New Optical/UV Counterparts and the Spectral Energy Distributions of Nearby, Thermally Emitting, Isolated Neutron Stars

TL;DR

This study presents optical and ultraviolet observations of nearby thermally emitting neutron stars, revealing significant optical excesses inconsistent with simple thermal models, and discusses potential explanations for these unexpected fluxes.

Contribution

First comprehensive optical/UV spectral energy distributions for all seven known nearby thermally emitting neutron stars, highlighting unexpected flux excesses and challenging existing emission models.

Findings

Most neutron stars show optical/UV fluxes exceeding thermal predictions.

Optical excesses range from 5 to over 50 times the X-ray extrapolation.

Standard explanations like atmospheric effects or magnetospheric emission are insufficient.

Abstract

We present Hubble Space Telescope optical and ultraviolet photometry for five nearby, thermally emitting neutron stars. With these measurements, all seven such objects have confirmed optical and ultraviolet counterparts. Combining our data with archival space-based photometry, we present spectral energy distributions for all sources and measure the "optical excess": the factor by which the measured photometry exceeds that extrapolated from X-ray spectra. We find that the majority have optical and ultraviolet fluxes that are inconsistent with that expected from thermal (Rayleigh-Jeans) emission, exhibiting more flux at longer wavelengths. We also find that most objects have optical excesses between 5 and 12, but that one object (RX J2143.0+0654) exceeds the X-ray extrapolation by a factor of more than 50 at 5000 A, and that this is robust to uncertainties in the X-ray spectra and absorption. We consider explanations for this ranging from atmospheric effects, magnetospheric emission, and resonant scattering, but find that none is satisfactory.