Old but Still Warm: Far-UV Detection of PSR B0950+08

We report on a Hubble Space Telescope detection of the nearby, old pulsar B0950+08 (d similar or equal to 262 pc, spin-down age 17.5 Myr) in two far-ultraviolet (FUV) bands. We measured the mean flux densities (f) over bar (nu) = 109 +/- 6 and 83 +/- 14 nJy in the F125LP and F140LP filters (pivot wavelengths 1438 and 1528 angstrom). Using the FUV data together with previously obtained optical-UV data, we conclude that the optical-FUV spectrum consists of two components-a nonthermal (presumably magnetospheric) power-law spectrum (f(nu) proportional to nu(alpha)) with slope alpha similar to -1.2 and a thermal spectrum emitted from the bulk of the neutron star (NS) surface with a temperature in the range of (1-3) x 10(5) K, depending on interstellar extinction and NS radius. These temperatures are much higher than predicted by NS cooling models for such an old pulsar, which means that some heating mechanisms operate in NSs. A plausible mechanism responsible for the high temperature of PSR B0950+08 is the interaction of vortex lines of the faster rotating neutron superfluid with the slower rotating normal matter in the inner NS crust (vortex creep heating).