Scintillation‐induced Intermittency in SETI

We consider interstellar scintillations as a cause of intermittency in radiosignals from extraterrestrial intelligence (ETI). We demonstrate thatscintillations are very likely to allow initial detections of narrowbandsignals from distant sources (> 100 pc), while making redetections improbable.We consider three models in order to assess the non-repeating, narrowbandevents found in recent SETI and to analyze large surveys in general: (I)Radiometer noise; (II) A population of constant Galactic sources undergoinginterstellar scintillation,; and (III) Real, transient signals (or hardwareerrors) of either terrestrial or ET origin. We apply likelihood and Bayesian tests of the models to The PlanetarySociety/Harvard META data. We find that Models II and III are both highlypreferred to Model I, but that Models II and III are about equally likely.Ruling out Model II in favor of Model III requires many more reobservationsthan were conducted in META *or* the reobservation threshold must be much lowerthan was used in META. *We cannot rule out the possibility that META events arereal, intrinsically steady ETI signals.* We recommend that future surveys use thresholds far below the typicalfalse-alarm threshold to lessen the effects of intermittency. The thresholdlevel is best defined in terms of the recording and computational technologythat is available at a cost commensurate with other survey costs.Comment: 59 pages, LaTeX using aaspp4 style file, 12 figures in 14 PostScript figures, ApJ, in press, 1997 Oct.