Hubble space telescope observations of isolated pulsars

Using the COSTAR corrected Faint Object Camera on the Hubble Space Telescope, we have obtained deep ultraviolet and optical images of the fields containing three nearby radio pulsars spanning a range in age between ∼1 × 10⁵ and 2 × 10⁷ yr. Deep 7″.4 × 7″.4 images were obtained in the long-pass F130LP filter for the youngest, PSR B0656 + 14, and the oldest, PSR B0950+08, pulsars; each of the images contains only one pointlike source which is likely the UV-optical counterpart of the corresponding pulsar. PSR B1929 + 10 was imaged in the F130LP, F342W (U), and F430W (B) filters, and it was apparently detected in the former two images. The AB magnitudes in the F130LP filter are m_130LP = 25.2, 27.1, and 26.9 for B0656 +14, B0950 + 08, and B1929 + 10, respectively. For B1929 + 10 we also obtained m_U = 26.8 and m_B > 26.2. Comparison of the flux observed from B0656 + 14 with ROSAT data and model spectra of neutron star atmospheres shows that it is predominantly of a nonthermal origin, exceeding by several orders of magnitude predictions from phenomenological theoretical models. We predict that the thermal component of the flux should be observable in the far-UV domain. The fluxes observed from the older pulsars B0950+08 and B1929+09 are consistent with the assumption that they are thermal-like fluxes arising from the entire neutron star surface. Under this assumption, the observed (blackbody) temperature of B0950+08 is T_∞^bb = (7 ± 1) × 10⁴ K for the distance d= 127 ± 13 pc and the neutron star radius R_∞ = 13 km. The fluxes observed from B1929 + 10 correspond to T_∞^bb = (1-3) × 10⁵ K for the distance and interstellar extinction in the ranges d = 150-180 pc and E(D-V) = 0.0-0.1. These relatively high temperatures for the old pulsars require (re)heating mechanisms to be operating in old neutron stars. They can be explained by frictional heating due to dissipation of energy of differential rotation. The deep exposures set very stringent upper limits on the temperatures of neutron stars at these distances if our candidates are not the pulsars. For example, the field around B0950+08 has a 3 σ limit corresponding to 2 × 10⁴ K at d = 130 pc.