Radio and X-ray variability in the Seyfert galaxy NGC 4051
  We present intensive quasi-simultaneous X-ray and radio monitoring of the
narrow line Seyfert 1 galaxy NGC 4051, over a 16 month period in 2000-2001.
Observations were made with the Rossi Timing X-ray Explorer (RXTE) and the Very
Large Array (VLA) at 8.4 and 4.8 GHz. In the X-ray band NGC 4051 behaves much
like a Galactic black hole binary (GBH) system in a `soft-state'. In such
systems, there has so far been no firm evidence for an active, radio-emitting
jet like those found in `hard state' GBHs. VLBI observations of NGC 4051 show
three co-linear compact components. This structure resembles the core and outer
hot spots seen in powerful, jet-dominated, extragalactic radio sources and
suggests the existence of a weak jet.
  Radio monitoring of the core of NGC 4051 is complicated by the presence of
surrounding extended emission and by the changing array configurations of the
VLA. Only in the A configuration is the core reasonably resolved. We have
carefully removed the contaminations of the core by extended emission in the
various arrays. The resulting lightcurve shows no sign of large amplitude
variability (i.e. factor 50 %) over the 16 month period. Within the most
sensitive configuration (A array) we see marginal evidence for radio core
variability of ~25% (~0.12 mJy at 8.4GHz) on a 2-week timescale, correlated
with X-ray variations. Even if the radio variations in NGC 4051 are real, the
percentage variability is much less than in the X-ray band. Within the B
configuration observations, where sensitivity is reduced, there is no sign of
correlated X-ray/radio variability. The lack of radio variability in NGC 4051,
which we commonly see in `hard state' GBHs, may be explained by orientation
effects. Another possibility is that the radio emission arises from the X-ray
corona, although the linear structure of the compact radio components here is
hard to explain.
