Jet vs Disc: the Optical Output of Flat-Spectrum Quasars

Abstract

Flat-Spectrum Radio Quasars (FSRQs) are commonly thought of as being dominated at optical/infrared wavelengths by synchrotron emission. Such emission is believed to come from the relativistic jet observed at radio frequencies in many of these objects. We have been studying a large sample of FSRQs in the optical and near-infrared and have found that the picture is rather more complicated. We find that ~40% of the sample are indeed dominated by a synchrotron component that turns over at rest-frame optical wavelengths. A further 40% however show no sign of optical synchrotron emission and instead show spectra consistent with emission from an accretion disc -- similar to optically-selected quasars. There is a smaller percentage of the sample that have both components present.

The identification of significant accretion disc emission in FSRQs has important implications for the overall energy budget particularly for multi-wavelength modelling of quasars that are known gamma-ray emitters.

Finally while the BL Lacs in the sample are all synchrotron-dominated they are not the only such sources. The fact that some quasars have spectra just as dominated by synchrotron emission indicates that the lack of strong emission lines in BL Lacs cannot wholly be due to strong non-thermal emission Show more