillation patterns over timescales of a few minutes.

Gravitational waves detectors

         There are 3 consortia around the world which use pulsars to search for gravitational waves. In Europe, there is the European Pulsar Timing Array (EPTA); there is the Parkes Pulsar Timing Array (PPTA) in Australia; and there is the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) in Canada and the US. Together, the consortia form the International Pulsar Timing Array (IPTA). The pulses from Millisecond Pulsars (MSPs) are used as a system of Galactic clocks. Disturbances in the clocks will be measurable at Earth. A disturbance from a passing gravitational wave will have a particular signature across the ensemble of pulsars, and will be thus detected.


Written by Krishna Pandit.
Post-graduate student
Dept. of Mathematics
Cotton College State University


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