Extensive photometry of the intermediate polar V1033 Cas (IGR J00234+6141)

Abstract

To measure the spin period of the white dwarf in V1033 Cas with high precision, we performed extensive photometry. Observations were obtained over 34 nights in 2017. The total duration of the observations was 143 h. We found that the spin period of the white dwarf is equal to $563.116\,33\pm 0.000\,10~\text{s}$. Using this period, we derived the oscillation ephemeris with a long validity of 100 years. The spin oscillation semi-amplitude was stable and was equal to $95.5\pm 1.3~\text{mmag}$. This is a very large semi-amplitude of the spin oscillation among intermediate polars, which have similar and lesser spin periods. This large semi-amplitude suggests that the system is noticeably inclined. The spin pulse profile was sinusoidal with high accuracy. This may mean that the spin oscillation is produced by a single accretion curtain whereas the second accretion curtain may be obscured by the accretion disc. Despite the large amount of our observations, we did not detect sidebands. The semi-amplitudes of the undetected sideband oscillations do not exceed 10 mmag. The absence of sideband oscillations seems puzzling. We detected the orbital variability of V1033 Cas with a period of $4.0243\pm 0.0028~\text{h}$ and with a semi-amplitude of $55\pm 4~\text{mmag}$. The orbital variability semi-amplitude seems large and also suggests that the system is noticeably inclined. Using our oscillation ephemeris and the times of spin pulse maximum obtained in the past, we found that the spin period is very stable. $\mathrm{d}P/{\mathrm{d}}t$ is most probably less than $-4\times 10^{-12}$. This contradicts the assumption that the white dwarf in V1033 Cas is not spinning at equilibrium. Our spin period and our oscillation ephemeris can be used for further investigations of the stability of the spin period in V1033 Cas.