Thermal and nonthermal emission in the optical-UV spectrum of PSR B0950+08

TL;DR

This study combines new HST optical observations with previous data to analyze the thermal and nonthermal emission components of the old pulsar PSR B0950+08, revealing a lower surface temperature than earlier estimates and implications for neutron star heating mechanisms.

Contribution

It provides a refined measurement of the pulsar's optical-UV spectrum and surface temperature, highlighting the presence of heating processes in an old neutron star.

Findings

Spectral slope $eta=-0.3\, ext{to}\,0.3$ with new calibration

Surface temperature estimated at $(6-12)\times 10^4$ K

Temperature higher than passive cooling predictions

Abstract

Based on recent Hubble Space Telescope (HST) observations in far-UV and groundbased 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 $(1$--$3)\times 10^5$ 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 ($f_\nu\propto \nu^\alpha$) and a thermal blackbody components. We obtained the spectral slope $\alpha=-0.3\pm 0.3$, considerably flatter than found from groundbased observations, and the best-fit temperature in the range of $(6$--$12)\times 10^4$ 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 interaction of the faster rotating neutron superfluid with the slower rotating normal matter in the inner crust of the NS.