Recently we looked at James and Gregory Benford’s thoughts on interstellar beacons, noting that using cost as a likely constraint allowed the authors to discuss how cost would affect design, and therefore the parameters of any beacon we would be likely to observe. But what is it about interstellar beacons that sets them apart from transient phenomena? After all, it was no longer ago than 1963 that Nikolai Kardashev proposed that the radio source CTA 102 could be evidence of a Type II or III extraterrestrial civilization (i.e., one that is able to use the entire energy output of its star, or in the most extreme case, of its entire galaxy).
When Gennady Sholomitskii announced his observation that CTA 102’s radio emission was varying, something of a sensation ensued. Those of us of a certain age can recall Roger McGuinn’s song ‘CTA 102,’ written and performed by McGuinn’s group The Byrds. It was on their Younger Than Yesterday LP, released in 1967. A sample:
Year over year receiving you
Signals tell us that you’re there
We can hear them loud and clear
and so on. We soon learned, of course, that the source of these emissions was a quasar, one that has since been observed by a huge range of instruments. But the question that lingers is how we would separate out an atypical pulsar that might be producing odd transients from a genuine interstellar beacon. It’s a problem James Benford attacks in a new paper.
A Puzzling Transient Analyzed
Take the case of PSR J1928+15, a transient bursting source observed in 2005 near the galactic disk at 1.44 GHz. A two-minute observation by the Arecibo dish noted the signal but did not find it again despite 48 minutes of revisits. Three pulses were received, according to Benford’s paper, the first and third down a factor of ten from the 0.180 Jy central pulse. The source is roughly 24,000 light years away, putting it close to galactic center.
A pulsar? Pulsars are marked by radiation from a rotating neutron star’s magnetosphere. One explanation for this event is an asteroid falling into the neutron star from a circumpulsar disk, perturbing its magnetosphere. But because we’re trying to learn how to distinguish a beacon from a natural source, Benford looks at how we might analyze the observational data in ways that would allow us to deduce a beacon’s parameters. It’s a fascinating exercise:
We make two working assumptions:
1) The Beacon is a ‘lighthouse’ scanning the galactic plane. The source is a scanning beacon and, as it swept past, Arecibo caught the central pulse, the true beam. The first and third pulses are at the edges of the antenna’s acceptance angle, which is 3.5 arcmin=1 mrad.
2) The beam bandwidth covers all channels of the 100 MHz span of the detector array. (The channel BWs are 0.39 MHz, with total BW 100 MHz.) This assumption drives the Beacon power estimate.
If this is the case, then we can start to plug in values to make sense of the signal. Benford tries out a beacon antenna diameter of 100 kilometers, working out a total power of 190,000 TW. Think of this as a beacon and you are dealing with a civilization much more advanced and powerful than our own. It’s one that ranks above Kardashev Type I but falls far short of Kardashev Type II. Benford would rank it at Kardashev 1.13 (Earth is 0.73 on this scale).
Moreover, if the beacon is scanning the disk at a thickness of 1300 light years (roughly what the disk thickness is at our distance), then the signal cycle can be estimated. Benford works out a cycle around the galactic circumference of roughly fifteen hours, noting “It’s understandable that 48 minutes of revisits hasn’t seen it again, as that is only 5% of the revisit time. Of course, it could be scanning a smaller area, so that the revisit time would be sooner.”
Playing with Beacon Assumptions
Assume an antenna diameter of 1 kilometer and interesting changes to the conclusions occur:
The beamwidth is reduced by a factor of 10 to 5 x 10-4 rad. Spot size diameter falls to 12.2 ly, As~117 ly2. Power in the spot falls to 1900 TW, Kardashev scale falls to K=0.93. This is a civilization intermediate between ours and the planetary scale civilization of the previous example.
The spot moves at the same rate, 30 ly/sec. But since the spot is smaller, the number of strips in the scan increases to 1350ly/12.2ly = 110. So the Beacon will return in 110 x 5 x 103 sec = 5.5 x 105 sec = 150 hours. Observers have revisited the site for 48 minutes, only 0.5% of the revisit time, and haven’t seen it again.
So a civilization lower on the Kardashev scale, i.e., K= 0.93 will have a narrower beam, revisit less frequently, be harder to observe…
To reach a civilization at this level given the energy growth rate we see on Earth during the 20th Century would require about 2000 years, a small time on the cosmic scale. This corresponds to a civilization of Kardashev 0.93, not yet a full Type I.
The broader principles: We can begin to distinguish beacons from pulsars by bandwidth, for pulsars have large bandwidths. But bandwidth by itself is not definitive, because advanced methods of microwave generation might allow very broadband emissions with huge data transmission rates. We can also add in pulse length (“[c]ost optimized Beacons will likely be pulsed to lower cost, with a preference for shorter pulses due to source physics”) and frequency. Benford notes about the latter that pulsar searches cluster in the lower end of the microwave, but beacons are more likely to appear at higher frequencies “due to the favorable scaling of cost with frequency.”
Learning how we would set about observing a candidate beacon signal is not only an ingenious exercise in itself, but a necessary warm-up in case of future detections of even more puzzling transients than PSR J1928+15. The fact that natural phenomena can produce some of the same observables as an interstellar signal behooves us to sharpen our tools for analyzing and differentiating between such signals. The paper is James Benford, “How can we distinguish transient pulsars from SETI beacons?” (preprint available).