Astron. Astrophys. 322, 576-590 (1997)

3. IUE observations

IUE observations were mostly obtained as a Target-of-Opportunity (TOO) programme in coordination with the ROSAT observations. The last observations were made within the approved programme SI047 for the 19th IUE observing episode. Table 3 summarizes the IUE low resolution observations made in the period June 1994 - September 1995.

Table 3. Journal of AG Dra low resolution IUE observations

In general, priority was given to the low resolution spectra for line and continuum fluxes. High resolution spectra were also taken at all dates, but they will not be discussed in this paper. For most of the low resolution images, both IUE apertures were used in order to have in the Large Aperture the continuum and the emission lines well exposed and in the Small Aperture (not photometric) the strongest emission lines (essentially HeII 1640 Å) not saturated. The fluxes from the Small Aperture spectra were corrected for the smaller aperture transmission by comparison with the non-saturated parts of the Large Aperture data. Some representative IUE spectra are shown in Fig. 1.

Fig. 1. IUE spectra of AG Dra at three different dates during the 1994-1995 activity phase: during the 1994 outburst (top), during the "quiescent" state between the outbursts (middle) and during the second outburst (bottom). The strongest line in the three spectra is HeII 1640 Å. The lines marked in the spectra are: NV 1240 Å, the blend SiIV + OIV] 1400 Å, NIV] 1486 Å, CIV 1550 Å, OIII] 1663 Å, HeII 2733 Å, MgII 2800 Å, OIII 3133 Å and HeII 3202 Å.

The flux of the strongest emission lines and of the continuum flux at 1340 and 2860 Å is given in Table 4. All the fluxes are corrected for an interstellar absorption of E(B-V)=0.06 (Viotti et al. 1983). The feature at 1400 Å is a blend, unresolved at low resolution of the SiIV doublet at 1393.73 and 1402.73 Å, the OIV] multiplet (1399.77, 1401.16, 1404.81 and 1407.39 Å), and SIV] 1402.73 Å. OIV] 1401.16 Å is the dominant contributor to the blend. Because of the variable intensity of the lines, at low resolution the 1400 Å feature shows extended wings with variable extension and strength (Fig. 1). The two continuum regions were chosen, as in previous works, for being less affected by the emission lines, and for being the 1340 Å representative of the continuum of the hot source (nearly the Rayleigh-Jeans tail of a 10⁵ K blackbody), and the 2860 Å region representative of the HII Balmer continuum emission (see for instance Fernández-Castro et al. 1995).

Table 4. Ultraviolet of AG Dra in outburst. Typical quiescent fluxes are given in the first line (Viotti et al. 1984, Meier et al. 1994).

The behaviour of the continuum and line intensities is shown in Fig. 2. In July 1994 the UV continuum flux increase was larger than that of the HeII 1640 Å line, suggesting a decrease of the Zanstra temperature at the time of the outburst, and a recovering of the high temperature in September 1994. In addition, the NV/continuum (1340 Å) and the HeII/continuum (1340 Å) ratios reached a maximum at the time of the second outburst (July 1995). The HeII Zanstra temperatures are given in Fig. 3.

Fig. 2. Evolution of the UV continuum and emission lines of AG Dra during the 1994-1995 outburst. Units of continuum fluxes are 10-14 erg cm-2 s-1 Å-1, and of line intensities 10-12 erg cm-2 s-1. All the measurements are corrected for an interstellar reddening of E(B-V)=0.06.

Fig. 3. The X-ray and optical light curve of AG Dra over the past 5 years. The two top panels show the U and B band variations as observed at Sonneberg Observatory (filled lozenges = photoelectric photometry, open lozenges = photographic sky patrol), Skalnate Pleso Observatory (Hric et al. 1994; filled triangles), and italian amateurs (Montagni et al. 1996; stars). The large middle panel shows the X-ray intensity as measured with the ROSAT satellite: filled squares denote PSPC observations, filled triangles are HRI observations with the countrate transformed to PSPC rates (see text), and the open triangle is the Boron filter observation corrected for the filter transmission. Statistical errors (1 ) are overplotted; those of the PSPC pointings are smaller than the symbol size. The vertical bars at the top indicate the minima of the U band lightcurve (Skopal 1994). The dotted line shows the fit of an expanding and cooling envelope to the X-ray decay light curve. The lower panel shows temperature estimates from blackbody fits to the ROSAT X-ray data (filled circles) and from the HeII ( 1640) flux to continuum flux at 1340 Å as determined from the IUE spectra and assuming E(B-V)=0.06 (=Zanstra temperatures, filled lozenges).

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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