PKS 0558–504 is the brightest radio-loud Narrow-line Seyfert 1 galaxy (NLS1) at X-ray energies. Here we present results from the radio, optical, UV, and X-ray bands obtained with Swift, XMM-Newton, and the Australia Telescope Compact Array (ATCA) during a ten-day monitoring campaign in 2008 September. The simultaneous coverage at several wavelengths makes it possible to investigate in detail the broadband spectral energy distribution (SED) and the energetics of this source. The main results can be summarized as follows. The ATCA reveals the presence of an extended radio emission in PKS 0558–504 with two lobe-like structures ~7'' from the bright central source. The extended radio structure and the low value of the radio loudness similar to radio-quiet Seyfert galaxies coupled with constraints from higher energy bands argue against a jet-dominated emission in PKS 0558–504. The study of the SED, which is dominated by a nearly constant optical-UV emission, supports the conclusion that PKS 0558–504 is accreting at a super-Eddington rate. This conclusion was reached by assuming M BH = 2.5 × 108 M ☉, which was obtained with a new scaling method based on X-ray spectral variability results. A comparison between the accretion luminosity and the kinetic power associated with the jet suggests that in this source, the accretion power dominates in agreement with the results obtained from radiation-magnetohydrodynamic simulations of Galactic black holes (GBHs) accreting at the Eddington rate. The combined findings from this panchromatic investigation strongly suggest that PKS 0558–504 is a large-scale analog of GBHs in their highly accreting intermediate state. More importantly, PKS 0558–504 may also be the prototype of the parent population of the very radio-loud NLS1s recently detected at γ-ray energies.
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