The nature of the rapid variability of CI Cam (XTE J0421+560)

E.A. Barsukova (SAO, Russia), V.P. Goranskij (SAI, Moscow Univ.)

Short report was published in Astronomer's Telegram #1381.

CI Cam is an X-ray transient and a multiple system which consists of a B4 III-V star displaying B[e] phenomenon, a compact object, probably a white dwarf on an eccentric orbit with the period of 19.407 day (ATEL #416), and a third massive companion of the unknown nature which causes the slow shift of the wind lines of B[e] star (ATEL #1036). As reported elsewhere, CI Cam showed different types of optical variability: large optical outburst accompanied the X-ray event in 1998 April, long-term changes in the scales of a few years, variation with a period of 372+/-15 day (ATel #820), 19.4 day orbital periodic changes, mini-outbursts and dips, and irregular intranight microvariability. Total amplitude of brightness variations in the quiet state is equal to 0.30 mag in the V band.

We found that the nature of the intranight variability of CI Cam is the double-mode pulsation of its B4 type companion. We analyzed a set of V-band photometry taken using the SAI Crimean Station 50-cm Maksutov telescope and Meade Pictor-416 CCD during 18 nights in 2006 December. The set includes 2366 observations. The duration of night monitoring reached 13.2 hours in some nights, and the accuracy of measurements was between 0.003 and 0.002 mag.

CI Cam showed the rapid variability in the time scales of hours with the maximum amplitude of 0.07 mag. Method of analysis was discrete Fourier transform and decomposition into periodic components including prewhitening procedure. The set was cleaned for the low-frequency noise with f<=0.2 c/d.

In the light curve, two waves predominate with periods of 0.4152 and 0.2667 day with the full amplitudes of 0.019 and 0.017 mag, correspondingly. The ratio of periods is close to 3:2. There is a third wave with the period of 0.5216 day (Ampl.=0.008) which is approximately double relative to the second period. Additionally, other lower amplitude waves can be extracted from the light curve. Using 651 observations of CI Cam taken with the same CCD and telescope before December 2006 we established the coherence of dominant pulsations in the range of one and half year. Owing to resonant interaction of these waves, sometimes the pulsations are seen as mini-flares with the amplitude up to 0.07 mag. Pulsations of CI Cam resemble those of Be stars (Astron. & Astrophys. V.476, 927, 2007), but they are observed in a B[e] star for the first time. This result may be asteroseismologic clue to study an internal structure of stars with B[e] phenomenon.

Figure 1. Fragment of the light curve and model. Black points show observational data, solid line is a model which presents a sum of seven periodical component waves.