A synchrotron self-Compton model with low energy electron cut-off for the blazar S5 0716+714

TL;DR

This paper presents a synchrotron self-Compton model with a low-energy electron cut-off to explain high brightness temperatures and the spectral energy distribution of the blazar S5 0716+714, fitting observations with a simple electron distribution.

Contribution

It introduces a low-energy electron cut-off in SSC modeling, successfully explaining high T_B and the observed SED of S5 0716+714 with a homogeneous source.

Findings

Hard radio spectrum explained by low-energy cut-off electrons

Doppler factor D≥65 needed for full spectrum fit

Electron distribution with low-energy cut-off accounts for high T_B

Abstract

Rapid inverse Compton cooling sets in when the brightness temperature (T_B) of a self-absorbed synchrotron source with power-law electrons reaches ~10^{12} K. However, T_B inferred from observations of intra-day variable sources (IDV) are well above the "Compton catastrophe" limit. This can be understood if the underlying electron distribution cuts off at low energy. We approximate a low-energy cut-off with monoenergetic electrons. We compute the synchrotron self-Compton (SSC) spectrum of such distribution, and using the IDV source S5~0716+714 as an example, we compare it to the observed SED of S5~0716+714. The hard radio spectrum is well-fitted by this model, and the optical data can be accommodated by a power-law extension to the electron spectrum. We therefore examine the scenario of an injection of electrons that is a double power law in energy with a hard low-energy component that does not contribute to the synchrotron opacity. We show that the double power-law injection model is in good agreement with the observed SED of S5~0716+714. For intrinsic variability, we find that a Doppler factor of D\geq30 can explain the observed SED provided that low-frequency (<32 GHz) emission originates from a larger region than the higher-frequency emission. To fit the entire spectrum, D\geq65 is needed. We find the constraint imposed by induced Compton scattering at high T_B is insignificant in our model. We confirm that electron distribution with a low-energy cut-off can explain the high T_B in compact radio sources. We show that synchrotron spectrum from such distributions naturally accounts for the observed hard radio continuum with a softer optical component, without the need for an inhomogeneous source.