Sudden strengthening of He II emission line in the spectrum of B[e] star CI Cam

ATel #14362; E. A. Barsukova, A. N. Burenkov (Special Astrophysical Observatory, Russia), V. P. Goranskij (SAI, Moscow University)
on 31 Jan 2021; 19:43 UT
Credential Certification: Barsukova E.A. (bars@sao.ru)

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CI Cam is an optical counterpart of the X-ray transient XTE J0421+560 flared up in 1998 in all wavelength ranges. In the quiet state, its spectrum shows weak emission line He II 4686 A. This is a single line in the optical spectrum with large radial velocity variations. The amplitude of variations is of about 400 km/s. The source of this radiation - the secondary companion - moves in an elliptical orbit with the period of 19.407 day and the eccentricity of 0.49. In the course of the spectral monitoring of CI Cam on 2021 January 25.675 UT, we found its He II 4686 A emission line being unusually strong and wide. The spectrum was taken with the SAO 1 meter Zeiss telescope and UAGS spectrograph. The exposure was 2400 s long, the spectral range 3850-5260 A, the resolution 2.4 A. Parameters of the line were the following: EW = -1.9 A, FWHM = 390 km/s, and FWZI = 700 km/s (corrected for spectral resolution), heliocentric radial velocity -254 km/s. An hour later, we have taken a noisy spectrum through clouds. The emission had EW ~ -1.3 and the same radial velocity. Other emission lines have not shown significant variations. So far, we have analyzed 270 blue spectra of CI Cam obtained in the time range between December 1998 and December 2020. We found a rapid variability of the equivalent width of He II line on a time scale of tens of minutes in the full range between zero and -1.15 A. The amplitude of variability was progressively increasing for 20 years. However, the new observed intensity of the line is the strongest after the 1998 outburst. The equivalent widths of He II line also depend on the orbital phase. The maximum widths appear when the component passes the descending node of the orbit, while the smaller ones are associated with the apastron of the orbit. This fact suggests that the source of He II line radiation is extended. Really, in the descending node, the velocity of the source relative to surrounding circumstellar gas is more than 200 km/s, and the heated surface of the source colliding with this gas is located face-on to the observer. Our December 2020 photometry of CI Cam also confirms pulsations of the B star with the 0.406 day period and the amplitude of 0.04 mag in the V band. The rapid variability of He II line equivalent width may be explained by the interaction of the extended surface of the component with the pulsation waves in the envelope of the B star.

He II equivalent widths variations 1998-2021