Orbital period of CI Cam (XTE J0421+560).

Barsukova, E.A., Borisov, N.V., Burenkov, A.N., Klochkova, V.G. (Special Astrophysical Observatory, Russian Academy of Sciences);
Goranskij, V.P., and Metlova, N.V. (Sternberg Astronomical Institute, Moscow University)

Short report is published in Astronomer's Telegram No.416 (2005). Detailes.

CI Cam is known to be a B[e] star and X-ray transient source. Its powerful X-ray, radio and optical outburst occured in April, 1998. Photometric monitoring is carried out by us with an electrophotometer and CCDs in the UBVR system since the 1998 outburst at the Crimean station of SAI (Moscow University), and SAO (Russian Academy of Sciences). Spectroscopic observations continue routinely with the 1-m Zeiss reflector of SAO RAN equipped with UAGS spectrograph, some additional spectra were taken with the 6-m BTA reflector using UAGS spectrograph. Spectral resolution is in the range of 4 and 8A. We used also two spectra of CI Cam with resolution of 0.23 and 0.08A for our analysis, both taken with BTA using LYNX, and NES echelle spectrographs. A total number of nightly averaged spectra is 39.

Figure 1. The light curve of CI Cam in residuals relative to the mean level of the slow variability (top). The radial velocity curve of the HeII emission (middle). The velocity curve of the nearby close HeI doublet (bottom). The curves were build versus phase of 19.4 day period.

To probe the low-amplitude light variability, we have taken dense 28 day long sets of photoelectric and CCD data of CI Cam in November and December 2002. New observations did not confirm small amplitude 11.7 day variations first noted by A.S.Miroshnichenko AsAp. Trans. V.6, 251 (1995). But these observations demonstrate a wave with the longer period, 19.4 days. On the base of our photometry taken in the time range 1998-2005 in quiescence (172 nights, average values), one may improve its value to 19.41+/-0.02 days. The V light curve is shown in Figure 1 (top). The procedures of reduction these ununiform data to a single system and elimination of slow trend were performed to present this curve. Periodic composition of the light curve has amplitude of 0m.032. The ephemeris is the following

Max =JD hel. 2452200.75 +19d.407*E.

The majority of lines in the spectrum of CI Cam do not show noticeable variability of radial velocity in the quiet state. FeII emissions forming in the stellar wind have rectangular profile of 1.2A width, and centroid radial velocity -51 km/s. This velocity does not vary in our spectra. However we noted that very faint HeII 4686A line (EW~0.3A) changes its position relative to nearby lines by more than 400 km/s (see Fig.2). We measured radial velocities and found that these changes depend on the phase of 19.4 day period (Fig. 1, middle). Sometimes the line looks very faint, and vanishes in the noise of spectrum. So, the quality of its profile was estimated visually in tenfold scale. If the profile is bell-shaped and strong enough, the quality was 10. On the contrary, if it looks like a weak fluctuation on the noise level, the value was 1. Non symmetric or noise distorted profiles estimated by intermediate values depending on degree of distortion. The radii of black circles in the Figure 1 (middle) agree with these estimates, and depend on the quality of the spectrum. Open circles mark the observations made during the outburst. Let us note that the intensity of HeII emission increased in the peak of the outburst by 300 times.

Figure 2. Comparison of two CI Cam spectra in the region of HeII line. The maximum displacement of HeII line profile due to Doppler effect is seen. Two FeII lines are marked to be a reference points to improve the accuracy of velocities, and HeI doublet is chosen to check the accuracy.

The radial velocity curve of HeII in CI Cam has a saw-tooth shape that may be an evidence of an elliptical orbit. Half-amplitude Kx~230 km/s. The best-fit model drawn in the Figure 1 (middle) implies the orbit eccentricity close to 0.62. The orbit major semiaxis a*sin( i ) ~ 42 million km. At the inferior conjunction, the source of emission is located near periastron, and light maximum occurs later with the shift of 0.06 P. The moment of inferior conjunction may be dated by JD 2452199.6+/-0.2, although the observations are absent in this phase. Based on our spectra, the primary companion of CI Cam system is a B4 III-V star with the expected value of mass being no more than 7 solar masses, and of radius being no more than 6 solar radii. Mass function of HeII emission source is 12 solar masses. This value is a lower limit of the primary star mass, and this is almost twice more than expected one for normal star of such type. The radial velocity semi-amplitude of this star is imperceptibly small (less than 10 km/s). Then the mass of the HeII emission source star in this limiting case may be less than 0.5 solar masses. Very likely, the fast moving companion is a white dwarf, and the emission source is an accretion disk around it.

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