A Probable Optical Counterpart to the Isolated Neutron Star RX J1308.6+2127

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Abstract
Using a very deep observation from the Hubble Space Telescope Space Telescope Imaging Spectrometer, we have searched for an optical counterpart to the nearby radio-quiet isolated neutron star RX J1308.6+2127 (RBS 1223). We have identified a single object in the 90% Chandra error circle that we believe to be the optical counterpart. This object has m50CCD = 28.56 ± 0.13 mag, which translates approximately to an unabsorbed flux of Fλ = (1.7 ± 0.3) × 10-20 ergs s-1 cm-2 Å -1 at 5150 Å or an X-ray-to-optical flux ratio of log(fX/fopt) = 4.9. This flux is a factor of ≈5 above the extrapolation of the blackbody fit to the X-ray spectrum, consistent with the optical spectra of other isolated neutron stars. Without color information, we cannot conclude that this source is indeed the counterpart to RX J1308.6+2127. If not, then the counterpart must have m50CCD > 29.6 mag, corresponding to a flux that is barely consistent with the extrapolation of the blackbody fit to the X-ray spectrum.