## 3. Time series and frequency analysisInspecting the time series in Fig. 1 we can identify a periodic light variation in all curves with a period of 18 min and amplitude varying from 10 mmi (run ro093) to 70 mmi (run ro074). The time series of the comparison star which was observed in channel two do not display any periodic light variation. Moreover, it is noteworthy that the regular light variation was detected both in 1995 and in 1996, i.e., we can state that this variation is not due to transparency or sky variations. We can also see on run ro093 what seems to be beating between pulsations with closely spaced frequencies. The time series shown in Fig. 1 therefore strongly suggests that BPM 24754 has a multiperiodic light curve. Obviously, this periodic light variation must appear in the amplitude spectrum as a peak at the corresponding frequency. We analysed the time series of fractional intensity using the well-known Fourier Transform method. The Deeming (1975) algorithm was applied and a frequency spectrum was obtained. The output Fourier spectrum is in units of amplitude modulation (ma), we therefore call it an amplitude spectrum. We calculated these spectra for each run separately and the results are shown in Fig. 3. The frequency range spans from 0.1 to 5.0 mHz and the amplitude varies from 0 to 9 mma or 25 mma (ro074). We clearly note in Fig. 3 the presence of the same peak in every spectra with a frequency around 0.92 mHz or a period of 18 min - confirming the light variation period displayed by light curves (Fig. 1). Table 2 shows the observed frequencies and the corresponding period for the largest amplitude peak. The different frequencies and amplitude of this peak can be caused by the presence of other smaller amplitude pulsations, beating of closely spaced frequencies, and the spectral resolution - approximately the inverse of the data length.
© European Southern Observatory (ESO) 1998 Online publication: November 24, 1997 |