Dwarf nova oscillations and quasi-periodic oscillations in cataclysmic variables - III. A new kind of dwarf nova oscillation, and further examples of the similarities to X-ray binaries

Abstract

We present measurements of the periods of dwarf nova oscillations (DNOs) and quasi‐periodic oscillations (QPOs) in cataclysmic variable stars (CVs), many culled from published literature, but also others newly observed (in VZ Pyx, CR Boo, OY Car, Z Cha, AQ Eri, TU Men, HX Peg, CN Ori, V893 Sco, WX Hyi and EC2117−54). These provide data for 26 systems. We show that in general PQPO∼ 15 PDNO and that the correlation for CVs extends by three orders of magnitude lower in frequency the similar relationship found for X‐ray binaries. In addition, we have found that there is a second type of DNO, previously overlooked, which have periods ∼4 times those of the regular DNOs (as well as those mined from publications, we have observed them in VW Hyi, OY Car, AQ Eri, V803 Cen, CR Boo, VZ Pyx, HX Peg and EC2117−54). Often both types of DNO coexist. Unlike the standard DNOs, the periods of the new type, which we refer to as longer‐period DNOs (lpDNOs), are relatively insensitive to accretion luminosity and can even appear in quiescence of dwarf novae. We interpret them as magnetically channelled accretion on to the differentially rotating main body of the white dwarf primary, rather than on to a rapidly slipping equatorial belt as in the case of the standard DNOs. This is supported by published measurements of v sin i for some of the primaries. Some similarities of the DNOs, lpDNOs and QPOs in CVs to the three types of QPO in X‐ray binaries (burst pulsations, high‐ and low‐frequency QPOs) are noted.

An explanation for the kHz-QPO twin peaks separation in slow and fast rotators

“…This strongly supports the idea of one and same general mechanism at hand in this accretion disks. The twin peaks ratio around 3:2 for the kHz-QPOs is naturally explained not only for black hole candidates or neutron stars but also for white dwarfs as reported by Warner et al (2003). Indeed, the presence or the absence of a solid surface, a magnetic field or an event horizon play no relevant role in the production of the X-ray variability (Wijnands 2001).…”

Long-term photometry of the eclipsing dwarf nova V893 Scorpii

“…Medium frequencies and the interpretation of peaks in power spectra I will start the discussion concentrating on medium frequencies because this is the range where the most interesting results are observed and where previous studies reported the detection of oscillations (Paper I, Warner et al 2003). Apparently significant signals were also found in the power spectra of other light curves of the present study.…”

“…They exhibit a wide range of coherence: Over the course of hours the periods can change by many seconds, but they also may remain stable on the millisecond level. Warner et al (2003) also introduce a second kind of DNOs which they term "longer period DNOs (lpDNOs)" with periods typically 4 times longer than normal DNOs.…”

“…Mukai et al (2009) observed V893 Sco in X-rays with the Suzaku satellite and found partial eclipses. Warner et al (2003) report on the detection of quasi-periodic oscillations (QPOs) (which have also been seen by Bruch et al 2000) and dwarf nova oscillations (DNOs), and Pretorius et al (2006) claim to have observed an instant with DNO activity.…”

“…The detection of QPOs and DNOs by Bruch at al. (Paper I), Warner et al (2003) and Pretorius et al (2006) justifies a frequency analysis of the light curves which is performed in Sect. 7.…”