Period switching in the symbiotic star BX Mon

ABSTRACT We report on a detailed analysis of the optical light curve of the symbiotic system BXMon, the data of which were gathered from the literature. The light curve covers theperiod December 1889 March 2009, with a gap of no observations between March 1940and February 1972. The light curve is characterized by strong oscillations of peak topeak amplitude of 2 to more than 3 magnitudes. Before the gap the fluctuations weremodulated mainly by a period P a =1373±4 d. After the gap the dominant periodicityis P b =1256±16. Higher harmonics as well as a few beats of the two major periodicitiescan also be identified in the light curve. We identify one of the beat periods, P r =656d, as the sidereal rotation period of the giant component of the system . The periodswitching that took place during the gap in the observations was possibly associatedwith a certain cataclysmic event, hints of which may be recognized in the LC in thefirst 11 years after the gap.We suggest that the origin of the major oscillations is in periodic episodes of massaccretion from the M giant onto the hot component of the system. After the gap theyare correlated with the periastron passage of the system, and therefore appear withthe binary period. Before the gap the oscillations appeared with the diurnal cycle ofan observer on the surface of the rotating M giant, whose sun is the hot component.The event of the period switching is possibly related to an intensive magnetic activityin the outer layers of the giant star.Keywords: binaries: symbiotic – stars: individual: BX Mon – stars: magnetic fields– stars: rotation.