Old but still warm: Far-UV detection of PSR B0950+08

TL;DR

This paper reports the first far-ultraviolet detection of the old pulsar PSR B0950+08, revealing a higher-than-expected surface temperature and suggesting ongoing internal heating mechanisms.

Contribution

It presents the first FUV detection of PSR B0950+08 and analyzes its spectrum, indicating unexpected thermal emission and possible internal heating processes.

Findings

Detected FUV flux densities in two bands.

Identified a thermal component with higher temperature than cooling models predict.

Suggested vortex creep heating as a plausible internal heating mechanism.

Abstract

We report on a Hubble Space Telescope detection of the nearby, old pulsar B0950+08 ($d\simeq 262$ pc, spin-down age 17.5 Myr) in two far-ultraviolet (FUV) bands. We measured the mean flux densities $\bar{f}_\nu = 109\pm 6$ nJy and $83\pm 14$ nJy in the F125LP and F140LP filters (pivot wavelengths 1438 and 1528 \AA). 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 ($f_\nu\propto \nu^\alpha$) with slope $\alpha\sim -1.2$ and a thermal spectrum emitted from the bulk of the neutron star surface with a temperature in the range of $(1-3)\times 10^5$ K, depending on interstellar extinction and neutron star radius. These temperatures are much higher than predicted by neutron star cooling models for such an old pulsar, which means that some heating mechanisms operate in neutron stars. 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 neutron star crust (vortex creep heating).