We present spectroscopy and imaging with the Very Large Telescope ( VLT ) of the neutron star RX J1856.5 $ - $ 3754 .
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 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 \pm 0.06 ) \times 10 ^ { -19 } ( \lambda / 5000 { { \hbox { \AA } } } ) ^ { -4 % } 10 ^ { -0.4 ( 0.12 \pm 0.05 ) ( A _ { \lambda } / A _ { V } -1.138 ) } { erg } { s ^ { -1 } } { % cm ^ { -2 } } { { \hbox { \AA } } ^ { -1 } } , where A _ { \lambda } / A _ { V } is the reddening curve ; implied V = 25.58 \pm 0.02 ) .
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 .