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Ask Dr. SETI ®

Chapter 6: Technology

Following Up on Past Detections

Dear Dr. SETI:

I see from your website that Project Argus has plateaued at ~150 telescopes, and has switched to a strategy of intermittently scanning the whole sky. There have been a number of signals of apparently extraterrestrial origin that have lasted a brief time and never been seen again. The WOW! signal, at 30 sigmas, is the standard for Project Argus. The 2010 signal from TYC 1220-91-1, a solar twin older than the sun, was at 300 sigmas IIRC and at Pi * 1420 MHz. Carl Sagan, in PALE BLUE DOT, mentions 11 such signals and that was about 20 years ago.
Benford has argued well that the best strategy for contacting another planetary system is to scan the Milky Way with a narrow beam, or sending quick bursts to nearby likely stars at high power. Such Benford Beacons would give such short bursts at long repetition periods, Would it be possible to devote some of the Project Argus telescopes to monitor the strongest signal locations continuously, so we can get a repetition period and confirm them?
Anonymous, via email

The Doctor Responds:

You raise some important questions about follow-up detection and signal verification. Unfortunately, the answer is not a simple one.

First off, the validity of the very kind of ongoing monitoring of known coordinates of past detections is widely recognized, and such activities are vigorously pursued. The coordinates of the Ohio State University "Wow!" signal, for example, are probably the most widely observed of any in the sky. Thousands of hours of observation have been conducted over the past 37 years, from all of the world's great radio telescopes (I was privileged to conduct one such set of observations myself, from NRAO Green Bank WV). They have all come up empty, but that does not invalidate the follow-up monitoring strategy.

The two most widely practiced SETI search strategies are the all-sky survey and the targeted search. They are compared and contrasted in this brief article:

The two strategies require different instrumentation designs. An all-sky survey is best conducted with relatively small, low-gain antennas, which exhibit a wide angular beamwidth, maximizing sky coverage. They tend to be fixed in orientation, operating in drift scan, or meridian-transit, mode.

Targeted searches require much larger, higher gain antennas with incredibly narrow beamwidths, which are operated in tracking mode to remain fixed upon a single set of celestial coordinates as the Earth rotates on its axis. Since the kind of follow-up activity you suggest is in effect a targeted search, it is best performed with instrumentation of the latter type.

Since Project Argus was designed as an all-sky survey, its stations are of the former design, optimized for drift-scan use, and thus not well suited to the activity which you propose. But, all is not lost.

Probably the best instruments currently in existence for long-term follow-up monitoring of past known sources are the 42 dishes of the Allen Telescope Array, aimed either individually or collectively. These larger, higher-gain antennas are narrow-beamwidth, fully steerable for continuous tracking, and have feed designs and receivers which are sufficiently frequency-agile to concentrate on the specific portion of the spectrum at which a given candidate signal was initially detected. And, in fact, the SETI Institute has been devoting a portion of the ATA's operating schedule to conducting the very kind of activity, and using the very targets, you have suggested! So, your idea certainly has merit.

As for Project Argus, its meridian-transit instruments will continue to scan the skies for other intermittent but interesting candidate signals. You can be sure that any found will warrant follow-up observations from the ATA and other such targeted-search instruments.

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