On the optical variability of a be star, HDE 245770, — the optical counterpart of the transient X-ray pulsar A0535+26

Astronomy and Astrophysics – Astronomy

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X-Ray Binary Systems, Photoelectric Photometry, Optical Variability

Scientific paper

Photoelectric WBVR observations of Be star HDE 245770=V 725 Tau, the optical counterpart of the transient X-ray pulsar A0535+26, having a pulse period of about 104 s, were conducted for more than 10 years. An irregular long-term optical variability of the star with amplitudes of the order of a few tenths of magnitude was found to be a usual phenomenon. In some cases rapid changes of the star's optical luminosity with a characteristic period of a few tens of minutes or a few hours, and an amplitude of several hundredths of magnitude in all the spectral bands used, which have practically coincided or correlated with the X-ray pulsar outbursts detected by X-ray satellites, were observed. Photoelectric recording of the optical flux from HDE 245770 were made in 1981 1982 with a time resolution of 1 second and 10 s, respectively, in theR spectral band (λ0 ≃ 7000 Å) and in the narrowH α-emission-line band (Δλ1/2 ≃ 75 Å) using a 48-cm reflector of High-Mountain Tien-Shan observatory of the Sternberg Astronomical Institute near Alma-Ata. An analysis of autocorrelation functions of the flux changes from object under study and a comparison with the star BD+26° 876 indicated the variability of luminosity of V 725 Tau in theR spectral band on a time scale of a few tens of second; this variability resembles shot noise with a characteristic time of stochastic bursts of about 15 20 s and their amplitudes of about a few tenths of a percent. InH α-emission-line radiation autocorrelation functions and power spectra show quasiperiodic variability of luminosity of HDE 245770 with a characteristic period of about 100 150 s and an amplitude in the neighbour-hood of 0.5%. The latter result is not quite reliable because of not quite fine weather conditions during the observations; independent observations and check-up are required.

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