ZTF’s fast areal survey rate means that it will obtain large numbers of repeated exposures across the night sky—more than 250 per field per year averaged over the sky area visible from Palomar. This data will enable detailed study of a wide variety of variable sources. ZTF will discover periodic variable stars such as RR Lyrae and Cepheid variables throughout the Galaxy, mapping its extent and evolution. ZTF will find large numbers of stellar and compact binaries, enabling studies of stellar evolution and fundamental physics. ZTF will characterize the variability of AGN on timescales from hours to years. ZTF-derived rotation periods can age-date stars through gyrochronology.
Some types of objects blur the boundaries between “transients” and “variables”. For example, CVs, AGN, and (some) M-dwarfs will persistently detected by ZTF but may exhibit scientifically interesting outbursts. The requirements imposed in order to discover such outbursts promptly are very similar to those discussed in Section \ref{sec:transients}, so we do not discuss them further here.
Our desire to identify and study large samples of variable objects drives several requirements. The first is to ensure that all sources are observed. Because CCDs are not perfectly buttable, there are gaps of \(\sim\)10 arcmin between adjacent CCDs. If ZTF observes on a single fixed pointing grid, sources in the gaps will never be observed, while adjacent areas will have nearly a thousand exposures at the end of the survey. This allocation is highly inefficient for variable studies.
SRD Requirement 1.1 To permit variability studies of variable sources across the entire sky accessible from Palomar, ZTF shall observe in a manner that provides regular observations of the sky falling in the chip gaps of the primary pointing grid.
Chip gaps may be filled in either by dithering—which may complicate image subtraction—or by observing using two offset field grids. The latter approach also helps equalize exposure in the vignetted corners of the CCD area and maximizes tiling efficiency for TOO exposures.
Variability studies improve with increasing number of exposures. For strictly periodic sources, with adequate sampling the period can usually be detected with roughly 50 exposures in a single band. More exposures over a long time baseline are still valuable, however, to search for secular changes, outburst, and non-periodic variations. To ensure ZTF can detect periodic variables over the largest possible area, we impose minimum exposure coverage requirements as well as uniformity goals:
SRD Requirement 1.1 ZTF shall observe all areas of the sky above Dec of -20\({}^{\circ}\) (goal: -30\({}^{\circ}\)) (inclusive of chip gaps) at least 50 times (goal: 100 times) in each of \(r\) and \(g\) bands.
Goal: ZTF should attempt to cover the sky to roughly uniform numbers of exposures during each year of its survey.
The observing cadence influences the effectiveness of searches for periodic variability. In particular, it is valuable to observe at a variety of timescales and to avoid introducing aliases from overly strict cadencing.
SRD Requirement 1.1 : ZTF should observe each field with a range of cadences from \(<\)1 hour to years, and should avoid strict regularity in its time sampling (e.g., at same time each sidereal day).
Variability science is often conducted using large-scale searches of photometric databases. We impose several requirements on the Data System to enable such searches:
SRD Requirement 1.1 The ZTF Data System shall include a database of sources detected by PSF photometry from each epochal image.
SRD Requirement 1.1 The ZTF Data System shall maintain a database (or databases) with lightcurves generated by positionally matching the source-detected photometry from different epochs. The lightcurve database shall support cone search queries for single objects, crossmatches to target lists and other catalogs with positional uncertainties, and queries based on simple lightcurve statistics (cf. \cite{2012ApJS..203...32R}).
Identifying variable source periods and other signals requires good relative photometric precision and rejection of outliers. We specify these requirements in the ZTF Photometric Requirements Document.
Faint sources with known positions from other surveys or from ZTF’s deep coadded reference frames can be searched for variability even when individual detections are below threshold using “forced photometry.” With ZTF’s large number of images, such searches could extend ZTF’s sensitivity for detecting periodic sources such as RR Lyrae.
Goal: The ZTF Data System should obtain forced photometry fluxes in each epochal image for every source detected by PSF photometry in the coadded reference image.