Sd. Bloom et al., Multiwaveband observations of quasars with flat radio spectra and strong millimeter-wave emission, ASTROPH J S, 122(1), 1999, pp. 1-27
We present multiwaveband observations of a well-selected sample of 28 quasa
rs and two radio galaxies with flat radio spectra and strong millimeter-wav
e emission (referred to here as FSRQs). The data are analyzed to determine
the radio to infrared and X-ray to gamma-ray properties of FSRQs and the re
lationships between them. Specifically, the synchrotron. self-Compton (SSC)
process is examined as a likely common radiation mechanism. For most sourc
es, the broadband spectra are still incomplete, especially in the far-infra
red and ultraviolet range. Therefore, precise analysis, such as model fitti
ng of spectra, is not usually possible. To compensate partially for this, w
e have taken a statistical approach and examine the relationship between hi
gh- 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 f
requencies than those of previous surveys - in conjunction with nearly simu
ltaneous radio to submillimeter-wave observations to determine the paramete
rs of the synchrotron spectrum and to examine the compact angular structure
of a subset of sources from our sample. These parameters are used to predi
ct the SSC X-ray flux densities. Seven of 30 sources have predicted self-Co
mpton X-ray flux densities well above the observed flux densities obtained
with the ROSAT satellite unless one assumes that the radiating plasma exper
iences bulk relativistic motion directed toward the observer's line of sigh
t. Three of these seven sources are detected at gamma-ray frequencies. Mode
l spectra show that the X-rays are consistent with the first-order SSC proc
ess, with the simultaneous multiwaveband spectrum of the quasar 0836+710 ob
tained in 1992 March being very well fitted by SSC emission from a uniform,
relativistically moving source. The gamma-rays are not produced via second
-order self-Compton scattering but rather by either first-order self-Compto
n scattering or some other process.
A comparison of the ROSAT X-ray flux densities and those obtained earlier w
ith 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 variab
ility and lower VLBI core-to jet flux ratios. Detections at gamma-ray energ
ies 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 simi
lar to 0.65. The peak in the distribution of X-ray to millimeter spectral i
ndices also indicates a strong connection between millimeter-wave and X-ray
emission. Particularly interesting is a correlation between X-ray to milli
meter spectral index and fraction of flux density contained in the VLBI cor
e. 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.