WZ Sagittae:FUSESpectroscopy of the 2001 Outburst

Abstract

The short-period cataclysmic variable WZ Sge underwent its first dwarf nova outburst in 23 years in 2001 July. Here we describe 905-1187 Å spectra obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) during the outburst and within the first few months after the outburst. The first spectrum, obtained about 7 days after the outburst began and 6.4 days past optical maximum, is dominated by a broad (50-60 Å equivalent width) feature due to O vi and can be characterized in terms of emission from a highly inclined disk modulated by the effects of a highly ionized wind. WZ Sge is fainter in later observations (starting about 45 days after optical maximum) and increasingly '' red '' in the far ultraviolet. The spectral shape increasingly resembles that expected from a metal-enriched white dwarf photosphere, although it seems likely that some disk contribution remains at least through the second FUSE observation, when the visible light curve showed the system to be undergoing a rebrightening event. The absorption lines observed on September 29 and in early November, when the visible light curve was in a fairly steady decline toward quiescence, are narrow, with widths (FWHM) of the order of 0.7-1.3 Å (200-400 km s À1 ). The most natural explanation for these spectra is that the continuum arises from a relatively massive, slowly rotating white dwarf and that the lines are created by a combination of metals in the atmosphere and absorbing material along the line of sight to the white dwarf. If the white dwarf has log g ¼ 8:5, then the white dwarf cooled from $25,000 K on September 29 to 23,000 K in early November.

Far ultraviolet spectroscopy of (non-magnetic) cataclysmic variables

“…Among CVs, WZ Sge has one of the shortest binary periods (88 m), and one of the longest interoutburst periods (20-30 yrs). FUSE and HST observations during the outburst showed a high excitation spectrum dominated by broad O VI, NV, and C IV features, consistent with the high (75 o ) system inclination (Long et al, 2003;Knigge et al, 2002). Following the outburst, the spectrum acquired the characteristics of a WD-dominated system (Fig.…”

Hubble Space TelescopeSTIS Spectroscopy and Modeling of the Long‐Term Cooling of WZ Sagittae following the 2001 July Outburst

“…All the other best fits listed in Table 5 (with a different WD mass) fitted the observed cooling curve as well (except model 6, which we consider in the next subsection). From these simulations, the quiescence WD temperature is %13;500 AE 1000 K, and the average mass accretion rate of the outburst models (a few times 10 À8 M yr À1 ) is larger by about 1 order of magnitude than the value determined from the spectral fits to the observations during the outburst phase (%3 ; 10 À9 M yr À1 ; Long et al 2003). It could be that during the outburst the disk actually is selfabsorbing/self-occulting, and therefore the mass accretion rate derived from the observations during outburst might have been underestimated.…”

“…Long et al (2003) also estimated the temperature from two FUSE observations. The HST STIS derived temperatures were independently reassessed in Godon et al (2004) using methods (a) and (b) and the integrated flux as described in the previous section.…”

Hubble Space TelescopeSpectroscopy of the Unexpected 2001 July Outburst of the Dwarf Nova WZ Sagittae

“…The analysis of the FUV light curves obtained in the time-tag mode will be reported elsewhere. The September observation took place during a rebrightening, as shown in Figure 1 of Long et al (2003), where the placement of the Far Ultraviolet Spectrographic Explorer (FUSE) and HST STIS observations is compared with the optical light curve of the outburst and decline. This rebrightening was one of several closely spaced so-called echo outbursts.…”