A systematic search for orbital periods of polars with TESS. Methods, detection limits, and results

TL;DR

The paper systematically searches for orbital periods of polars using TESS two-minute cadence light curves, applying four complementary period-detection methods and a rigorous treatment of uncertainties, half-period aliases, and asynchronous systems. It introduces a noise-flattening technique and a probabilistic reliability framework based on the PSD signal-to-noise to assess detection robustness as a function of magnitude, establishing a practical reliability threshold at ${\rm S/N}_{\rm PSD,min}=0.004$. Out of 95 polars with suitable TESS data, 93 periods are measured, with 85 in agreement with literature; four asynchronous systems reveal spin, orbital, and beat frequencies, while four others show literature-consistent deviations and two remain inconclusive due to noise. The study demonstrates that TESS can reliably determine orbital periods for CVs, particularly for brighter targets ($T<17$ mag), and provides a broadly applicable methodology for period analysis and reliability assessment in large photometric surveys. It also contextualizes the polar population in the Gaia CMD, showing evolutionary trends from the main sequence toward the white-dwarf sequence as orbital periods shorten.

Abstract

Context. Determining the orbital periods of cataclysmic variable stars (CVs) is essential for confirming candidates and for the understanding of their evolutionary state. The Transiting Exoplanet Survey Satellite (TESS) provides month-long photometric data across nearly the entire sky that can be used to search for periodic variability in such systems. Aims. This study aims to identify and confirm the orbital periods for members of a recent compilation of magnetic CVs (known as polars) using TESS light curves. In addition, we set out to investigate their reliability, and hence the relevance of TESS for variability studies of CVs. Results. Ninety-five of the 217 polars in our sample have pipeline-produced TESS two-minute cadence light curves available. The results from our period search are overall in good agreement with the previously reported values. Out of the 95 analysed systems, 85 exhibit periods consistent with the literature values. Among the remaining ten objects, four are asynchronous polars, where TESS light curves resolve the orbital period, the white dwarf's spin period, and additional beat frequencies. For four systems, the periods detected from the TESS data differ from those previously reported. For two systems, a period detection was not possible. Our analysis of the flattened TESS light curves reveals a positive correlation between noise levels and TESS magnitude. Our noise level estimates resemble the rmsCDPP, a measure of white noise provided with the TESS pipeline products. However, our values for the noise level are systematically higher than the rmsCDPP indicating red noise and high-frequency signals hidden in the flattened light curves. Additionally, we present a statistical methodology to assess the reliability of period detections in TESS light curves. We find that for TESS magnitudes $\gtrsim$ 17, period detections become increasingly unreliable.

Figures (19)

• Figure 1: Properties of the sample of polars with two-minute time-resolution TESS light curves. Left panel: Histogram of TESS magnitudes. Right panel: Histogram of distances (from Bailer-Jones).
• Figure 2: Illustration of the results from different period detection methods for a case where the dominant period is $1/2 \,\text{P}_{\rm orb}$ (from left to right): Lomb-Scargle periodogram, ACF, and Fourier power spectrum. The example refers to TIC 124404442 (alias V1043 Cen) (sector 37). The red vertical lines indicate the signals at $1/2 \,\text{P}_{\text{orb}}$, while the green vertical lines indicate the true orbital period (4). In this light curve, all three methods failed to identify the orbital period. We note that although the autocorrelation coefficient at the true orbital period is higher than that at $1/2 \,\text{P}_{\text{orb}}$ in the ACF plot, the subsequent FFT applied to the autocorrelation coefficients yields a stronger signal in $1/2 \,\text{P}_{\text{orb}}$.
• Figure 3: Phase plots for the light curve of TIC 258799357 (alias EP Dra) (sector 26), an eclipsing polar, as an example of a light curve where the dominating modulation is at $1/2 \,\text{P}_{\text{orb}}$. Left panel: Light curve folded over the detection at $1/2 \,\text{P}_{\text{orb}}$. Right panel: Light curve folded over the true orbital period 41. The true orbital period (right panel) displays the pattern corresponding to one complete orbit of the primary star around the secondary, with the eclipse distinctly visible around phase 0.5.
• Figure 4: Zoomed-in view of the Lomb-Scargle periodograms of the TESS light curves of TIC 339374052 (alias BY Cam). Top panels: Sector 19. Bottom panels: Sector 59. The red lines mark the positions of the different periodicities that were measured (see Table \ref{['table:BYCam']}).
• Figure 5: Zoomed-in view of the Lomb-Scargle periodogram of the TESS light curve of TIC 228975750 (alias IGR J19552+0044) (sector 54). The red lines mark the positions of the different periods that were measured (see Table \ref{['table:IGRJ19552']}).