A probable optical counterpart to the isolated neutron star RX J1308.6+2127

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 m(50CCD) = 28.56 +/- 0.13 mag, which translates approximately to an unabsorbed flux of F-lambda = (1/7 +/- 0.3) x 10(-20) ergs s(-1) cm(-2) Angstrom (-1) at 5150 Angstrom or an X-ray-to-optical flux ratio of log (f(x)/f(opt)) = 4.9. This flux is a factor of approximate to5 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 mag, m(50CCD) > 29.6 corresponding to a flux that is barely consistent with the extrapolation of the blackbody fit to the X-ray spectrum.