In order to investigate whether the X-ray emission of the Tucanae stars is variable we have generated lightcurves for all detected sources from the arrival time information of the photons counted within the source extraction radius. The background counts were extracted from the background map as described in Sect. 2.1. An alternative method of background acquisition consists of constructing a separate background lightcurve at a source free position, and subtracting it from the source lightcurve. This procedure is sensitive to local variations in the background. We have applied both methods to the Tucanae stars and found no significant differences. The count rates have been corrected for vignetting, i.e. effects due to detector off-axis position and support structure.
For the RASS lightcurves the photons have been binned into 5600 s intervals (corresponding to the duration of one satellite orbit around the Earth). Note, however, that the actual exposure times in individual bins range between 10 - 30 s. For pointed observations we use a binsize of 400 s. This interval corresponds to the wobbling motion of the telescope. The wobble is performed to ensure that no source remains hidden behind the entry window's support structure, and may produce apparent variability in a lightcurve if the binning is smaller than that period.
Most of the lightcurves show strong variability. To examine the variations in a quantitative way we have computed the relative luminosity change between the bin with maximum () and minimum count rate () for each star, i.e.
and its 1 uncertainty from the errors of the respective count rates.
In Fig. 4 we display all RASS lightcurves for which the change in count rate in the course of the observation amounts to at least 3 . For clarity we provide the number of counts and the exposure time in each bin on top of each lightcurve. The variability is significant at the 3 level for 10 of the detected Tucanae members (77%). The values and its significance are given in Table 6 together with the minimum and maximum luminosity observed in the lightcurve.
Table 6. Variability in the RASS lightcurves of the Tucanae stars measured through the relative change in count rate during the observation (see text). Column 3 provides a measure of the significance of the variation. All changes larger than 3 have been considered significant, and the corresponding lightcurves are displayed in Fig. 4. Columns 4 and 5 are the minimum and maximum luminosity inferred from the lightcurve using distance and ECF as described in the text.
For variable stars the amplitude of the lightcurve is displayed in Fig. 5, where we have plotted maximum versus minimum count rate. Constant sources would lie on the dotted line. The dashed line corresponds to a factor of 10 change.
Among the pointed PSPC observations, only the lightcurve of HIP 92680 shows significant variability. However, the other two observations (ROR 200099p and 200404p) are rather short, and therefore long-term variations on HIP 100751 and HIP 103438 (an improbable member) might have been missed. The lightcurve of HIP 92680 is displayed in Fig. 6. Remarkably, this star was among the few sources not found to be variable (at the 3 level) during the RASS (see Table 6).
© European Southern Observatory (ESO) 2000
Online publication: October 2, 2000