Photons’ Scattering in Relativistic Plasma with Velocity Shear: Generation of High Energy Power-law Spectra

Mukesh K. Vyas, Asaf Pe’er

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A high energy power law is a common feature in the spectra of many astrophysical objects. We show that the photons in an unmagnetized relativistic plasma composed of electrons and protons with a variable Lorentz factor (or a velocity shear) go through repeated scattering with electrons to gain energy. The escaped population of photons naturally produces a power-law-shaped spectrum making it a photon’s analog to the conventional Fermi acceleration mechanism for charged particles. Thus, this mechanism provides a natural alternative to current explanations of high energy power-law spectra via synchrotron or thermal Comptonization. The model is applicable to any relativistic plasma beam with an arbitrary Lorentz factor profile. We implement the theory to the gamma-ray burst prompt phase and show that the obtained range of the photon indices is compatible with the observed values and the results of Monte Carlo simulations that we carry out independently. Therefore, the observed high energy spectral indices provide a unique indicator of the jet structure.

Original languageEnglish
Article numberL3
JournalAstrophysical Journal Letters
Issue number1
StatePublished - 1 Jan 2023

Bibliographical note

Publisher Copyright:
© 2023. The Author(s). Published by the American Astronomical Society.


A.P. acknowledges support from the European Union (EU) via ERC consolidator grant 773062 (O.M.J.). M.K.V. acknowledges the PBC Fellowship by the government of Israel as well as part of funding obtained by the abovementioned ERC grant.

FundersFunder number
Government of Israel
European Commission
European Research Council773062


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