Pulsars Table of Contents Introduction... 1 Discovery...2 Observation...2 Binary Pulsars...3 Pulsar Classes... 3 The Significance of Pulsars... 3 Sources...4 Introduction Pulsars are neutron stars which appear to emit electromagnetic radiation at regular intervals, in 'pulses', from which the name is derived (though the stars do not actually pulsate). The short periods and regularity have led to the interpretation that the pulses are generated by neutron stars formed in supernova explosions from normal stars. Since the normal stars are rotating, the neutron stars also rotate, except they now have a very high magnetic field. They also rotate faster than normal stars because they are smaller, so conservation of angular momentum results in higher rotational speeds. The period of rotation is Illustration 1: Schematic view of a pulsar. The sphere in the middle represents the neutron star, the curves indicate the
almost constant to a very high degree of accuracy, and plasma (ionised gas) is blown out by the huge magnetic field as it rotates. electrons are accelerated in the magnetic field and emit synchrotron radiation (ie generated by the acceleration of ultrarelativistic charged particles moving through a magnetic field), giving rise to a strong beam of electromagnetic radiation at all wavelengths. If the earth is in the path swept out by this beam, it will be observed by us as a pulse, in the same way that light-houses appear to flash from far away. There are over a thousand pulsars known, and several have been identified in the optical or x-ray band. Discovery All stars appear to twinkle in the sky in the optical spectrum due to refraction of light by the atmosphere, however they also display this phenomenon in the radio band, which arises from density fluctuations in the solar wind. It was while studying this scintillation in 1967 when Jocelyn Bell, during her phd studies (under professor anthony hewish), discovered signals in the metre waveband which occurred with incredible regularity and unknowingly recorded the first observed data for a pulsar. The pulses were so regular that, at first, the group tried to find a local source of the radiation, and even entertained the idea of extraterrestrial intelligence, but once more pulsing radio sources had been observed in different parts of the sky, it became clear that they were a natural phenomenon. Observation It is expected that the luminosity of a source of synchrotron radiation will have a power-law dependence on frequency, v, given by L where ~0.75. Since v > 1 and β < 1, as v increases, v β will increase and v - β will decrease. this means that signal strength of pulses will be higher when observed at lower frequencies. Due to the fact that each pulse detected on the earth will correspond to the rotation of the source pulsar, it is possible to measure the period of rotation by measuring the time between pulses. Pulsars do not have a defined characteristic luminosity when observed through an optical telescope (and many cannot be seen anyway), so absolute magnitudes cannot be used to determine their distances from us; other methods must be employed, such as measurment of dispersion. All electromagnetic radiation moves at a constant speed through a vacuum, but is slowed proportionally to it's wavelength when it travels through a medium. the interstellar medium is often regarded as a vacuum, but in actual fact is occupied by atoms and free electrons, albeit at extremely low densities. Therefore lower wavelength pulses will be detected on earth slightly sooner than higher wavelength pulses (however, this does not affect the period). This time difference can be measured and used to find the distance of the
pulsar from the earth using the equation t=4150n e d 1 f 1 2 1 f 2 2 where Δ t is the time delay (seconds), n e is the interstellar electron density in cm -3, d is the distance in parsecs and f 1 and f 2 are the observation frequencies in MHz. This equation is derived from the expression for the velocity of propagation of a radio wave through an ionised medium u=c 1 K n e f 2. Binary Pulsars Most stars in our galaxy are part of a binary or multiple star system. These usually remain intact, even in the case of one star becoming a supernova, so a pulsar could exist in a binary system. these can be identified as their periods have a large cyclic change, owing to the change in redshift as the pulsar periodically moves towards and away from us. Pulsar Classes There are three distinct classes of pulsars, each one corresponding to a source of energy powering the radiation. All pulsars are neutron stars, but the classes each have very distinct, observable behaviour. Rotation powered pulsars are powered by the loss of rotational energy. This is the first group to have been discovered, and were once known as radio pulsars, but rotation powered pulsars that emit x-rays have since been found. Accretion-powered pulsars (also known as x-ray pulsars) are powered by the gravitational potential energy of the accreted matter, producing x-rays that are observable from earth. Magnetars are powered by the decay of an extremely strong magnetic field, emitting x-rays and gamma rays. The Significance of Pulsars The study of pulsars has yielded observations which are highly applicable to several areas of Physics and Astronomy. For example, the observation of pulsars in radio wavelengths have pushed the limits of our understanding of gravitational waves, confirming predictions made by general relativity. It was also via the observation of a distant pulsar which led to the very first detection of an exarsolar planetary system. I can therefore say, without hesitation, that I believe pulsars to be one of the most fascinating, intricate, and indeed beautiful celestial bodies to have been observed in our universe.
Illustration 2: Composite Optical/X-ray image of the Crab Nebula, showing synchrotron emission in the surrounding pulsar wind nebula, powered by injection of magnetic fields and particles from the central pulsar. Sources Zeilik & Gregory, Introductory Astronomy & Astrophysics Duncan R. Lorimer, Binary and Millisecond Pulsars at the New Millennium D. R. Lorimer & M. Kramer, Handbook of Pulsar Astronomy
Ingrid H. Stairs, Testing General Relativity with Pulsar Timing ULO materials 910 words (not including contents & sources)