Phase-resolved IUE and Optical Observations of the Polar BY Camelopardalis

Abstract

Time-resolved lUE observations of the asynchronous polar BY Cam (H0538 +608) over a period of 11 days show that the N V 1240 Å emission-line velocity varies on the orbital period, with an amplitude of 368±27 km s<SUP>-1</SUP>. The N V flux also varies on the orbital period, with maximum flux corresponding to maximum blue-shift. This behavior is seen in He II 1640 Å as well, although with a smaller velocity amplitude (244±26 km s<SUP>-1</SUP>). Variation on the orbital (rather than the white dwarf spin) period implies that the emission must originate outside the magnetosphere of the white dwarf, in the accretion stream, the heated face of the secondary, or some combination of the two. The rise, peak, and fall in line emission take place over ∼0.3 of an orbit, constraining models of the system geometry; we discuss a number of potential explanations for the observed phenomena, including optically thick (in the lines) emission from the accretion stream, occultation of the emission region by the secondary, and combinations of the two processes. We also propose an alternative scenario, in which X-ray illumination from the environment of the white dwarf gives rise to anisotropic line emission from the accretion stream. Ultraviolet continuum intensities on different nights appeared to be in either low or high flux states; we obtained three nights of optical photometry during the same time period, and the optical light curves corresponding to the high and low flux states were distinct, supporting the idea that the pattern of accretion flow changes. Finally, we note that spectroscopic attempts to observe Na I λλ8183, 8194 in the photosphere of the secondary were not successful.