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Astron. Astrophys. 327, 245-251 (1997)

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4. The X-ray light curves

V2301 Oph must have been in a very similar state during the pointed PSPC and HRI observations - the ratio of the mean count rates roughly corresponds to that of the effective areas of the instruments for a hard spectrum and the light curves are very similar. The differences between the RASS and the pointed observations suggest that V2301 Oph shows several distinct X-ray states with different flux levels and degrees of light curve variability, presumeably related to the overall accretion rate: a minimum state during which the optical Zeeman lines from the white dwarf can be seen; a state with fairly steady accretion like that seen during the PSPC and HRI observations; and the high state with the irregular accretion seen in the RASS data.

When the eclipses in the PSPC and HRI mean light curves are compared with the smoothed mean R-band light curve from BRB (Fig. 5), it is clear that the X-ray eclipse is due to the eclipse of the accretion region on or around the white dwarf. The other major optical eclipse feature is not seen in the X-rays and is due to the accretion stream while or after it is deflected onto the magnetic field lines of the white dwarf. Unfortunately, the number of photons is not large enough to see if the lengths of the ingress and egress phases are equal (as would be expected for the simple eclipse of a small accretion region on the white dwarf) or not (as is observed in the optical during the normal photometric state of V2301 Oph; BRB).

[FIGURE] Fig. 5. The mean X-ray (histograms) and optical R-band (continuous line) eclipse light curves. The phase-resolution of the HRI light curve is twice as high as that of the PSPC light curve.

The RASS data are too sparse to allow a precise determination of the hump position, but there is enough phase coverage in the PSPC and HRI data to permit a simple sinusoidal fit to the portions of the mean light curves uncontaminated by either the eclipse or the "dips". The fitted phase positions of the assumed sinusoidal peaks are [FORMULA] and [FORMULA] for the PSPC (taken in 1993) and HRI (1995) mean light curves, respectively. These results are consistent with the overall shape of the light curve in the RASS data taken in 1990/1991.

In order to search for a hypothetical spin-pulse, we pre-whitened the PSPC and HRI data using the fits to the orbital humps and then binned them into light curves with 40 sec time resolution. Analysis-of-Variance periodograms (AoV: Schwarzenburg-Czerny 1989) were then calculated, making sure to bin consecutive points falling in the same phase bin. No signal with periods greater than 150 sec are present above the expected noise level ([FORMULA]). The spacing of the OBI's and the loss of the dip and eclipse phase data limit our sensitivity: only hypothetical pulses with semi-amplitudes of more than 0.1 and 0.08 counts s-1 for the PSPC and HRI data sets, respectively, would have been seen above the statistical noise level. Thus, we can only rule out the presence of a 100% modulated signal at the 95% confidence level.

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998