Hubble Space Telescope observations of the old pulsar PSR J0108-1431

We present results of optical-UV observations of the 200 Myr old rotation-powered radio pulsar J0108\(-\)1431 with the Hubble Space Telescope. We found a putative candidate for the far-UV (FUV) pulsar counterpart, with the flux density \(f_\nu = 9.0\pm 3.2\) nJy at \(\lambda = 1528\) \AA. The pulsar was not detected, however, at longer wavelengths, with \(3\sigma\) upper limits of 52, 37, and 87 nJy at \(\lambda =\) 4326, 3355, and 2366 \AA, respectively. Assuming that the pulsar counterpart was indeed detected in FUV, and the previously reported marginal \(U\) and \(B\) detections with the Very Large Telescope were real, the optical-UV spectrum of the pulsar can be described by a power-law model with a nearly flat \(f_\nu\) spectrum. Similar to younger pulsars detected in the optical, the slope of the nonthermal spectrum steepens in the X-ray range. The pulsar's luminosity in the 1500--6000 \AA wavelength range, \(L \sim 1.2\times 10^{27} (d/210\,{\rm pc})^2\) erg s\(^{-1}\), corresponds to a high efficiency of conversion of pulsar rotation energy loss rate \(\dot {E}\) to the optical-UV radiation, \(\eta = L/\dot{E} \sim (1\)--\(6)\times 10^{-4}\), depending on somewhat uncertain values of distance and spectral slope. The brightness temperature of the bulk neutron star surface does not exceed 59,000 K (\(3\sigma\) upper bound), as seen by a distant observer. If we assume that the FUV flux is dominated by a thermal component, then the surface temperature can be in the range of 27,000--55,000 K, requiring a heating mechanism to operate in old neutron stars.