M10 High Energy Astrophysics

Topic

The central element of this model is the population of relativistic electrons residing within a magnetized astrophysical source.

Synchrotron emission occurs when these electrons—possessing very high Lorentz factors—spiral while trapped within a magnetic field. Because the frequency at which this emission peaks increases with both the square of the electron's Lorentz factor and the strength of the magnetic field itself, sufficiently energetic electrons end up emitting in the radio and soft X-ray bands, even within magnetic fields as weak as those of the interstellar medium.

Inverse Compton scattering, by contrast, is the reverse process: a relativistic electron from that same population transfers energy to a low-energy photon, boosting the photon's energy by approximately the square of the electron's Lorentz factor. Driven by either synchrotron photons from the source itself or photons from the cosmic microwave background, this mechanism is precisely what produces the very-high-energy gamma rays observed in blazars.

The power-law spectrum exhibited by this population of relativistic electrons is the signature characteristic of Fermi shock acceleration; consequently, the power-law index directly determines the spectral index of the resulting synchrotron radiation. Thus, from the relativistic electron population with which this document begins to the spectral index with which it concludes—encompassing synchrotron emission and inverse Compton scattering—high-energy radiative processes ensure that measuring a source's radio or X-ray spectrum provides direct information about the particle acceleration mechanism that produced it.

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