The Radio-X-ray Connection in Young Stellar Objects in the Orion Nebula
  Cluster
  (Abridged) Context: Both X-ray and radio observations offer insight into the
high-energy processes of young stellar objects (YSOs). The observed thermal
X-ray emission can be accompanied by both thermal and nonthermal radio
emission. Due to variability, simultaneous X-ray and radio observations are a
priori required, but results have been inconclusive. Aims: We use archival
X-ray and radio observations of the Orion Nebula Cluster (ONC) to significantly
enlarge the sample size of known YSOs with both X-ray and radio detections.
Methods: We study the ONC using multi-epoch non-simultaneous archival Chandra
X-ray and NRAO Very Large Array (VLA) single-band radio data. The multiple
epochs allow us to reduce the impact of variability by obtaining approximated
quiescent fluxes. Results: We find that only a small fraction of the X-ray
sources (7%) have radio counterparts, even if 60% of the radio sources have
X-ray counterparts. The radio flux density is typically too low to distinguish
thermal and nonthermal radio sources. Only a small fraction of the YSOs with
detections in both bands are compatible with the empirical "Guedel-Benz" (GB)
relation. Most of the sources not compatible with the GB relation are proplyds,
and thus likely thermal sources, but only a fraction of the proplyds is
detected in both bands, such that the role of these sources is inconclusive.
Conclusions: While the radio sources appear to be globally unrelated to the
X-ray sources, the X-ray dataset clearly is much more sensitive than the radio
data. We find tentative evidence that known non-thermal radio sources and
saturated X-ray sources are indeed close to the empirical relation, even if
skewed to higher radio luminosities, as they are expected to be. Most of the
sources that are clearly incompatible with the empirical relation are proplyds
which could instead plausibly be thermal radio sources.
