Analyzing the Low State of EF Eridani with Hubble Space Telescope Ultraviolet Spectra

TL;DR

This study uses Hubble UV spectra to analyze the low accretion state of EF Eri, revealing complex magnetic and thermal features of its white dwarf and hot spot, with implications for magnetic field structures in polars.

Contribution

First detailed UV spectral analysis of EF Eri during low state, exploring models of white dwarf temperature, hot spots, and cyclotron emission to understand its magnetic and accretion properties.

Findings

White dwarf temperature around 10,000K with a hot spot at 15,000K fits some spectral features.

A 9500K white dwarf with a 100 MG cyclotron component explains some flux variations.

Evidence suggests complex magnetic field structures in EF Eri, with higher fields than optical/IR fits indicate.

Abstract

Time-resolved spectra throughout the orbit of EF Eri during its low accretion state were obtained with the Solar Blind Channel on the Advanced Camera for Surveys onboard the Hubble Space Telescope. The overall spectral distribution exhibits peaks at 1500 and 1700A, while the UV light curves display a quasi-sinusoidal modulation over the binary orbit. Models of white dwarfs with a hot spot and cyclotron emission were attempted to fit the spectral variations throughout the orbit. A non-magnetic white dwarf with a temperature of ~10,000K and a hot spot with central temperature of 15,000K generally matches the broad absorptions at 1400 and 1600A with those expected for the quasimolecular H features H2 and H+2 . However, the flux in the core of the Lyalpha absorption does not go to zero, implying an additional component, and the flux variations throughout the orbit are not well matched at long wavelengths. Alternatively, a 9500K white dwarf with a 100 MG cyclotron component can fit the lowest (phase 0.0) fluxes, but the highest fluxes (phase 0.5) require an additional source of magnetic field or temperature. The 100 MG field required for the UV fit is much higher than that which fits the optical/IR wavelengths, which would support previous suggestions of a complex field structure in polars.