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CCD Photometry of the Intermediate Polar FO Aquarii A. Kruszewski And

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CCD Photometry of the Intermediate Polar FO Aquarii A. Kruszewski And ACTA ASTRONOMICA Vol. 43 (1993) pp. 127±138 CCD Photometry of the Intermediate Polar FO Aquarii by A. Kruszewski and I. S e m e n i u k Warsaw University Observatory, Al. Ujazdowskie 4, Warszawa, 00-478 Poland Received April 26, 1993 ABSTRACT New optical photometry of the intermediate polar FO Aquarii is reported. We con®rm the presence of partial eclipses. We cannot reach any de®nite conclusion about the sign of the spin period derivative because of an uncertain cycle count over the last four years. Key words: Stars: individual: FO Aquarii ± binaries: close ± cataclysmic variables 1. Introduction FO Aqr (H2215-086) is known as a very spectacular intermediate polar system due to the high amplitude (around 0.4 mag) of its dominantphotometric modulation with theperiod of20.9min (Pattersonand Steiner1983). This modulation observed also in X-rays and in infrared (Cook et al. 1984, Sherrington et al. 1984, Chiapetti et al. 1989, Norton et al. 1992) is generally accepted to be a direct manifestation of the white dwarf rotation with the period of 20.9 min. The orbital period equal to 4.85 hr is also clearly seen in optical photometry, X-ray and spectroscopy (Osborne and Mukai 1989, Norton et al. 1992, Shafter and Targan 1982) with its photometric amplitude equal about 0.3 mag. Photometric optical and X-ray observations show in addition a few weaker modulations corresponding to higher harmonics of the spin frequency, and to var- ious combinations of the spin and the orbital frequencies (Patterson and Steiner 1983, KaøuznyÇ and Semeniuk 1988, Chiappetti et al. 1989, Martell and Kaitchuck 1991, Norton et al. 1992). H No circular polarization has been detected in the photometric V and bands by Berriman et al. (1986). Intermediate polars are generally considered to be a link between polars and non-magnetic cataclysmic variables. A natural question arises whether that link is an evolutionary one, and, if this is so, what is the direction of the evolution. 128 A. A. In other words, will intermediate polars become polars or vice versa? There are theoretical arguments for a direction of evolution from intermediate polars to polars (Chanmugan and Ray 1984, King, Frank and Ritter 1985) but the more recent investigations (Lamb and Melia 1987, Hameury et al. 1988, Chanmugam et al. 1990, King and Lasota 1991) seem to indicate that the problem is more complicated. In particular Hameury, King and Lasota (1989) have suggested that a magnetic cataclysmic variable may be at different times either polar or intermediate polar depending on the changingrate of the mass transfer. They consider, however, that such a switching behavior between polar and intermediate polar characteristics P can occur only in the orbital period range 3 hr 4 hr. According to the compilation of King and Lasota (1991) the variations in the primary star rotation period have been detected for 5 objects with three cases of the rotation period decrease and two cases of increasing period. FO Aquarii is listed there as an object with increasing rotation period what has been shown ®rst by Pakull and Beuermann (1987) and con®rmed by Shafter and Macry (1987) and KaøuznyÇ and Semeniuk (1988). However, further photometric investigations (Steinman-Cameron et al. 1989, Osborne and Mukai 1989) have indicated that the rate of the rotation period increase is decreasing itself so that the time derivative of the white dwarf rotation period may be just changing its sign. The orbital period of FOAquariiis notin theperiodrangeindicatedbyHameury, King and Lasota(1989) but nevertheless a demonstration of transition from the rotation period spin-down to spin-up would call for a caution when comparing the theoretical evolutionary considerations with observations. Our observations have been planned to bring some contribution to the knowl- edgeofthewhitedwarfspinbehaviorofFOAquariiinahopethat new observations may con®rm or disprove the claimed transition from the rotation period spin-down to spin-up. 2. Observations The presentobservations of FO Aqr were carried out on three nights of Septem- ber 25, 26 and 27, 1992 with TK512 CCD at the Cassegrain focus of the 0.6 m telescope at the Ostrowik station of the Warsaw University Observatory. The used CCD camera has been described by Udalski and Pych (1992). The durations of observational runs on the three nights were 3.6, 5.2 and 6.3 hr, respectively. Ex- posure times were 30 seconds with a dead time of around 20 