Brightness suppression of relativistic radio jets of quasars: the role of the lower electron energy cutoff

Abstract

Although the possibility of a low-energy cutoff (LEC) of the relativistic electron population (E_min ~ 0.1 GeV) in the different components of radio galaxies has been discussed in the literature, from both theoretical and observational viewpoints, its possible role in causing a distinct spectral flattening, and thus reducing the apparent brightness temperature of the radio nucleus, has thus far not been explicitly discussed. Here we point out that such an effect may, in fact, be quite significant for the parsec-scale, superluminal emission regions associated with the inner radio jets. This is because the observed frequency of the spectral turnover arising from the LEC increases linearly with the bulk Doppler factor (d) of the jet flow, whereas the frequency of synchrotron self-absorption (the well-known cause of spectral flattening and turnover at low frequencies) shows a mild decrease as d rises. For an observationally relevant range of source parameters, we investigate here the role of the LEC. We argue that some statistical trends revealed by the recent very long baseline interferometry (VLBI) surveys can, in fact, be understood in terms of effects arising from LEC, including the faster superluminal motion found in the VLBI surveys at higher frequencies and the apparent inability of most radio cores to even attain the brightness temperatures consistent with the equipartition condition. We also point out some possible implications of the LEC for X-ray observations of kiloparsec-scale relativistic jets of quasars.