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 ± 0.00047 s, 1276.3424 ± 0.0022 s and 4.71942 ± 0.00016 h, respectively. Comparing the measured spin period with the spin period of MU Cam in the past, we detected the spin period change with dP/dt = −(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 doublepeaked 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 quasisinusoidal, 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.

Extensive photometry of the intermediate polar V2069 Cyg (RX J2123.7+4217)

“…From the rms error of the spin period, we found that the formal validity of this ephemeris is equal to 36 years (a 1σ confidence level). Although this formal validity seems large, it is noticeably less than the formal validities of the spin ephemerides, which we obtained for V455 And, V647 Aur and MU Cam (85-100 years) by applying roughly the same observational coverage (Kozhevnikov 2012(Kozhevnikov , 2014(Kozhevnikov , 2016. The reason consists in noticeably less amplitude of the spin oscillation in V2069 Cyg.…”

“…Spin oscillations of many IPs are directly visible in the light curves (see, e.g., figure 1 in Kozhevnikov 2016). In contrast, the oscillation of V2069 Cyg with a period of about 743 s, which is detected in X-rays and in optical light and obviously corresponds to the spin period (Bernardini et al 2012), is inconspicuous in the light curves (Fig.…”

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

“…12 in Patterson 1994), they must have large amplitudes. The spin oscillation in V1033 Cas was the strongest oscillation compared with the oscillations in the 7 later discovered IPs, which we observed by using the same technique (Kozhevnikov 2001(Kozhevnikov , 2010(Kozhevnikov , 2012(Kozhevnikov , 2014(Kozhevnikov , 2015(Kozhevnikov , 2016(Kozhevnikov , 2017. Indeed, among these 7 IPs, only MU Cam revealed a bit lesser semi-amplitude of 90 mmag.…”

“…In addition, MU Cam shows orbital variations with a similar semi-amplitude of 42-59 mmag (see Fig. 5 in Kozhevnikov 2016). The similar amplitudes of orbital variations in V1033 Cas and in MU Cam can mean that the inclinations of these two systems are also similar.…”