High Energy Emission from Pulsars and Pulsar Wind Nebulae
Pulsars are accidentally discovered by the Cambridge scientists [48]. Shortly thereafter, Gold [39] and Pacini [72] proposed that pulsars are rotating neutron stars with surface magnetic fields of around 1012 G. Gold [39] pointed out that such objects cou
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High Energy Emission from Pulsars and Pulsar Wind Nebulae Kwong Sang Cheng
18.1 Introduction Pulsars are accidentally discovered by the Cambridge scientists [48]. Shortly thereafter, Gold [39] and Pacini [72] proposed that pulsars are rotating neutron stars with surface magnetic fields of around 1012 G. Gold [39] pointed out that such objects could account for many of the observed features of pulsars, such as the remarkable stability of the pulsar period, and predicted a small increase in the period as the pulsar slowly lost rotational energy. With the discovery of the Vela pulsar with a period of 88 ms [65], the identification of the Crab pulsar with a period of 33 ms [86] and the discovery of slowdown of Crab pulsar [77], it was essentially confirmed that pulsars are rapidly rotating neutron stars. So far, over 1,500 radio pulsars have been found (see the most updated list of pulsars in www.atnf.csiro.au/research/pulsar/). The radio luminosities of these pulsars are small compared with the energy loss rate due to the pulsar spin down (∼10−6 –10−5 ). Strong high-frequency radiation in the X-ray band has been observed from about two dozens pulsars (for recent review cf. [6, 7]), but only eight pulsars have been confirmed to emit high energy γ -rays (cf. [94] for a recent review). The observed radiated power for the γ -ray pulsars is concentrated mainly in the γ -ray range and the γ -ray luminosities are a substantial fraction (10−3 –10−1 ) of the spin-down power. This makes studies of high energy radiation from a pulsar a promising way to better understand the physical processes which result in their non-thermal radiation. Theoretically, a common idea is that emissions ranging from radio to γ -rays are produced in different regions of the pulsar magnetosphere. To an excellent approximation, the pulsar may be considered as a non-aligned rotating magnet with a very strong surface magnetic field. Just outside the surface of the neutron star, the Lorentz force on a charged particle is very strong and far exceeds the force of gravitational K.S. Cheng Department of Physics, University of Hong Kong e-mail: [email protected] W. Becker (ed.), Neutron Stars and Pulsars, Astrophysics and Space Science Library 357, c Springer-Verlag Berlin Heidelberg 2009
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attraction, i.e., e((v/c) × B)/(GMm/r2 ) 1. As a result, the structure of the magnetosphere of the neutron star is completely dominated by electromagnetic forces. Because the induced electric fields at the surface of a neutron star are so strong that the force on the charged particle in the surface exceeds the work function of the surface material, there must be a plasma surrounding the neutron star. In this way, there is a fully conducting plasma surrounding the neutron star, and electric currents can flow in the magnetosphere (e.g., [69]). If the component of the electric field E = E · B/B along the magnetic field direction (B) is non-zero in the pulsar magnetosphere, and this component of the electric field can accelerate particles to ultra
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