Thermal and Nonthermal Emission in the Optical-UV Spectrum of PSR B0950+08

Based on recent Hubble Space Telescope (HST) observations in the far-UV and ground-based observations in optical bands, Pavlov and colleagues have revealed a thermal component in the spectrum of the old pulsar B0950+08 (spin-down age 17.5 Myr) and estimated a neutron star (NS) surface temperature of (1-3) × 105 K. Our new HST observations in the optical have allowed us to resolve the pulsar from a close-by galaxy and measure the optical fluxes more accurately. Using the newly measured fluxes and a new calibration of the HST's far-UV detector, we fit the optical-UV pulsar's spectrum with a model that consists of a nonthermal power law (f ν ∝ ν α ) and thermal blackbody components. We obtained the spectral slope α = -0.3 ± 0.3, considerably flatter than found from ground-based observations, and the best-fit temperature in the range of (6-12) × 104 K (as seen by a distant observer), depending on interstellar extinction and NS radius. The temperature is lower than reported previously, but still much higher than predicted by NS passive cooling scenarios for such an old pulsar. This means that some heating mechanisms operate in NSs, e.g., caused by the interaction of the faster-rotating neutron superfluid with the slower-rotating normal matter in the inner crust of the NS.