Blazh effect

RR Lyra star's light curve : visible magnitude versus ripple phase.

The Blazhko effect , sometimes called long-period modulation, is a variation in the period and amplitude of variable stars of the RR Lyra type . This effect was first observed by Sergei Blazhko in 1907 with the star RW Dragon ^{[1] [2]} . For these stars, the moments of maximums of brightness deviate from the linear formula in one direction or another - they are late or ahead of the ephemeris. The shape of the light curve also changes synchronously. A characteristic feature of the Blazhko effect in stars of the RR Lyra type is that the period of the Blazhko effect is about two orders of magnitude longer than the period of the main oscillation. For example, for two stars discovered by Blazhko himself, these periods are as follows: P = 0 ^d .4665 for XZ Swan and P = 0 ^d .4429 for RW Dragon . The main prototype, the RR Lyra variable itself, also demonstrates the Blazhko effect . The most remarkable example of the Blazhko effect was found in 2000 by E. Schmidt and K. Lee in the Hercules variable V422 : its amplitude in the V rays varies from 0.27 ^m to 1.39 ^m . Interestingly, at high amplitudes, the star shows a characteristic curve of the RRA type for its period, while at low amplitudes, the light curve resembles the RRC type ^[3] .

The observed phenomena give the impression that under the Blazhko effect , beats of two oscillations with close periods occur. For some time, such an interpretation was hindered by the conclusion reached by V.P. Tsesevich and B.A. Ustinov in the 1950s. They studied in great detail the brightness changes of three variables of the RR Lyra type with the Blazhko effect and concluded that changes in the shape of the light curve cannot be represented as the result of beating of two different-period elementary vibrations. Subsequently, however, it turned out that this conclusion was based on a misunderstanding. Tsesevich and Ustinov simply tried to add brightness changes, and for a pulsating star, only radius changes can be added directly, on which, of course, temperature changes are superimposed. However, it remains unclear why stars with the Blazhko effect can simultaneously be excited with two very close periods (say, for AR Hercules , one of the stars studied by Tsesevich and Ustinov, vibrations with P ₀ = 0 should be involved in beats ^d .470 and P ₁ = 0 ^d .463). The theory does not predict the coexistence of such oscillations. For example, simultaneous instability in the fundamental tone and in the first overtone of radial pulsations would give a beating of about 4: 3, as is observed in stars of the RR (B) type and in some variables of the type δ Shield . Of the many proposed explanations for the Blazhko effect , the most attractive are those that use the notion of the role of rotation and the magnetic field in the observed phenomena. In 1987, Yu.S. Romanov et al., Having performed spectral observations of the RR Lyra star, found her a magnetic field variability with a pulsation period, as well as the dependence of the magnetic field intensity averaged over the cycle of pulsations on the phase of the Blazhko effect . A connection with the phase of the Blazhko effect was also found for the strength of the lines of some elements. The relationship between stars of the RR Lyra type with the Blazhko effect and magnetic variables of the a2 type of the Hounds of Dogs is outlined here. The result of Romanov et al. Needs to be verified using more extensive material ^[3] .

The physics behind the Blazhko effect is currently still in debate, and there are three main hypotheses. In the first case, in the so-called resonance model, the reason for the modulation is the nonlinear resonance of both the main and the first overtone of the star pulsation mode and a higher mode ^{[4] [5]} . The second hypothesis, known as the magnetic model, suggests that the change is caused by the inclination of the magnetic field to the axis of rotation, deforming the main radial mode ^[6] . The third model assumes that cycles in convection cause alternations and modulations ^[7] .

Observational data based on observations of the Kepler space telescope indicate that the modulation of the two-beam light curve with the Blazhko effect is due to a simple doubling of the period. Many stars of the RR Lyrae type have a variability period of approximately 12 hours, and ground-based astronomers usually make night observations with a period of 24 hours: thus, doubling the period leads to brightness maxima during night observations, which differ significantly from the day maximum ^[8] .