seconds between I the frames. Two ®lters, V and , were used. The total number of observations obtained with the yellow V ®lter was 384 and the number of frames with the infrared I ®lter was 452. All the data reductions have been performed with the ESO-MIDAS data reductions system. The frames, after usual de-biasing, dark- subtracting and ¯at-®elding, have been analyzed with help of the INVENTORY photometric package (Kruszewski 1989). Relative magnitudes of FO Aquarii were Vol. 43 129 obtainedas the difference of magnitudes of the variable and a little brighter compar- 00 ison star situated 74 00 north and 64 east with respect to the variable star position. The atmospheric conditions varied during the observations and so varied the accu- racy of measurements. It has ranged from better than 0.01 mag at clear sky to as bad as 0.05 mag when the sky transparency was reduced. It has been checked by comparing the measurements of other constant stars that the variable atmospheric conditions produced no systematic effects in the differential photometry. ∆V 1.2 25 Sep 1992 1.6 .25 .3 .35 .4 .45 ∆V 1.0 26 Sep 1992 1.4 ∆I 1.8 1.2 1.6 2.0 .25 .3 .35 .4 .45 ∆V 1.0 27 Sep 1992 1.4 ∆I 1.8 1.2 1.6 2.0 .25 .3 .35 .4 .45 .5 HJD I Fig. 1. V and light curves of FO Aqr measured in September 1992. V Fig. 1 presents the light curves of FO Aqr for the three September nights. I V I and denote the difference of the instrumental and magnitudes between 130 A. A. the variable and the comparison. The most conspicuous feature in all three runs is the 20.9 min spin modulation. Its peak-to-peak amplitude reaches about 0.5 mag I in V and 0.4 mag in . 3. The Power Spectra FO Aqr is known to demonstrate in its light curve also some weaker modula- tions of various origin (Patterson and Steiner 1983, KaøuznyÇ and Semeniuk 1988, Chappetti et al. 1989, Steinmen-Cameron et al. 1989, Hellier 1992, Norton et al. 1992). In order to search for these periodicities in our observations we have com- puted power spectra using the Lomb-Scargle (Lomb 1976, Scargle 1982) method of spectral analysis for unevenly spaced data. .10 V .08 .06 .04 .02 Power .06 I .04 .02 .4 .8 1.2 1.6 Frequency (mHz) Fig. 2. Mean values of power spectra calculated for individual nights. The upper panel displays the average from the individual V spectra of 25 and 26 Sep. nights, while the lower panel shows the average from individual I spectra for the nights of 26 and 27 Sep. Vol. 43 131 V .10 .08 .06 .04 .02 Power .06 I .04 .02 .2 .4 .6 .8 Frequency (mHz) Fig. 3. Power spectra for the merged observations of consecutive nights; the upper panel is for the I V data from the nights of 25, 26 and 27 Sep., while the lower panel for the data of the nights of 26 and 27 Sep. Fig. 2 presents mean values of the individual night power spectra. The upper panel shows the mean of two individual runs spectra in the V ®lter (nights of 25 and 26 Sep), and the lower panel presents the mean of the individual I runs (26 and 27 Sep) spectra. We can see in the ®gure that only the orbital frequency and the rotational frequency accompanied by its two sidelobes are clearly present in our data. The higher overtones of the spin and orbital frequencies reported by Norton et al. (1992) for GINGA X-ray observations are not visible in our spectra. One can conclude that the optical oscillations more closely resemble sinusoidal variations than it is in the case of X-ray variability. We do not ®nd any detectable power in the range of frequencies 2±7 mHz where Steinman-Cameron et al. (1989) have reported occurance of quasi periodic oscillations. 132 A. A. Fig. 3 shows the spectra computed for the merged V observations of all three nights (upper panel) and the merged I observations (lower panel). The power P = = corresponding to the spin 20 9 min at the frequency 0 8 mHz is almost = equal in both colors, while the power at the orbital frequency 0 057 mHz I is twice as large in V as it is in . There are also two sidebands present at the + frequencies and . They are of the same height in the case of ®lter I what can be understood as originating from the amplitude modulation of the rotational variability in the course of the orbital revolution (Warner 1986). In the + case of ®lter V the sidebandis clearly present while the opposite one was presentonly in a single night so it is weakerin Fig. 2 and absent in Fig. 3 what is similar to what was observed earlier with the ®lter B by KaøuznyÇ and Semeniuk (1988). We can say that at the shorter wavelengths the sideband is mainly due to the re¯ected or reprocessed light and the contribution from the amplitude modulation is small or absent. 4.
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