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

Abstract

Context: In a self-absorbed synchrotron source with power-law electrons, rapid inverse Compton cooling sets in when the brightness temperature of the source reaches T_B∼10¹² K. However, brightness temperatures 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.
Aims: We examine the compatibility of the synchrotron and inverse Compton emission of an electron distribution with low-energy cut-off with that of IDV sources, using the observed spectral energy distribution of S5 0716+714 as an example.
Methods: We compute the synchrotron self-Compton (SSC) spectrum of monoenergetic electrons and compare it to the observed spectral energy distribution (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, which is a double power law in energy, with a hard low-energy component that does not contribute to the synchrotron opacity.
Results: 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≥30 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≥65 is needed. We find the constraint imposed by induced Compton scattering at high T_B is insignificant in our model.
Conclusions: We confirm that electron distribution with a low-energy cut-off can explain the high brightness temperature 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. The required low energy electron distribution is compatible with a relativistic Maxwellian.