| Description |
Seven of the nearby isolated neutron stars (INS) known till date show predominantly thermal spectrum with $\sim ~10^{6}$ K temperatures. Being relatively nearby and isolated makes them particularly attractive to constrain important neutron star properties such as mass, radius, cooling and possibly the equation of state of matter at ultra-high densities. However, interpreting their thermal spectra has turned out to be far more difficult than thought initially. Therefore, we carried out HST observations and identified their optical/UV counterparts. When compared to their X-ray spectra most of these sources appear to be brighter in optical/UV, in one case by a factor of 40 ! We find that the optical/UV SEDs show a range of slopes that are inconsistent with what is expected from thermal (Rayleigh-Jeans) emission. Several explanations including atmospheric effects, magnetospheric emission, and resonant scattering were considered, but none seems sufficient. These observations show that the puzzle of INSs, and possibly of all neutron stars, is far from over.
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