TESS Extended Mission 10 Minute Cadence Retains Nyquist Aliases

Abstract

During its two-year prime mission, the Transiting Exoplanet Survey Satellite (TESS;Ricker et al. 2014) is obtaining full-frame images with a regular 30-minute cadence in a sequence of 26 sectors that cover a combined 85% of the sky. While its primary science case is to discover new exoplanets transiting nearby stars, TESS data are superb for studying many types of stellar variability, with the number of publications using TESS data for other areas of astrophysics keeping pace with exoplanet papers. 1 Following the conclusion of its prime mission in July 2020, TESS will revisit the sky in an extended mission that records full-frame images at a faster ten-minute cadence. In this note, I demonstrate that choosing a large submultiple of the original exposure times for the new cadence limits the synergy between prime and extended TESS mission data since both sampling rates produce many of the same Nyquist aliases. Adjusting the extended mission exposure time by as little as one second would largely resolve Nyquist ambiguities in the combined TESS data set.Recording any periodic signal with a regular time sampling of ∆t will produce an infinite number of identical alias peaks in a periodogram that are reflected off of the Nyquist frequency, f Nyq = (2∆t) −1 . This means that there are an infinite number of candidate frequency solutions that do an equally good job of explaining the regularly sampled signal. Without a physical argument for choosing one of these candidates over the others, analyses of TESS data are ambiguous at best and inaccurate at worst. In these cases, it is typically necessary to obtain follow-up observations with a different cadence to resolve which peaks correspond to intrinsic frequencies (e.g., Bell et al. 2017). This strains observing resources, limits results to researchers with abundant telescope access, and is impractical for the millions of stars that TESS is observing. TESS itself could eliminate the need for such follow-up if it were to carry out its extended mission observations with a sample spacing that is not precisely a small submultiple of the original cadence. Figure 1 depicts the Lomb-Scargle periodograms of simulated light curves sampled at three rates: (a) once every 30 minutes; (b) once every 10 minutes; and (c) once every 11 minutes. The underlying source exhibits a single signal with an intrinsic frequency of 1000 µHz, resulting in an infinite set of aliases in each periodogram. Because the spacing between samples in (b) is a submultiple of the sampling of (a), all aliases in periodogram (b) are exactly coincident with aliases in (a), and the solution from the combined data set remains ambiguous. However, since the sampling of (c) is not so simply related to (a), the peak at the intrinsic frequency is easily identified as the only one that appears at the same location in both periodograms (a) and (c).The same conclusion holds for periodic signals that are not strictly sinusoidal, since all harmonics in the periodogram are also aliased to the same frequencies for 30-and 10-minu...

HD 183986: A High-contrast SB2 System with a Pulsating Component

“…In 2020 July, the mission was extended for the following two years and slightly modified. FFIs were obtained every 10 minutes (Bell 2020), and the number of targets with SC data was increased and, for very interesting targets, a data cadence as short as 20 s was also collected. HD 183986 was observed by TESS in Sector 14 from 2019 July 18 to August 15.…”

HD 183986: a high-contrast SB2 system with a pulsating component

“…In July 2020, the mission was extended for the following two years and slightly modified. The FFI are obtained every 10 minutes (Bell 2020), and the number of targets with SC data was increased and, for very interesting targets, the data cadence, as short as 20 seconds, was also collected.…”