Searching for the mHz variability in the TESS observations of nova-like cataclysmic variables

Abstract

AIMS: We investigated fast optical variability of selected nova-like cataclysmic variables observed by TESS satellite. We searched for break frequencies (f_b) in the corresponding power density spectra (PDS). The goal is to study whether these systems in almost permanent high optical state exhibit preferred f_b around 1 mHz. METHODS: We selected non-interrupted light curve portions with duration of 5 and 10 days. We divided these portions into ten equally long light curve subsamples and calculated mean PDS. We searched for f_b in frequency interval from log(f/Hz) = -3.5 to -2.4. We defined as positive detection when the f_b was present in at least 50% of the light curve portions with a predefined minimum number of detections. RESULTS: We measured f_b in 15 nova-like systems and confirmed that the value of this frequency is clustered around 1 mHz with a maximum of the distribution between log(f/Hz) = -2.95 and -2.84. The confidence that this maximum is not a random feature of a uniform distribution is at least 96%. This is considerably improved since previous value of 69%. We discuss the origin of these f_b in the context of sandwich model where central hot X-ray corona surrounds central optically thick disc. This scenario could be supported by correlation between white dwarf mass and f_b; the larger the mass, the lower the frequency. We see such tendency in the measured data, however the data are too scattered and based on low number of measurements. Finally, it appears that systems with detected f_b have lower inclination than 60-75 degrees. In higher inclination binaries the central disc is not seen and the PDS is dominated by red noise. This also supports the inner disc regions as source of the observed f_b.