Astron. Astrophys. 324, 109-120 (1997)

Astron. Astrophys. 324, 109-120 (1997)

1. Introduction

Polars (AM Her type systems) are cataclysmic binaries in which a magnetic white dwarf accretes matter from a red dwarf star filling its Roche lobe. The accretion occurs along the magnetic field lines down to the magnetic poles of the white dwarf. The X-rays and the optical polarized flux, both originating close to the surface of the white dwarf are modulated at the orbital period which proves the synchronism of the white dwarf rotation with the orbital motion. A comprehensive review of these objects has been done by Cropper (1990). In the case of BY Cam, the situation concerning the different periods in the system is not clear. The X-ray source H0538+608 (BY Cam) has been identified as a polar by Remillard et al.(1986), on the basis of the detection of a circularly polarized optical flux. This source shows two brightness states (Szkody et al. 1990), such a behaviour being shared by most polars. However this object is atypical by several aspects. In the UV it reveals an abnormal emission line spectrum with an enhanced NV line and a weak CIV line (Bonnet-Bidaud and Mouchet 1987). This could be linked to the chemical composition of a secondary whose outer layers have been lost during the evolution of the system (Mouchet et al. 1991). This is also reminiscent of what is found in some novae, suggesting a possible unnoticed nova-like event (Bonnet-Bidaud and Mouchet 1987). However no nova outburst is recorded in archive plates (Silber et al. 1992 (hereafter SBIOR)). Noteworthily, X-ray spectra of BY Cam obtained with the BBXRT experiment revealed an oxygen absorption edge near 0.6 keV, which intensity either requires an overabundance of oxygen or partially ionized material (Kallman et al. 1993).

Contemporaneous X-ray and optical observations (Ishida et al. 1991, SBIOR) have revealed the presence of two very close periods around 3.3h, suggesting the asynchronism of the system (SBIOR). This was the second polar sharing this peculiarity: the first one being V1500 Cyg, a nova which is expected to become synchronous again in less than 200 years (Schmidt and Stockman 1991, Katz 1991). A third one, RX J1940.1-1025, has been added recently, but contrary to both previous ones, the spin period is slightly longer than the orbital period (Friedrich et al. 1996). From a compilation of previous optical polarimetric and photometric measurements of BY Cam, combined with new observations, Piirola et al. (1994) obtained a new determination of the shortest period (3.3308h) which differs significantly from the original determination (Mason et al. 1989, hereafter MLS). The presence of two close periods in this system led to search for a longer period of the order of fourteen days which would be the beat period. A large set of photometric data collected over a period of 66 days, seem to reveal a period of seven days (Silber 1995), but a beat period of 14 days has also been suggested by Mason et al. (1995a, b).

Recently, time-resolved UV spectroscopy revealed a modulation of the line fluxes and of the radial velocities with the longest period identified as the orbital one (Zucker et al. 1995), implying an origin far from the accretion column, contrary to the common idea that the high ionization UV resonance lines are formed close to the white dwarf. In the optical emission lines are very complex and can show up to four components (MLS).

In this paper we present an analysis of high temporal resolution spectroscopic and photometric optical data obtained at three different dates. Partial results have been presented by Bonnet-Bidaud et al. (1992). The optical emission lines, composed of components arising from different regions, are good tools to constrain the different periods present in this system and to trace the geometry of the accreting flow. The optical results are compared to the UV spectroscopic results. A critical review of the different period determinations is also presented in Sect. 5 and their interpretation is discussed in Sect. 6 in terms of phase-drifts introduced by an asynchronous rotating magnetosphere.

Online publication: May 26, 1998

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