Memo 97 The Square Kilometre Array as a Radio Synoptic Survey Telescope: Widefield Surveys for Transients, Pulsars and ETI

This document explores some of the issues involved in searching for transients, pulsars, and ETI signals with a widefield instrument. We assume also that steady sources are surveyed at the same time, including spectral lines and continuum radiation from galaxies and AGNs. We present a survey metric that quantifies survey completeness and incorporates time-variable sources with arbitrary durations and event rates. We discuss the role of propagation effects in surveys. Some radio sources are intensely modulated, such as giant pulses from the Crab pulsar with structure as short as 0.4 ns, variations in active galactic nuclei on time scales of days and longer, and GRB afterglows on day to month time scales. Apart from pulsars, we know about such variations from observations that target previously detected sources, as opposed to discovering them via their time variability in blind surveys. The transient or dynamic radio sky is rich enough to make it plausible that blind surveys will discover many sources of both known and unknown types. Schemes for conducting searches are outlined in the context of the Large-N-Small-D concept for the Square Kilometer Array, though they are applicable to large single reflectors and aperture array concepts with minimal changes. The particular science addressed is for mid-range frequencies, e.g. 0.3 to 10 GHz. The Square Kilometer Array (SKA), properly designed, can provide the capabilities needed — high sensitivity, wide field of view, and flexibility in processing the time and frequency domains — to serve as a radio synoptic survey telescope (RSST). An example synoptic cycle is presented that includes different scan rates for extragalactic and Galactic science; it also includes options for non-survey projects and targets of opportunity triggered by other telescopes. Further work is neeeded to confirm that surveys for transients, pulsars and ETI can be done simultaneously with those for steady sources, such as galaxy HI surveys and continuum source surveys, including polarimetry to yield Faraday rotation measures. The example synoptic cycle underscores the need for field-of-view expansion through use of multiple-pixel receivers in order to conduct a large-scale galaxy survey in a reasonable amount of time. In addition, investigation is needed on whether follow-up monitoring observations, such as pulsar timing, can be encompassed in the survey mode or instead require special pointings outside the survey paradigm. Finally, requirements for calibration and backend processing systems need to be identified.