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Astron. Astrophys. 355, 1041-1048 (2000)

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2. Optical photometry

We carried out new photometric observations of RX J0537.7-7034 at La Silla on January 18/19 1998 and January 24-26 1999 with the 1.5 m Danish telescope of the European Southern Observatory (ESO) equipped with the spectrograph-imager DFOSC. Observations were primarily done in the V band, with one run also in B. Exposure times typically were 2-3 min. each, and our total temporal coverage amounts to more than 17 hours (see details in Table 1). Photometric reduction in this very crowded field was done using the profile-fitting algorithm of DOPHOT (Mateo & Schechter 1989). On January 24 and 25, 1999 a set of Landolt (1992) standards were observed confirming the absolute photometric calibration derived by Orio et al. (1997).


[TABLE]

Table 1. Log of all observations of RX J0537.7-7034.


The system is very blue and has a mean magnitude and color of V = 19.7 mag and [FORMULA] = -0.05. In Fig. 1 we show the V light curves of all individual nights. The light curves of both 1998 nights and of January 26, 1999 are characterized by an overall variation of [FORMULA] 0.2 mag amplitude and rather smooth minima. In contrast, the system was much more variable on January 24 and 25, 1999 (peak-to-peak amplitude [FORMULA] mag) and the minima appeared more asymmetric or skewed. Also, minimum light was substantially reduced down to [FORMULA].

[FIGURE] Fig. 1. V band light curves of RX J0537.7-7034 obtained during our 5 nights. The top panel includes in addition the B band data (filled squares) and the zero point of the x-axis is 0.05. Note the strong variability from night to night.

In order to derive an orbital period we carried out a period search using the analysis-of-variance method (Schwarzenberg-Czerny 1989). The resulting periodogram (Fig. 2) shows a variety of maxima which are predominantly caused by the 1 day and 1 year sampling rates. In addition, the strong night-to-night variations of the system prevent any clear signal to emerge. The most probable periods (Fig. 2) are 0.147275 d, 0.172157 d, 0.206914 d and 0.258718 d.

[FIGURE] Fig. 2a and b. Results of the period-search using the analysis-of-variance method for the photometry (top) and radial velocities (bottom). The adopted orbital period is that peak in the top panel which best coincides with the spectroscopic peak (0.147275 d).

There is a clear difference between the short and the long period possibilities: while for the above two shorter periods (0.147275 d and 0.172157 d) the folded light curve is basically sinusoidal, it is double humped for the two longer periods. It is important to note that in both cases the primary minima have different brightnesses (differing by 0.2 mag).

For completeness we have also checked all other optical objects within the X-ray error circle for possible variability, but found no other variable star with an amplitude [FORMULA]0.2 mag down to 22 mag.

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© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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