  X-ray observations performed with ROSAT led to the discovery of 7
radio-silent isolated neutron stars (INSs) which are detected only through the
relatively dim and purely thermal X-ray emission from the cooling star surface.
A few of these INSs (a.k.a. X-ray Dim INSs, or XDINSs) have been also detected
at optical wavelengths where they seem to feature thermal spectra. Optical
studies of XDINSs thus play a crucial role in mapping the temperature
distribution on the neutron star surface and in investigating the existence of
an atmosphere around the neutron star. The aim of this work is to investigate
the optical identification of the XDINS RX J0420.0-5022, tentatively proposed
by Haberl et al. (2004) based on VLT observations. We re-analysed the original
observations of Haberl et al. to assess the detection significance of the
proposed counterpart and we performed deeper VLT observations aiming at a
higher confidence detection. With a ~2 sigma detection significance and a
re-computed flux of B=27.52+/-0.61, we can not rule out that the proposed
counterpart was spurious and produced by the halo of a very bright nearby star.
While we could not detect the proposed counterpart in our deeper VLT
observations, we found evidence for a marginally significant (~3.9 sigma)
detection of a similarly faint object (B= 27.5+/-0.3), ~ 0.5" north of it and
coincident with the updated Chandra position of RX J0420.0-5022. Interestingly,
the angular separation is consistent with the upper limit on the RX
J0420.0-5022 proper motion (Motch et al. 2009), which suggests that we might
have actually detected the Haberl et al. proposed counterpart. From the flux of
the putative RX J0420.0-5022 counterpart we can rule out a > 7 optical excess
with respect to the extrapolation of the XMM-Newton spectrum.
