[Paper Review] EUVE Observations of Nonmagnetic Cataclysmic Variables

Christopher W. Mauche|arXiv (Cornell University)|Sep 7, 2001

Solar and Space Plasma Dynamics2 references3 citations

TL;DR

This paper summarizes EUVE observations of high-accretion-rate nonmagnetic cataclysmic variables, focusing on SS Cyg, U Gem, VW Hyi, and OY Car in outburst. It demonstrates that the boundary layer emission is primarily radiated in the EUV and is observable only through scattered radiation from an extended accretion disk wind, with models indicating a boundary layer temperature of 8–11 eV and a mass-accretion rate ≤10⁻⁸ M☉ yr⁻¹.

ABSTRACT

We summarize EUVE's contribution to the study of the boundary layer emission of high accretion-rate nonmagnetic cataclysmic variables, especially the dwarf novae SS Cyg, U Gem, VW Hyi, and OY Car in outburst. We discuss the optical and EUV light curves of dwarf nova outbursts, the quasi-coherent oscillations of the EUV flux of SS Cyg, the EUV spectra of dwarf novae, and the future of EUV observations of cataclysmic variables.

Motivation & Objective

• To investigate the nature of boundary layer emission in high-accretion-rate nonmagnetic cataclysmic variables using EUVE observations.
• To understand the origin of EUV flux in dwarf novae, particularly the role of scattering in extended accretion disk winds.
• To overcome observational challenges posed by interstellar absorption and the need for target-of-opportunity scheduling.
• To provide the first detailed follow-up of full outburst evolution in multiple dwarf novae using deep EUV photometry and spectroscopy.
• To assess the feasibility and future prospects of EUV observations for studying accretion physics in nearby CVs.

Proposed method

• Utilized EUVE's deep survey photometer (DS) and short-wavelength spectrometer (SW) to obtain long-duration, high-sensitivity EUV light curves and spectra of outbursting CVs.
• Combined EUVE data with optical and UV observations from AAVSO, VSS/RASNZ, HST, Voyager, Chandra, RXTE, and EXOSAT for multiwavelength context.
• Applied binary-phase spectroscopy to detect persistent emission lines during eclipses, indicating extended emission regions beyond the continuum source.
• Constructed detailed EUV light curves from DS and SW data, with integration times up to 274 ks, enabling full outburst monitoring.
• Developed radiative transfer models of scattering in disk winds to interpret the observed line profiles and flux behavior.
• Used the relation $ L_{\rm bl} \approx L_{\rm disk} \approx 3\times10^{34} (\dot{M}/10^{-8}~{}\rm{M_\odot~{}yr^{-1}})~{}\rm{erg~{}s^{-1}} $ to estimate accretion rates from luminosities.

Experimental results

Research questions

• RQ1 What is the origin of the observed EUV flux in high-accretion-rate nonmagnetic cataclysmic variables?
• RQ2 Why is the EUV emission in dwarf novae in outburst not directly visible despite being the dominant energy source?
• RQ3 How do the observed emission lines and light curves constrain the geometry and physical conditions of the accretion disk wind?
• RQ4 What is the role of scattering in the disk wind in making the boundary layer emission detectable?
• RQ5 What are the implications of the observed EUV spectra for the mass-accretion rate and wind properties in systems like OY Car and U Gem?

Key findings

• The EUV flux in high-accretion-rate CVs like SS Cyg, U Gem, VW Hyi, and OY Car is primarily due to scattering of boundary layer radiation in an extended accretion disk wind, not direct emission.
• The boundary layer temperature is constrained to $ kT \approx 8 $–11 eV, with luminosity $ L_{\rm bl} \leq 4\times10^{34} (d/85~{}\rm{pc})^2~{}\rm{erg~{}s^{-1}} $, implying $ \dot{M} \leq 10^{-8}~{}\rm{M_\odot~{}yr^{-1}} $.
• Emission lines in the EUV spectra of OY Car (e.g., N V, O V–VI, Ne V–VII, Fe XXIII) are moderately broad with FWHM ≈ 1 Å, indicating a high-velocity wind.
• The wind mass-loss rate is estimated at $ \dot{M}_{\rm wind} \leq 10^{-10}~{}\rm{M_\odot~{}yr^{-1}} $, or about 1% of the accretion rate, requiring magnetic forces to drive it beyond radiation pressure alone.
• The absorbing column density is $ N_{\rm H} \approx 1.6 $–$ 3.5\times10^{19}~{}\rm{cm^{-2}} $, consistent with low interstellar absorption in nearby systems.
• The persistence of emission lines through orbital eclipses (e.g., in U Gem) confirms that the line-forming region is spatially extended beyond the central source, supporting the wind-scattering model.