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

Using a very deep observation with HST/STIS, we have searched for an opticalcounterpart 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 circlethat we believe to be the optical counterpart. This object has$m_{50CCD}=28.56\pm0.13$ mag, which translates approximately to an unabsorbedflux of $F_{\lambda}=(1.7 \pm 0.3)e-20$ ergs/s/cm^2/A at 5150 A or anX-ray-to-optical flux ratio of $log(f_X/f_opt)=4.9$. This flux is a factor of$\approx 5$ above the extrapolation of the black-body fit to the X-rayspectrum, consistent with the optical spectra of other isolated neutron stars.Without color information we cannot conclude that this source is indeed thecounterpart of RX J1308.6+2127. If not, then the counterpart must have$m_{50CCD} > 29.6$ mag, corresponding to a flux that is barely consistent withthe extrapolation of the black-body fit to the X-ray spectrum.Comment: 4 pages, 2 figures. Uses emulateapj5.sty, onecolfloat5.sty. Accepted by ApJ Letter