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Astron. Astrophys. 350, 517-528 (1999)

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6. The sideband modulations

The beat [FORMULA] modulation, although weak among the detected sidebands, shows an anti-phased behaviour between line fluxes and continuum (Fig. 4a). The UV line fluxes, with average fractional amplitudes of [FORMULA], show a minimum when the UV continuum is maximum. Their maximum shows a dip-like feature centered on the UV continumm minimum. Colour effects are also encountered, the far-UV pulses being stronger ([FORMULA]) than the near-UV ([FORMULA]) ones and lagging by [FORMULA] 0.2 in phase the near-UV. The trend of a decrease in the amplitudes at increasing wavelengths is confirmed in the optical where an upper limit of [FORMULA] 2[FORMULA] can be set to the fractional amplitudes.

[FIGURE] Fig. 4. a  The beat [FORMULA] modulation in the far-UV, near-UV continua and line fluxes of C IV and He II. b  The [FORMULA] and c  the [FORMULA] light curves in three UV bands and zero order light. Fluxes are fractional. d  From bottom to top, the pulsation spectra at the beat [FORMULA], [FORMULA] and [FORMULA] frequencies (solid line) together with broad band UV fluxes derived from the multi-frequency fits (filled circles).

A similar colour effect is also observed in the [FORMULA] sideband pulsation with fractional amplitudes ranging from [FORMULA] in the far-UV to [FORMULA] in the near-UV, displaying a broadening of the maximum towards longer wavelengths (Fig. 4b).

The strong [FORMULA] pulsation only shows a wavelength dependence in the fractional amplitudes, being in the far-UV [FORMULA] and decreasing to [FORMULA] in the near-UV (Fig. 4c) and to [FORMULA] (upper limit) in the optical.

The modulated spectra at the three frequencies have been derived following the same procedure as for the rotational pulsed spectrum. The anti-phased behaviour of the emission lines is seen in the beat pulsed spectrum, where these are seen as absorption features, except for residuals in the C IV and He II lines (Fig 4d, bottom panel). The modulation spectra at [FORMULA] and [FORMULA] frequencies show weak emissions at Si IV, C IV and He II indicating a marginal variability at these frequencies. The UV continuum energy distributions of these variabilities are best represented by power laws [FORMULA] with spectral index [FORMULA], rather than blackbodies (20 000-25 000 K), possibly suggesting that more than one component is acting. Given the low level of confidence of the sideband variabilities it is not possible to derive further information.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999