Hunt for the mHz variability in the TESS and XMM-Newton observations of nova-like cataclysmic variables

TL;DR

This work expands the census of break frequencies $f_{ m b}$ in the flickering of nova-like cataclysmic variables by analyzing optical data from TESS and X-ray data from XMM-Newton. Using a consistent segment-based PDS approach and broken-power-law fits, the authors identify three new optical detections and two tentative X-ray counterparts of $f_{ m b}$, with frequencies clustering near log($f$/Hz) ≈ $-3.2$ (≈1 mHz). The results reveal that $f_{ m b}$ can vary with long-term brightness and disc state, and that detections favor systems with lower inclinations, while WD mass shows no robust correlation. The cross-band (optical and X-ray) detections support a common, central origin for flickering—most plausibly a sandwich corona or inner-disc boundary layer—contributing to our understanding of accretion disc variability in nova-like CVs.

Abstract

We analysed the flickering of selected nova-like cataclysmic variables observed by the TESS satellite and XMM-Newton. We searched for break frequencies ($f_{\rm b}$) in the corresponding power density spectra (PDS), and for any long-term evolution. We found a new optical $f_{\rm b}$ in three nova-like systems and confirmed that the value of this frequency is clustered around 1 mHz. V504 Cen and V751 Cyg show possible X-ray counterparts of $f_{\rm b}$ that had previously only been seen in MV Lyr. This points towards the very central disc for source localisation. We investigated a previously proposed correlation between white dwarf mass and $f_{\rm b}$, but thanks to the new measurements we do not conclude its existence. V3885 Sgr and V1193 Ori show flaring activity in the long-term light curve during which TESS observations were made. The corresponding PDSs show changes in shape and disappearance of $f_{\rm b}$. TT Ari and SGRt 062340.2-265715 exhibit smooth changes in the long-term optical light curve, and the corresponding TESS observations show variable $f_{\rm b}$ during these changes. $f_{\rm b}$ is higher for lower brightness, which was seen only in MV Lyr so far.

Figures (6)

• Figure 1: PDSs of three systems with positive detection of $f_{\rm b}$. Upper panel - PDSs with positive detection of $f_{\rm b}$ for $n_{\rm d} = 10$ and 5 light curves (points with errors of the mean). Red lines represent fits yielding $f_{\rm b}$ values with error lower than 0.1. PDSs are vertically offset for better visualisation. Individual $f_{\rm b}$ are shown as blue dots. The grey area around blue dots represents the natural scatter due to TESS light curve quality. The point style is varying just for clarity of the figure. Lower panel - all PDSs with (black lines) and without (red lines) positive detection of $f_{\rm b}$. Dashed lines represent low(er) optical state.
• Figure 2: Number of nova-likes with detected and non-detected $f_{\rm b}$ for various parameters. The values are from this work and from dobrotka2024. Left panel - number of detected $f_{\rm b}$ per frequency interval for nova-like CVs observed by TESS with condition $n_{\rm m} \geq 2$. Middle panel - Number of systems per magnitude interval with detected and non-detected $f_{\rm b}$. Right panel - Number of systems brighter than 14 mag per inclination interval with and without detection of $f_{\rm b}$.
• Figure 3: PDSs calculated from XMM-Newton observations using $n_{\rm subs} = 2$ (left column) and 3 (right column). Red lines are the fits to the PDSs with lower PDS end of log($f$/Hz) = -4.0 while blue is from log($f$/Hz) = -4.5.
• Figure 4: ASAS-SN light curves of selected systems with detected $f_{\rm b}$. Vertical lines represent time of TESS light curve portions with detected (black lines) and non-detected (red lines) $f_{\rm b}$. Blue vertical line shows the time of XMM-Newton observation. Dashed lines represent observations taken during specific times discussed in the text (corresponding to dashed PDSs in Fig. \ref{['pds_tess']} and \ref{['pds_tt_v1193_hs']}).
• Figure 5: PDSs of TT Ari, V1193 Ori and HS 0506+7725. Black lines represent PDSs with $f_{\rm b}$ detection, and red lines without. Dashed lines represent PDSs taken during specific times shown as dashed lines in Fig. \ref{['lc_asassn']} (see text for details).