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

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5. The rotational modulation

UV continuum, zero-order light, and UV emission line fluxes as well as their V/R ratios have been folded in 56 phase bins along [FORMULA]. The spin pulses, pre-whitened from all other frequency variations, are shown in Fig. 3 together with the optical B band pulses folded in 28 phase bins. A strong colour effect is observed in both amplitudes and phasing. Fractional amplitudes (amplitudes of the sinusoid [FORMULA] divided by the average value) decrease from 26[FORMULA] in the far-UV to [FORMULA] in the near-UV and to [FORMULA] in the optical. The far-UV pulse maximum is broader and lags by [FORMULA] and 0.186[FORMULA]0.008 the near-UV and optical maxima respectively. While the UV line fluxes follow the near-UV modulation, their V/R ratios are in phase with the far-UV, with a maximum blue shift when the far-UV pulse is at maximum. The pulsation in the line fluxes are 18[FORMULA], 12[FORMULA], and 10[FORMULA] in He II, C IV and Si IV. The V/R ratios are generally [FORMULA]1 indicating the presence of a dominant blue component in the lines, which is also visible from the extended blue wings in the average spectrum. The velocity displacements indicate the presence of a spin S-wave in the line profiles similar to the optical (Hellier et al. 1990). Both continuum and emission lines therefore strongly indicate the presence of two components which affect the rotational modulation in FO Aqr.

[FIGURE] Fig. 3. Upper left: Spin pulses in the far-UV, near-UV, zero order and B band (upper left panel). Upper right: C IV and He II flux and V/R rotational curves. Fluxes are fractional as described in the text, the average value has been subtracted. Bottom panel: Rotational broad band UV and optical energy distribution together with the best fit composite function: a hot (37 500 K) (dotted line) and a cool (12 000 K) (long-dashed line) blackbody function (sum: solid line). A composite function (solid line) consisting of a 36 000 K white dwarf model spectrum and of the same 12 000 K blackbody absorbed by [FORMULA] is shown in the insert figure together with the FOS rotational pulsed spectrum described in the text.

The 797 FOS spectra have been spin-folded into 20 phase bins. A total of 780 light curves, each sampling a wavelength bin of [FORMULA] 1.8 Å , were then produced and fitted with a sinusoid. The resulting amplitudes define the rotational pulsed spectrum as [FORMULA]. This spectrum (shown in the enlargement of the lower panel of Fig. 3), has to be regarded as an upper limit to the modulated flux since no pre-whitening could be performed given the low S/N of each 780 light curves. This spectrum gives evidence of modulation not only in the main emission lines and [FORMULA] absorption but also in the weaker emission features identified in Sect. 3. Broad band UV continuum and optical photometric spin pulsed fluxes, obtained from the multi-frequency fit and reported in Fig. 3, provide a correct description of the rotational pulsed energy distribution. A spectral fit to the broad band UV and optical spectrum, using a composite spectral function, consisting of two blackbodies, gives 37 500[FORMULA]500 K and 12 000[FORMULA]400 K ([FORMULA]). The projected fractional area of the hot component is [FORMULA], while the cool one covers [FORMULA], for [FORMULA], and d=325 pc (Paper 1).

The FOS rotational pulsed spectrum shows the presence of a [FORMULA] absorption feature which gives a hydrogen column density of [FORMULA]. On the other hand, assuming [FORMULA], as derived from the grand average spectrum, a composite function consisting of a white dwarf model atmosphere with at 36 000 K and of the same 12 000 K blackbody gives an equally satisfactory fit to the whole FOS spectrum. For a distance of 325 pc the radius of the white dwarf is 4.9[FORMULA] cm, in agreement with that of the hot blackbody component.

While the detection of the hot component is new, a comparison with previous spectral analysis of the optical and IR spin pulses observed in 1990 shows that the temperature of the cool component has not changed with time but instead it suffered a decrease in area by a factor of [FORMULA] 1.5.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999