1999ApJS..122....1B

We present multiwaveband observations of a well-selected sample of 28 quasars and two radio galaxies with flat radio spectra and strong millimeter-wave emission (referred to here as FSRQs). The data are analyzed to determine the radio to infrared and X-ray to γ-ray properties of FSRQs and the relationships between them. Specifically, the synchrotron self-Compton (SSC) process is examined as a likely common radiation mechanism. For most sources, the broadband spectra are still incomplete, especially in the far-infrared and ultraviolet range. Therefore, precise analysis, such as model fitting of spectra, is not usually possible. To compensate partially for this, we have taken a statistical approach and examine the relationship between high- and low-energy emission by using the data set for the entire sample.

We use very long baseline interferometry (VLBI) at 8.4 and 22 GHz–higher frequencies than those of previous surveys–in conjunction with nearly simultaneous radio to submillimeter-wave observations to determine the parameters of the synchrotron spectrum and to examine the compact angular structure of a subset of sources from our sample. These parameters are used to predict the SSC X-ray flux densities. Seven of 30 sources have predicted self-Compton X-ray flux densities well above the observed flux densities obtained with the ROSAT satellite unless one assumes that the radiating plasma experiences bulk relativistic motion directed toward the observer's line of sight. Three of these seven sources are detected at γ-ray frequencies. Model spectra show that the X-rays are consistent with the first-order SSC process, with the simultaneous multiwaveband spectrum of the quasar 0836+710 obtained in 1992 March being very well fitted by SSC emission from a uniform, relativistically moving source. The γ-rays are not produced via second-order self-Compton scattering but rather by either first-order self-Compton scattering or some other process.

A comparison of the ROSAT X-ray flux densities and those obtained earlier with the Einstein Observatory show that several FSRQs are X-ray variables on timescales of about a decade. Several sources that were observed more than once with ROSAT also show variability on timescales of 1-2 yr, with the X-ray variability in these cases often associated with millimeter-wave variability and lower VLBI core-to-jet flux ratios. Detections at γ-ray energies also appear to be related to increases in the radio to millimeter-wave flux densities.

Statistical analysis shows that the millimeter-wave and X-ray luminosities for the sample are strongly correlated, with a linear regression slope ∼0.65. The peak in the distribution of X-ray to millimeter spectral indices also indicates a strong connection between millimeter-wave and X-ray emission. Particularly interesting is a correlation between X-ray to millimeter spectral index and fraction of flux density contained in the VLBI core. This tendency toward higher X-ray fluxes from sources with stronger jet emission implies that the knots in the jet are prominent sources of X-rays.