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

Advanced Technologies Research Institute, Faculty of Materials Science and Technology in Trnava, Slovak University of Technology in Bratislava, Bottova 25, 917 24 Trnava, Slovakia

^⋆ Corresponding author; andrej.dobrotka@stuba.sk

Received: 5 June 2024
Accepted: 13 September 2024

Abstract

Aims. We investigated the fast optical variability of selected nova-like cataclysmic variables observed by the 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 an almost permanent high optical state exhibit preferred f_b around 1 mHz.

Methods. We selected non-interrupted light curve portions with durations 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 the frequency interval from log(f/Hz) = −3.5 to −2.4. We defined as a 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 have 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 a considerable improvement on the previous value of 69%. We discuss the origin of these f_b in the context of the sandwich model in which a central hot X-ray corona surrounds a central optically thick disc. This scenario could be supported by a correlation between the white dwarf mass and f_b; the larger the mass, the lower the frequency. We see such a tendency in the measured data; however, the data are too scattered and based on a low number of measurements. Finally, it appears that systems with detected f_b have a lower inclination than 60–75°. 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 being the source of the observed f_b.