Astrophysics, abstract
astro-ph/9707039
From: Jim Cordes <cordes@spacenet.tn.cornell.edu>
Date: Wed, 2 Jul 1997 19:13:32 -0400
Scintillation-Induced Intermittency in SETI
Authors: James M. Cordes, T. Joseph W. Lazio, Carl Sagan (Cornell)
Comments: 59 pages, LaTeX using aaspp4 style file, 12 figures in 14 PostScript
figures, ApJ, in press, 1997 Oct. 1
We consider interstellar scintillations as a cause of intermittency in radio signals
from extraterrestrial intelligence (ETI). We demonstrate that scintillations are very
likely to allow initial detections of narrowband signals from distant sources (> 100
pc), while making redetections improbable. We consider three models in order to assess the
non-repeating, narrowband events found in recent SETI and to analyze large surveys in
general: (I) Radiometer noise; (II) A population of constant Galactic sources undergoing
interstellar scintillation,; and (III) Real, transient signals (or hardware errors) of
either terrestrial or ET origin.
We apply likelihood and Bayesian tests of the models to The Planetary Society/Harvard
META data. We find that Models II and III are both highly preferred 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 reobservations than were conducted in META *or* the reobservation
threshold must be much lower than was used in META. *We cannot rule out the possibility
that META events are real, intrinsically steady ETI signals.*
We recommend that future surveys use thresholds far below the typical false-alarm
threshold to lessen the effects of intermittency. The threshold level is best defined in
terms of the recording and computational technology that is available at a cost
commensurate with other survey costs.
|