Deep VLT infrared observations of X-ray dim isolated neutron stars

Context. X-ray observations have unveiled the existence of a family of radio-quiet Isolated Neutron Stars whose X-ray emission is purely thermal, and hence are dubbed X-ray dim isolated neutron stars (XDINSs). While optical observations have allowed thermal emission to be related to the neutron star cooling and the neutron star surface thermal map to be built, IR observations are critical to pinpoint a spectral turnover produced by an as yet unseen magnetospheric component, or the presence of a fallback disk. The detection of such a turnover can provide further evidence of a link between this class of isolated neutron stars and the magnetars, which show a distinctive spectral flattening in the IR. Aims. We present the deepest IR observations reported to date of five XDINSs, which we use to constrain a spectral turnover in the IR and the presence of a fallback disk. Methods. We used archived VLT observations of these neutron stars performed with the ISAAC instrument in the H-band (1.65 μm) and the available fallback disk models. Results. It was not possible to identify the IR counterpart down to limiting magnitudes H ∼ 21.5-22.9 for any of our targets. Although these limits are the deepest ever obtained for neutron stars of this class, they are not deep enough to rule out the existence and the nature of a possible spectral flattening in the IR. We also derive, using disk models, the upper limits on the mass inflow rate in a fallback disk. We find the existence of a putative fallback disk consistent (although not confirmed) with our observations.