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Perry, Roy

MSc student
Perry, Roy
Research type


  • 2017- 2021, MSc Electron-Positron beams in intergalactic medium
    with Yuri Lyubarsky

    Abstract/Description: Modern age Cerenkov telescopes discovered the strong emissivity of TeV gamma
    rays from blazars, active galactic nuclei which emit relativistic jets. In the
    intergalactic medium, ultra-high energy gamma rays interact with the extragalactic
    background light, producing electron-positron beams with the energies of
    100GeV - 10TeV. It has been suggested that due to the inverse Compton scattering
    upon the cosmic microwave background, these beams emit GeV photons giving
    rise to gamma-haloes around blazars. However, this prediction has been disproved
    by observations. One explanation for the disparity is the presence of an
    intergalactic magnetic field that deviates pairs from the line of site. Another
    possibility is that due to the two-stream instability, the pair beams lose energy on
    the excitation of plasma waves by resonance interaction. The last process has been
    studied intensively, both analytically and numerically, but conclusions are still
    controversial regarding the fate of the relativistic beams. We show that for the possible range of
    parameters of the extragalactic beams, the instability is strongly suppressed for all
    but quasi-perpendicular waves, due to inhomogeneity of the intergalactic medium.
    Hence the beam energy loss is negligible, while it mostly expands transversally.
    Therefore, the two-stream instability is unable to prevent the beam particles to
    produce the gamma halo via the inverse Compton scattering. The lack of the
    observed gamma haloes around blazars should be attributed to the extragalactic
    magnetic field.