Extensive photometry of the intermediate polar MU Cam: detection of a spin period change

Abstract

Intermediate polars with known rates of spin period changes are not numerous because such tasks require measurements performed for a long time. To measure a spin period change, MU Cam is a good candidate because it has a spin oscillation with a large amplitude enabling measurements with high precision. Fortunately, in the past the spin period of MU Cam was measured with high precision. To measure the spin period anew, in 2014–2015 we performed extensive photometric observations of MU Cam, spanning a total duration of 208 h within 46 nights. We found that the spin, sideband and orbital periods are equal to 1187.16245±0.00047s, 1276.3424±0.0022s and 4.71942±0.00016h, respectively. Comparing the measured spin period with the spin period of MU Cam in the past, we detected the spin period change with d P/ d t= − (2.17 ± 0.10) × 10 − 10. This rate of the spin period change was not stable and varied in a time scale of years. During four nights in 2014 April–May MU Cam was fainter than usual by 0.8 mag, and the amplitude of the sideband oscillation was five times larger, denoting significant fraction of disc-overflow accretion. The sideband oscillation showed a double-peaked pulse profile in the normal brightness state. When the star brightness was decreased by 0.8 mag, the sideband oscillation showed a single-peaked pulse profile. In contrast, the spin pulse, which was quasi-sinusoidal, remained remarkably stable both in profile and in amplitude. Moreover, the spin pulse was also remarkably stable in a time scale of years and even decades. MU Cam is of great interest because it represents a distinctive object with a large and unstable rate of the spin period change and exhibits a distinctive behaviour of the pulse profiles. © 2016, Springer Science+Business Media Dordrecht.