VLT observations of the candidate counterpart to PSR J0108-1431

Context. Optical-ultraviolet (UV) observations of > 100 Myr pulsars are crucial to understand the long-term evolution of neutron stars, including the late stages of the neutron star cooling. The 166 Myr old pulsar PSR J0108-1431 is one of the best targets since it is the oldest non-recycled pulsar with a candidate counterpart, detected with the Very Large Telescope (VLT). Aims. Aim of our observations is to obtain a firm detection of its candidate counterpart, only detected with marginal significance and to measure anew its flux in the U and B bands, for which we obtained only uncertain values. Methods. We observed the PSR J0108-1431 field with the FOcal Reducer/low dispersion Spectrograph (FORS2) at the VLT, exploiting the updated pulsar radio coordinates obtained from recent VLBI observations. Results. Due to non-optimal seeing conditions, we only reached 3σ detection limits of U ∼ 26.5 and B ∼ 27.2, i.e. not incompatible with the fluxes of the candidate counterpart (U = 26.4 ± 0.3; B = 27.9 ± 0.5) that we measured in our previous VLT observations. Conclusions. We can not rule out that the proposed counterpart, detected at the edge of an elliptical galaxy, is real and that we could not detect it just because its flux is close to the detection limit of our new VLT observations. Due to its blue spectrum and proximity to the galaxy, UV observations with the Hubble Space Telescope are more suited to confirm the pulsar identification.