VLT/FORS2 observations of the optical counterpart of the isolated neutron star RBS 1774

Context. The X-ray observations performed with the Röntgen Satellite (ROSAT) have led to the discovery of a group (seven to date) of X-ray dim and radio-silent, middle-aged isolated neutron stars (a.k.a. XDINSs), which are characterised by pure blackbody spectra (kT ≈ 40-100 eV) and long X-ray pulsations (P = 3-12 s), and appear to be endowed with relatively high magnetic fields, (B ≈ 1013-1014 G). Optical observations of XDINSs are important, together with the X-ray ones, for studying the cooling of the neutron star surface and for investigating the relation between XDINSs and other isolated neutron star classes. RBS 1774 is one of the few XDINSs with a candidate optical counterpart, which we discovered with the Very Large Telescope (VLT). Aims: We aim at constraining the optical spectrum of RBS 1774, for which only two B-band flux measurements are available, and to determine whether its optical emission has either a thermal or a non-thermal origin. Methods: We performed deep observations of RBS 1774 in the R band with the VLT to disentangle a non-thermal power-law spectrum from a Rayleigh-Jeans, whose contributions are expected to be very different in the red part of the spectrum. Results: We did not detect the RBS 1774 candidate counterpart down to a 3σ limiting magnitude of R ~ 27. The constraint on its colour, (B - R) ≲ 0.6, rules out its being a background object, positionally coincident with the X-ray source. Our R-band upper limit is consistent with the extrapolation of the B-band flux (assuming a 3σ uncertainty) for a set of power laws Fν ∝ ν-α with spectral indices α ≤ 0.07. If the optical spectrum of RBS 1774 were non-thermal, its power-law slope would be very much unlike those of all isolated neutron stars with non-thermal optical emission, suggesting that it is most likely thermal. For instance, a Rayleigh-Jeans with temperature TO = 11 eV, for an optically emitting radius rO = 15 km and a source distance d = 150 pc, would be consistent with the optical measurements. The implied low distance is compatible with the 0.04 X-ray pulsed fraction if either the star spin axis is nearly aligned with the magnetic axis or with the line of sight or it is slightly misaligned with respect to both the magnetic axis and the line of sight by 5-10°. Conclusions: New observations, both from the ground and from the Hubble Space Telescope (HST), are important to characterise the optical/near-ultraviolet (UV) spectrum of RBS 1774, to better constrain the values of rO, d, and TO and measure the source's proper motion from which indirect constraints on the source distance can be inferred.