The pulsation frequency analyses were performed with a package of computer programs with single-frequency and multiple-frequency techniques (programs PERIOD, Breger 1990b; PERIOD98, Sperl 1998), which utilize Fourier as well as multiple-least-squares algorithms. The latter technique fits a number of simultaneous sinusoidal variations in the magnitude domain and does not rely on prewhitening. For the purposes of presentation, however, prewhitening is required if the low-amplitude modes are to be seen. Therefore, the various power spectra are presented as a series of panels in Fig. 4, each with additional frequencies removed relative to the panel above. The v and y data give identical results for the first 13 frequencies so that only the y results are presented in the left panel. For the additional frequencies, we present both the y and v (inverted) results in the right panel. Note that for the detected frequencies, the amplitude ratio, v/y, is near 1.5, as expected for these stars. This increases the confidence that the detected frequency peaks correspond to real pulsation modes, rather than noise artifacts.
^{(1)} Amplitude signal/noise limit is 4.00 for newly discovered pulsation modes and 3.50 for "expected'' combination frequencies. |
The average deviation of the observations from the fit are 3.0 mmag per single measurement in y and 4.0 mmag in v. This makes it possible to estimate the uncertainties of the amplitudes shown in Table 2. Based on the assumption that these residuals are random, we can apply the equation (a) = (m) (N/2)^{-1/2}, where a is the amplitude, (m) is the average residual of each data point, and N the number of measurements. We derive uncertainties of 0.09 and 0.12 mmag for the y and v amplitudes, respectively. Of course, in reality the sources of error are neither random nor independent of frequency (white noise). It is interesting to note that combining the two y, v data sets does not lower the noise level significantly and cannot improve the mode detection. Inspection of the data suggests two reasons: the computed noise in the frequency region under discussion is composed mainly of undetected additional modes and the measuring errors of y and v are not independent of each other. Although it is not possible to evaluate the errors in more detail, the present calculation can be useful to estimate whether or not observed amplitude variability is real.
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