Ultraviolet spectroscopy of the dwarf nova U Geminorum

Froning

Knigge

et al. 2005

ApJ

We present time-resolved FUV spectra of the dwarf novae SS Cyg and WX Hyi in quiescence from observations using the Hopkins Ultraviolet Telescope on the Astro-1 and Astro-2 space shuttle missions and the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Both dwarf novae are characterized by blue continua that extend to the Lyman limit punctuated by broad emission lines including transitions of O vi, N v, Si iv, and C iv. The continuum of WX Hyi can be fitted with a white dwarf model with physically reasonable model parameters, but neither system actually shows unambiguous signatures of white dwarf emission. The shape and flux of the spectrum of SS Cyg cannot be self-consistently reconciled with a white dwarf providing all of the FUV continuum flux. Combination white dwarf /disk or white dwarf /optically thin plasma models improve the fit but still do not give physically reasonable model parameters for a quiescent dwarf nova. Assuming that the UV emission lines arise from the disk, the line shapes indicate that surface fluxes fall roughly as R À2 in both systems. Fits to the double-peaked line profiles in SS Cyg indicate that the FUV line-forming region is concentrated closer to the white dwarf than that of the optical lines and provide no evidence of a hole in the inner disk. Although the flux from SS Cyg was relatively constant during all of our observations, WX Hyi showed significant variability during the GHRS observations. In WX Hyi, the line and continuum fluxes are (with the exception of He ii) highly correlated, indicating a link between the formation mechanisms of the line and continuum regions.

Section: Introductionmentioning

confidence: 59%

Far‐Ultraviolet Spectroscopy of the Dwarf Novae SS Cygni and WX Hydri in Quiescence

Froning

Knigge

et al. 2005

ApJ

“…Model fits to the Astro-1 observations implied an average WD temperature of ~ 38 000 K and near solar abundances in the WD atmosphere. This was higher than the 30 000 K estimated from IUE spectra far from outburst (Panek & Holm 1984) and use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0252921100038756 Wavelength(A) Figure 2.…”

Section: Dwarf Novae In Quiescencementioning

confidence: 56%

“…WDs have broad Lyman absorption lines as do HUT spectra of U Gem and VW Hyi. The UV flux from U Gem and VW Hyi is also about that expected from a WD at the estimated distances of U Gem and VW Hyi, given estimates of the temperature based on the widths of Lya, as indeed IUE observations had indicated (Panek & Holm 1984, Mateo k Szkody 1984. Because the strong gravitational field causes metals to settle into the atmosphere on a time-scale of 1... 3 yr (cf.…”

Section: Dwarf Novae In Quiescencementioning

confidence: 60%

Far Ultraviolet Observations of Dwarf Novae made with the Hopkins Ultraviolet Telescope

International Astronomical Union Colloquium

1996

Abstract.Observations with the Hopkins Ultraviolet Telescope on the Astro-1 and Astro-2 space shuttle missions have provided the first set of moderate (3 A) resolution far ultraviolet (FUV) spectra of dwarf novae to include the wavelength range between Lya and the Lyman limit. Important lines which are detected in the HUT spectra of dwarf novae in this wavelength range include S vi AA933,945, C in A977, O vi AA1032,1038, P v AA1118,1128 and C m A1176, as well as the higher order Lyman lines. The observations confirm earlier IUE observations that two dwarf novae -U Gem and VW Hyi -have quiescent FUV spectra dominated by the white dwarf, but suggest that the quiescent FUV emission from three other dwarf novae -SS Cyg, WX Hyi and YZ Cnc -are dominated by emission from a hot portion of the disk or a disk corona. The spectra obtained of the dwarf novae Z Cam and EM Cyg in outburst and also of the nova-like variable IX Vel can be modeled reasonably successfully in terms of steady state disks constructed by adding appropriately-weighted stellar model spectra.

“…In the far UV, the quiescent spectrum is dominated by the WD in the system (Panek & Holm 1984). The average temperature of the WD surface drops from ~ 38,000 K shortly after outburst to ~ 30,000 K far from outburst (Long et al 1993;.…”

Section: U Geminorummentioning

confidence: 99%

“…The ~ I M Q WD is rotating very slowly, if at all (vsin(i) < 100km s-1 ; Sion et al 1994). In outburst, the far UV flux rises by a factor of 100 and is dominated by the optically-thick accretion disk (Panek & Holm 1984 FIGURE 2. The fluxed spectrum of U Gem as observed with the SW spectrometer during the second observing interval.…”

Section: U Geminorummentioning

confidence: 99%

The EUVE Observations of Dwarf Novae

International Astronomical Union Colloquium

1996

In the standard theory of dwarf novae in outburst, the boundary layer region between the inner edge of the accretion disk and the white dwarf surface radiates primarily in the extreme ultraviolet. Using EUVE, observers have been able to obtain spectra with sufficient spectral resolution to characterize accurately the emission from several dwarf novae in outburst, including U Getninorum and SS Cygni. I present an overview of the observations and early analyses of the dwarf nova observations. The spectra obtained of dwarf novae are complex compared to the EUV spectra of magnetic cataclysmic variables and single white dwarfs. Detailed spectral modeling of an expanding atmosphere will most like be required to fully understand the spectra. Nevertheless, we already know there were significant differences in the effective temperatures and other properties of the EUV emissions. If we assume the EUV emission arises primarily from the boundary layer and parameterize the EUV spectrum in terms of a blackbody, then for U Gem the derived boundary luminosity is comparable to the disk luminosity, consistent with the standard theory, and the minimum size of the emitting region is about that of the white dwarf surface. The count rates from U Gem were modulated strongly with the orbital period; differences in the shape of the spectrum in eclipse and out of eclipse suggest that while the bulk of the emission arose from the vicinity of the white dwarf, there was an extended source of emission as well. For SS Cyg, however, the derived boundary layer luminosity was a small fraction of the disk luminosity. In U Gem, the effective temperature dropped during the decline from outburst. In contrast, in SS Cyg, the effective temperature remained constant as the count rate rose by a factor of 100 and the effective size increased. Thus while the observations of U Gem seem broadly consistent with the standard theory for the boundary layer emission from dwarf novae, SS Cyg appears to present fundamental challenges to that theory.