Alex Liu

Nova V4743 Sagittarii 2002: An Intermediate Polar Candidate

We present the results of 11 nights of CCD unfiltered photometry of V4743 Sgr (Nova Sgr 2002 No. 3) from 2003 and 2005. We find two periods of 0.2799 days (≈6.7 hr) and 0.01642 days (≈24 minutes) in the 2005 data. The long period is also present in the 2003 data, but only weak evidence of the shorter period is found in this year. The 24 minute period is somewhat longer than the 22 minute period that was detected from X-ray observations. We suggest that the 6.7 hr periodicity represents the orbital period of the underlying binary system and that the 24 minute period is the beat periodicity between the orbital period and the X-ray period, which is presumably the spin period of the white dwarf. Thus, V4743 Sgr should be classified as an intermediate polar (DQ Her star). About 6 months after the nova outburst, the optical light curve of V4743 Sgr seemed to show quasi-periodic oscillations, which are typical of the transient phase in classical novae. Therefore, our results support the previous suggestion that the transition phase in novae may be related to intermediate polars.

The Orbital Period of Nova V2540 Ophiuchi (2002)

We present the results of 26 nights of CCD photometry of the nova V2540 Oph (2002) from 2003 to 2004. We find a period of 0.284781 ± 0.000006 d (6.8347 ± 0.0001 h) in the data. Since this period was present in the light curves taken in both years, with no apparent change in its value or amplitude, we interpret it as the orbital period of the nova binary system. The mass-period relation for cataclysmic variables yields a secondary mass of about 0.75 ± 0.04 M� . From maximum magnitude-rate of decline relation, we estimate a maximum absolute visual magnitude of MV =− 6.2 ± 0.4 mag. This value leads to an uncorrected distance modulus of (m − M) = 14.7 ± 0.7. By using the interstellar reddening for the location of V2540 Oph, we find a rough estimate for the distance of 5.2 ± 0.8 kpc. We propose that V2540 Oph is either (1) a high-inclination cataclysmic variable showing a reflection effect of the secondary star, or having a spiral structure in the accretion disc, (2) a high-inclination intermediate polar system, or less likely (3) a polar.

A Photometric Study of the Newly Discovered Eclipsing Cataclysmic Variable SDSS J040714.78–064425.1

We present the results obtained from unfiltered photometric CCD observations of the newly discovered cataclysmic variable SDSS J040714.78–064425.1 made during seven nights in 2003 November. We establish the dwarf nova nature of the object as it was in outburst during our observations. We also confirm the presence of deep eclipses with a period of 0.17017 ± 0.00003 d in the optical light curve of the star. In addition, we found periods of 0.166 ± 0.001 d and possibly also 5.3 ± 0.7 d in the data. The 0.17017 d periodicity is consistent within the errors with the proposed orbital period of 0.165 and 0.1700 d. Using the known relation between the orbital and superhump periods, we interpret the 0.166 and 5.3 d periods as the negative superhump and the nodal precession period respectively. SDSS J040714.78–064425.1 is then classified as a negative superhump system with one of the largest orbital periods.

Detection of Orbital and Superhump Periods in Nova V2574 Ophiuchi (2004)

We present the results of 37 nights of CCD unfiltered photometry of nova V2574 Oph (2004) from 2004 and 2005. We find two periods of 0.14164 days (≈3.40 hr) and 0.14773 days (≈3.55 hr) in the 2005 data. The 2004 data show variability on a similar timescale, but no coherent periodicity was found. We suggest that the longer periodicity is the orbital period of the underlying binary system and that the shorter period represents a negative superhump. The 3.40 hr period is about 4% shorter than the orbital period and obeys the relation between superhump period deficit and binary period. The detection of superhumps in the light curve is evidence of the presence of a precessing accretion disk in this binary system shortly after the nova outburst. From the maximum magnitude-rate of decline relation, we estimate the decay rate t2 = 17 ± 4 days and a maximum absolute visual magnitude of M = -7.7 ± 1.7 mag.

