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

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 m_50CCD=28.56+/-0.13 mag, which translates approximately to an unabsorbed flux of F_λ=(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(f_X/f_opt)=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 m_50CCD>29.6 mag, corresponding to a flux that is barely consistent with the extrapolation of the blackbody fit to the X-ray spectrum.