Activity-period alias in radial-velocity sampling

An activity-period alias in radial-velocity sampling is an apparent period created when a stellar activity signal is measured at a restricted set of observing times. For observations exactly separated by $1/f_s$, a sinusoid of frequency $f$ is indistinguishable at those times from appropriate sinusoids at $|f\pm n{f}_s|$, with integer $n$. Thus a genuine stellar rotation signal can produce peaks at periods different from the rotation period.

Real observing times are irregular, so the aliases have different amplitudes and phases rather than being perfectly equivalent. Simulating a candidate signal at those same times helps test which observed peaks it predicts. The method applies to planetary signals as well as stellar noise; a periodogram peak alone does not identify a new exoplanet.^[ref]