Context : X-ray observations performed with the Röntgen Satellite ( ROSAT ) led to the discovery of seven 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 .

Aims : 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 Very Large Telescope ( VLT ) observations .

Methods : We re-analysed the original observations of Haberl et al .
( 2004 ) to assess the detection significance of the proposed counterpart and we performed deeper VLT observations aiming at a higher confidence detection .

Results : With a \sim 2 \sigma detection significance and a re-computed flux of B = 27.52 \pm 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 ( \sim 3.9 \sigma ) detection of a similarly faint object ( B = 27.5 \pm 0.3 ) , \approx 0 \aas@@fstack { \prime \prime } 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 .

Conclusions : High spatial resolution observations with the refurbished Hubble Space Telescope ( HST ) are the only way to confirm the detection of the putative candidate counterpart and to validate its identification with RX J0420.0 - 5022 .