The White Dwarf in EM Cygni: Beyond The Veil

TL;DR

This study analyzes FUSE spectra of EM Cygni, revealing a hotter white dwarf than previously thought, with phase-dependent flux variations indicating partial obscuration by stream material.

Contribution

It provides the first detailed spectral analysis of EM Cygni's white dwarf using FUSE data, showing higher temperatures and elemental abundances than earlier estimates.

Findings

White dwarf temperature estimated at 40,000K to 50,000K.

White dwarf contributes over 90% of FUV flux.

White dwarf shows suprasolar silicon, sulphur, and possibly nitrogen abundances.

Abstract

We present a spectral analysis of the FUSE spectra of EM Cygni, a Z Cam DN system. The FUSE spectrum, obtained in quiescence, consists of 4 individual exposures (orbits): two exposures, at orbital phases phi ~ 0.65 and phi ~ 0.90, have a lower flux; and two exposures, at orbital phases phi =0.15 and 0.45, have a relatively higher flux. The change of flux level as a function of the orbital phase is consistent with the stream material (flowing over and below the disk from the hot spot region to smaller radii) partially masking the white dwarf. We carry out a spectral analysis of the FUSE data, obtained at phase 0.45 (when the flux is maximual, using the codes TLUSTY and SYNSPEC. Using a single white dwarf spectral component, we obtain a white dwarf temperature of 40,000K, rotating at 100km/s. The white dwarf, or conceivably, the material overflowing the disk rim, shows suprasolar abundances of silicon, sulphur and possibly nitrogen. Using a white dwarf+disk composite model, we obtain that the white dwarf temperature could be even as high as 50,000K, contributing more than 90% of the FUV flux, and the disk contributing less than 10% must have a mass accretion rate reaching 1.E-10 Msun/yr.In both cases, however, we obtain that the white dwarf temperature is much higher than previously estimated.