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

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7. The orbital variability

The UV and optical orbital modulations have been investigated folding the FOS continuum broad band and emission line fluxes in 28 orbital phase bins. The light curves have been prewhitened by the other active frequencies using the results of the multi-frequency fits. For the IUE data, continuum broad band and emission line flux measures have been performed on each SWP and LWP spectrum. Three broad bands have been selected in each spectral range, five of them coinciding with the FOS selected bands and a sixth one in the range [FORMULA] 2900-2985. The contribution of the spin pulsation, as derived from the multi-frequency fit has been removed. The best fit blackbody spin pulsed spectrum has been used to allow prewhitening in the range [FORMULA]2900-2985. The FOS and IUE broad band continuum fluxes in the far-UV, mid-UV and near-UV as well as the zero order and B band light curves are reported in the left panel of Fig. 5, while the emission line fluxes of Si IV, C IV and He II are shown in the right panel. The orbital gaps due to the HST sampling are apparent.

[FIGURE] Fig. 5. Upper left: Orbital modulation in the far-UV, mid-UV and near-UV, zero order and B light (upper left panel). Upper right: Si IV, C IV and He II flux light curves. Fluxes are fractional as described in the text and the average value has been subtracted. The IUE measures are reported together with their phase coverage. Bottom panel: The orbital broad band UV and optical modulated fluxes represented with the best fit (solid line) composite function as described in the text. The two blackbodies are represented with dotted lines. A hot (21500 K) blackbody function describes the UV FOS spectrum (shown in the inserted figure.)

A strong colour dependence is encountered in the modulation amplitudes as well as the phasing. Fractional amplitudes range from 40[FORMULA] in the far-UV to 28[FORMULA] in the near-UV (IUE band) and 15[FORMULA] in the optical. The modulation amplitudes then have increased by a factor [FORMULA] 2.5 in the UV and [FORMULA] 1.5 in the optical with respect to 1990. However, the phasing of UV maximum and minimum has not changed with time, occurring at [FORMULA] and [FORMULA], respectively. The UV modulation is more sinusoidal, whilst the optical light curve is more structured with a double humped maximum between [FORMULA] = 0.75 and [FORMULA] = 0.0. A comparison with the optical behaviour in 1990 indicates an absence of a broad maximum centered at [FORMULA] = 0.0 and a less defined minimum. The current observations are inadequate to resolve the orbital dip due to a grazing eclipse of the accretion disc in either UV and optical ranges.

The orbital modulation in the UV emission line fluxes is strong with fractional amplitudes of 40[FORMULA] in N V, 20[FORMULA] in Si IV, 29[FORMULA] in C IV and 23[FORMULA] in He II and almost in phase with the UV continuum.

The spectrum of the UV orbital variability derived with the same procedure as described before is shown in the enlargement of Fig. 5 (bottom panel). From the inserted figure the [FORMULA] absorption feature is apparent and is consistent with the neutral hydrogen column density of [FORMULA] cm-2 inferred from the spin pulsed spectrum. Hence, while this absorption in the orbital pulsation spectrum is clearly circumstellar, the same nature in the spin pulsed spectrum cannot be excluded.

The UV FOS spectrum requires a hot component with a blackbody temperature of 21 500[FORMULA]500 K (Fig. 5 bottom panel enlargement). The composite UV and optical broad band energy distrubution confirms the previous results on the presence of two components, a hot at 19 500 [FORMULA] 500 K and a cool one at 5 700 [FORMULA] 200 K ([FORMULA]) (Fig. 5, bottom panel). With current UV observations, it is now possible to constrain the temperature of the hot emitting region. The temperature of the cool component is in agreement, within errors, with that inferred in Paper 1. A substantial increase by a factor of [FORMULA] in the area of the hot region is found when compared to the 1990 epoch, which is [FORMULA]. The emitting area of the cool component is instead similar to that previously derived (Paper 1).

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999