  We present spectroscopy and imaging with the Very Large Telescope (VLT) of
the neutron star RX J1856.5-2754. Little is known about the nature of this
source other than that it is a nearby hot neutron star. Our VLT spectrum does
not show any strong emission or absorption features. With considerable care to
photometric calibration, we obtain photometric measurements over the optical
and ultra-violet (UV) using our VLT observations and a detailed analysis of
archival {\em Hubble Space Telescope} data. We find that the entire optical to
UV spectral energy distribution is well described by a slightly reddened
Rayleigh-Jeans tail ($f_\lambda=(2.96\pm0.06)\times10^{-19}
(\lambda/5000\un{\AA})^{-4} 10^{-0.4(0.12\pm0.05)(A_\lambda/A_V-1.138)}
\un{erg}\un{s^{-1}}\un{cm^{-2}}\un{\AA^{-1}}$, where $A_\lambda/A_V$ is the
reddening curve; implied $V=25.58\pm0.02$). The reddening is consistent with
the interstellar absorption inferred from X-ray spectroscopy. The simplest
explanation for this Rayleigh-Jeans emission is that the optical-UV radiation
arises from thermal emission from the surface of the neutron star. The high
degree to which the data conform to the Rayleigh-Jeans tail significantly
limits contributions from other sources of emission. In particular, our
observations are inconsistent with the presence of an accretion disk and also
strongly constrain the amount of magnetospheric emission from this enigmatic
neutron star.
