Magnetic field structure and accretion regimes of the asynchronous polar by Cam

Abstract

Series of photometric CCD observations of the asynchronous polar BY Cam in a low accretion state (R = 14 m -16 m ) were made on the K-380 telescope at the Crimean Astrophysical Observatory (CrAO) over 100 hours in the course of 31 nights during [2004][2005]. A period of P 1 = 0.137120 ± 0.000002 days was found for the variations in the brightness, along with less significant periods of P 2 = 0.139759 ± 0.000003 and P 3 = 0.138428 ± 0.000002 days, where P 2 and P 3 are obviously the orbital and rotation periods, while the dominant period P 1 is the sideband period. A modulation in the brightness and an amplitude of 0.137 days in the oscillations at the orbital-rotational beat period (synodic cycle) of 14.568 ± 0.003 day are found for the first time. The profile of the modulation period is four humped. This indicates that the magnetic field has a quadrupole component, which shows up well during the low brightness state. Accretion takes place simultaneously into two or three accretion zones, but at different rates. The times of the times of maxima for the main accretion zone vary with the phase of the beat period. Three types of variation of this sort are distinguished: linear, discontinuous, and chaotic, which indicate changes in the accretion regimes. At synodic phases 0.25 and 0.78 the bulk of the stream switches by 180°, and at phase 0.55, by ~75°. At phases of 0.25-0.55 and 0.55-0.78, the O-C shift with a period of 0.1384 days, which can be explained by a retrograde shift of the main accretion zone relative to the magnetic pole and/or a change in the angle between the field lines and the surface of the white dwarf owing to the asynchronous rotation. For phases of 0.78-1.25 the motion of the accretion zone is quite chaotic. It is found that synchronization of the components occurs at a rate of less than dP rot /P rot~1 0 -9 day/day.

Idling magnetic white dwarf in the synchronizing polar BY Cam. The Noah-2 project

“…It is slightly less than a photometric period, which is expected for a non-sinusoidal curve [48]. The second period is close to P /4 confirming a 4-peak structure of the light curve, which was pointed out by Pavlenko [20]. The fourth peak occurs at 0 0056=8 min and agrees with smaller periods mentioned above.…”

“…Some results of this monitoring have been published separately [20]. However, in the present study, we have reanalyzed these data together with our new data to make a more complete study.…”

“…The log of observations is presented in Table 2 (available only electronically). In an addition to usually presented values, we have also added the phase for the mid-run according to the beat period ephemeris [20] T (HJD) = 2453089 2473 + 14 568E (1) where the initial epoch corresponds to the minumum of mean brightness. The light curves are modulated with this period, thus the corresponding phase is important when comparing light curves from different beat cycles (see below).…”

“…Here, ( ) is a signal at time P is an adopted period (we have used P = 0 137120 published by Pavlenko [20]), and C (where = 1 · · · (2 + 1)) is a parameter determined using the least squares method. A description of working formulae for the computer program was presented by Andronov 8 [45].…”

“…A description of working formulae for the computer program was presented by Andronov 8 [45]. We have applied the fixed value of = 4 after noting a 4-hump model of the light curve [20] which is often seen in our data. The individual light curves for different telescopes are shown in Figures 5-6 together with the corresponding fits.…”