Hard Synchrotron Spectra from Magnetically Dominated Plasma Turbulence

Luca Comisso, Emanuele Sobacchi, Lorenzo Sironi

Research output: Contribution to journalArticlepeer-review

22 Scopus citations


Synchrotron emission from astrophysical nonthermal sources usually assumes that the emitting particles are isotropic. By means of large-scale two- and three-dimensional particle-in-cell simulations, we demonstrate that the dissipation of magnetically dominated (σ 0≥ 1) turbulence in pair plasmas leads to strongly anisotropic particle distributions. At Lorentz factors ∼σ 0γ th0 (here,γ th0 is the initial Lorentz factor), the particle velocity is preferentially aligned with the local magnetic field; instead, the highest energy particles are preferentially oriented in the plane perpendicular to the field. This energy-dependent anisotropy leads to a synchrotron spectral flux ν Fναν s that is much harder than for isotropic particles. Remarkably, for σ 0≥ 1 we find that the angle-integrated spectral slope in the slow cooling regime is ∼ 0.5-0.7 for a wide range of turbulence fluctuations, despite significant variations in the power-law energy spectrum of nonthermal particles. This is because weaker turbulence levels imprint a stronger degree of anisotropy, thereby counteracting the effect of the steeper particle spectrum. The synchrotron spectral slope may be even harder, s≳ 0.7, if the observer is in the plane perpendicular to the mean magnetic field. Our results are independent of domain size and dimensionality. Our findings may help explain the origin of hard synchrotron spectra of astrophysical nonthermal sources, most notably the radio spectrum of pulsar wind nebulae.

Original languageEnglish
Article numberL40
JournalAstrophysical Journal Letters
Issue number2
StatePublished - 1 Jun 2020
Externally publishedYes

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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