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

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1. Introduction

Intermediate Polars (IPs) are a subclass of magnetic Cataclysmic Variables which consist of an asynchronously rotating ([FORMULA]) magnetized white dwarf accreting from a late type, main sequence, Roche-lobe filling secondary star (Patterson 1994; Warner 1995).

Except for a few systems for which polarized optical/IR emission is detected, the white dwarf is believed to possess a weak ([FORMULA] 2 MG) magnetic field which dominates the accretion flow only at a few radii from its surface. Within the magnetospheric radius, material is channeled towards the magnetic polar regions in an arc-shaped accretion curtain (Rosen et al. 1988). At larger distances, different accretion patterns can be present: a truncated accretion disc (disc-fed systems), direct accretion from the stream onto the magnetosphere (disc-less systems) as well as a combination of the two, where the stream material overpasses the disc (disc-overflow) (Hellier 1995 and references therein).

Due to the asynchronous rotation, IPs show a wide range of periodicities at the white dwarf spin ([FORMULA]), the orbital ([FORMULA]) and sideband frequencies (Warner 1986; Patterson 1994, Warner 1995), whose amplitudes can be different in different spectral ranges (de Martino 1993). FO Aqr (H2215-086) was known to show strong periodic X-ray, optical and IR pulsations at the spin frequency, [FORMULA], and lower amplitude variations at the orbital [FORMULA] and beat [FORMULA] frequencies (de Martino et al. 1994, hereafter Paper 1, and references therein; Marsh & Duck 1996; Patterson et al. 1998). The dominance of the spin pulsation at optical and high X-ray energies ([FORMULA]5 keV) can be accounted for by a disc-fed accretion, whose evidence was provided by a partial eclipse in the optical continuum and emission lines (Hellier et al. 1989; Mukai et al. 1994; Hellier 1995). FO Aqr was also found to possess a disc-overflow accretion mode (Hellier 1993), and the recent evidence of a long term variability in the amplitudes of the X-ray pulsations has been interpreted as changes in the accretion mode (Beardmore et al. 1998). This kind of variability, only recently recognized, is also observed in other IPs like TX Col (Buckley 1996; Norton et al. 1997) and BG CMi (de Martino et al. 1995).

The identification of the actual accretion geometry and the determination of energy budgets of the primary X-ray and secondary reprocessed UV, optical and IR emissions reside on multi-wavelength observations. The main modulations in FO Aqr have been studied in the X-rays and at optical/IR wavelengths. The optical spin pulsations, occurring mostly in phase with the X-ray ones, were found to arise from the outer regions of the accretion curtain (Paper 1; Welsh & Martell 1996). The orbital modulation from UV to IR was instead found to be multicomponent and attributed to the X-ray heated azimuthal structure of the accretion disc (henceforth bulge) and to the inner illuminated face of the secondary star (Paper 1). However the temperature of the UV emitting bulge could not be constrained due to the lack of a precise quantification of the spectral shape of the UV spin modulation. In this work we present high temporal resolution spectroscopy acquired with HST/FOS which provides the first detection of different UV periodicities in FO Aqr. For a comprehensive study, these data are complemented with low temporal resolution IUE spectra along the orbital period. Coordinated optical photometry extends the study to a wider spectral range, providing the link to investigate the long term behaviour of these variabilities.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999
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