Blazar synchrotron emission of instantaneously power-law injected electrons under linear synchrotron, non-linear SSC, and combined synchrotron-SSC cooling

TL;DR

This paper investigates how nonlinear synchrotron and SSC cooling processes affect the emission spectra of blazars with instantaneously injected power-law electrons, providing analytical and numerical solutions for different cooling scenarios.

Contribution

It introduces a new nonlinear cooling model for relativistic electrons in blazar jets and analyzes its impact on synchrotron emission spectra, including combined cooling effects.

Findings

Nonlinear cooling alters the spectral indices of synchrotron spectra.

Analytical solutions match numerical results for different cooling regimes.

Spectral indices are 1/2 for linear and 3/2 for nonlinear cooling at small frequencies.

Abstract

The broadband SEDs of blazars show two distinct components which in leptonic models are associated with synchrotron and SSC emission of highly relativistic electrons. In some sources the SSC component dominates the synchrotron peak by one or more orders of magnitude implying that the electrons mainly cool by inverse Compton collisions with their self-made synchrotron photons. Therefore, the linear synchrotron loss of electrons, which is normally invoked in emission models, has to be replaced by a nonlinear loss rate depending on an energy integral of the electron distribution. This modified electron cooling changes significantly the emerging radiation spectra. It is the purpose of this work to apply this new cooling scenario to relativistic power-law distributed electrons, which are injected instantaneously into the jet. We will first solve the differential equation of the volume-averaged differential number density of the electrons, and then discuss their temporal evolution. Since any non-linear cooling will turn into linear cooling after some time, we also calculated the electron number density for a combined cooling scenario consisting of both the linear and non-linear cooling. For all cases, we will also calculate analytically the emerging optically thin synchrotron fluence spectrum which will be compared to a numerical solution. For small normalized frequencies f < 1 the fluence spectra show constant spectral indices. We find for linear cooling a_SYN = 1/2, and for non-linear cooling a_SSC = 3/2. In the combined cooling scenario we obtain for the small injection parameter b_1 = 1/2, and for the large injection parameter b_2 = 3/2, which becomes b_1 = 1/2 for very small frequencies, again. This is the same behaviour as for monoenergetically injected electrons.