The 2005 July Superoutburst of the Dwarf Nova 2QZ J021927.9−304545: the SU UMa Nature Confirmed

We report on the time-resolved photometry of the 2005 July superoutburst of a dwarf nova, 2QZ J021927.9−304545. The resultant light curves showed conspicuous superhumps with a period of 0.081113(19) days, confirming the SU UMa nature of the object. Although we missed the maximum phase of the outburst, the amplitude of the superoutburst well exceeded 5mag. This value is slightly larger than that of typical SU UMa-type dwarf novae. The superhump period decreased as the time elapsed, as can be seen in most SU UMa-type dwarf novae. Based on the archive of ASAS-3, the recurrence time of a superoutburst of the variable turned out to be about 400 days. This value is typical of well-known SU UMa stars. The distance to this system was roughly estimated as 370(+20, −60)pc using an empirical relation.

V4745 Sgr – a nova above the period gap and an intermediate polar candidate

A period analysis of CCD unfiltered photometry of V4745 Sgr (Nova Sgr 2003 #1) performed during 23 nights in the years 2003 – 2005 is presented. The photometric data are modulated with a period of 0.20782± 0.00003 d (4.98768± 0.00072 h). Following the shape of the phased light curve and the presence of the periodicity in all data sets with no apparent change in its value, we interpret this periodicity as orbital in nature and this is consistent with a cataclysmic variable above the period gap. We found a probable short-term periodicity of 0.017238± 0.000037 d (24.82272± 0.05328 m) which we interpret as the probable spin period of the white dwarf or the beat period between the orbital and spin period. Therefore, we propose that nova V4745 Sgr should be classified as an intermediate polar candidate, supporting the proposed link between transition-oscillation novae and intermediate polars. The mass-period relation for cataclysmic variables yields a secondary mass of about 0.52± 0.05 M⊙.

The Peculiar 2004 Superoutburst in the Helium Dwarf Nova, 2003aw

We conducted a time-resolved photometric campaign of helium dwarf nova, 2003aw, in 2004 May–June. 2003aw stayed at 14.7–15.7mag for the first several days during this campaign, which is the plateau phase of this superoutburst. This variable then rapidly decayed to ∼ 18.0mag, still brighter by about 2mag than its quiescence magnitude, and maintained this brightness for about 20 days, having short flares of ∼2mag. A long fading tail followed it. We detected superhumps with a period of 0.02357(4)d [= 2036(3)s] during the plateau phase. The whole light curve of the superoutburst in 2003aw, taking into account the present data and those in the literature, perfectly resembles that of the 1996–1997 superoutburst of the peculiar WZ Sge-type hydrogen-rich dwarf nova, EG Cnc.

Evidence for Large Superhumps in TX Col and V4742 Sgr

Since the discovery of the largest positive superhump period in TV Col (6.4 h), we have started a program to search for superhumps in cataclysmic variables (CVs) with large orbital periods. In this work, we summarize preliminary results of our observations of TX Col and V4742 Sgr. TX Col is an intermediate polar with a 5.7-h orbital period. V4742 Sgr is a recent (2002) nova with no known periods. CCD unfiltered continuous photometry of these two objects was carried out during 56 nights (350 hours) in 2002–2003. The time series analysis reveals the presence of several periods in both power spectra. In TX Col, in addition to the orbital period of 5.7 h, we found peaks at 7.1 h and 5.0 h. These are interpreted as positive and negative superhumps correspondingly, although the effects of the quasi-periodic oscillations at ∼2 h (which may cause spurious signals) were not taken into consideration. In the light curve of V4742 Sgr two long periods are detected − 6.1 and 5.4 h as well as a short-term period at 1.6 h. This result suggests that V4742 Sgr is an intermediate polar candidate and a permanent superhump system with a large orbital period (5.4 h) and a superhump period excess of 13%. If these results are confirmed, TX Col and V4742 Sgr join TV Col to form a group of intermediate polars with extremely large superhump periods. There seems to be now growing evidence that superhumps can occur in intermediate polars with long orbital periods, which is very likely inconsistent with the theoretical prediction that superhumps can only occur in systems with mass ratios below 0.33. Alternatively, if the mass ratio in these systems is nevertheless below the theoretical limit, they should harbour undermassive secondaries and very massive white dwarfs, near the Chandrasekhar limit, which would make them excellent candidates for progenitors of supernovae type Ia.

Precessing accretion discs in binary systems

Superhumps – quasi-periodic oscillations at periods a few percent different than the orbital periods – have been observed in several dozen binary systems of various types. It is well-accepted now that they are caused by the precession of the accretion disc. New findings on TV Col, TX Col and V4742 Sgr are presented. These cataclysmic variables (CVs) have exceptional precessing periods that cast strong doubt on the predictions of the Tidal Disc Instability model.