Observation of the Far-ultraviolet Continuum Background with SPEAR/FIMS

TL;DR

This study characterizes the far-ultraviolet continuum background across the sky using SPEAR/FIMS data, revealing correlations with interstellar medium tracers, spectral softness variations, and spatial structures related to stellar distributions.

Contribution

First comprehensive analysis of the FUV continuum background over most of the sky using SPEAR/FIMS data, highlighting correlations and spectral properties.

Findings

Diffuse FUV intensity correlates with N_{HI}, 100 μm, and H-alpha intensities.

The FUV spectrum is relatively flat but softens at high Galactic latitudes.

The FUV intensity distribution follows a log-normal pattern.

Abstract

We present the general properties of the far-ultraviolet (FUV; 1370-1720A) continuum background over most of the sky, obtained with the Spectroscopy of Plasma Evolution from Astrophysical Radiation instrument (SPEAR, also known as FIMS), flown aboard the STSAT-1 satellite mission. We find that the diffuse FUV continuum intensity is well correlated with N_{HI}, 100 $\mu$m, and H-alpha intensities but anti-correlated with soft X-ray. The correlation of the diffuse background with the direct stellar flux is weaker than the correlation with other parameters. The continuum spectra are relatively flat. However, a weak softening of the FUV spectra toward some sight lines, mostly at high Galactic latitudes, is found not only in direct-stellar but also in diffuse background spectra. The diffuse background is relatively softer that the direct stellar spectrum. We also find that the diffuse FUV background averaged over the sky has about the same level as the direct-stellar radiation field in the statistical sense and a bit softer spectrum compared to direct stellar radiation. A map of the ratio of 1400-1510A to 1560-1660A shows that the sky is divided into roughly two parts. However, this map shows a lot of patchy structures on small scales. The spatial variation of the hardness ratio seems to be largely determined by the longitudinal distribution of spectral types of stars in the Galactic plane. A correlation of the hardness ratio with the FUV intensity at high intensities is found but an anti-correlation at low intensities. We also find evidence that the FUV intensity distribution is log-normal in nature.