Electron–positron jets associated with the quasar 3C279

Abstract

A long-standing question in extragalactic astrophysics is the composition of the relativistic jets of plasma that stream from the nuclei of quasars and active galaxiesÐdo they consist of a`normal' (electron±proton) plasma, or a`pair' (electron±positron) plasma? Distinguishing between these possibilities is crucial for understanding the physical processes occurring close to the putative supermassive black holes that are believed responsible for the jets. Here we report the detection of circularly polarized radio emission from the jets of the archtypal quasar 3C279. The circular polarization is produced by Faraday conversion, which requires the energy distribution of the radiating particles to extend to very low energies, indicating that electron±positron pairs are an important component of the jet plasma. Similar detections in three other radio sources suggest that, in general, extragalactic radio jets are composed mainly of an electron±positron plasma.The most luminous objects in the Universe are distant quasars and active galactic nuclei. A few per cent of these emit strongly at radio wavelengths, and are powered by twin jets of plasma streaming at close to the speed of light from the nucleus of the underlying galaxy 1 . The central`engine' in the nucleus is widely believed to be a supermassive black hole of some 10 8 ±10 9 solar masses 2 . At radio wavelengths the jets are visible from parsec to kiloparsec scales, emitting synchrotron radiation from ultra-relativistic electrons gyrating in a magnetic ®eld. The bulk kinetic energy carried by the jets powers the extended lobes of radio emission far outside the parent galaxy, while synchrotron and inverse Compton emission from the base of the jets generates radiation at optical, X-ray and even g-ray wavelengths 3,4 . The composition of the jet plasma has been an unresolved issue ever since the discovery of the jets. The two main candidates are a`normal' plasma consisting of protons and relativistic electrons (an e±p jet), and a`pair plasma' consisting only of relativistic electrons and positrons (an e + ±e -jet).Electrons and positrons emit synchrotron radiation of identical spectrum and linear polarization, so it is dif®cult to tell whether the radiating particles are mainly electrons or a mixture of electrons and positrons. Two lines of argument shed light on the problem. The ®rst comes from matching the total energy¯ux carried by the jet to the total dissipation observed in electromagnetic radiationÐthe energy stored in the extended radio lobes in magnetic ®elds and relativistic particlesÐand the mechanical work done against the surrounding medium 5,6 . The energy distribution of the radiating particles is assumed to be a power law, ng~g 2 2a1 for g min , g , g max , where n is the density, g 1 2 v 2 =c 2 2 1 2 is the Lorentz factor of a particle of velocity v, a is the observed spectral index of the synchrotron radiation, and c is the velocity of light. For an e±p jet, the energy distribution must cut off at g min < 100, or the jet may carry several orders of ...

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