SETI’s task challenges the imagination in every conceivable way, as Don Wilkins points out in the essay below. A retired aerospace engineer with thirty-five years experience in designing, developing, testing, manufacturing and deploying avionics, Don is based in St. Louis, where he is an adjunct instructor of electronics at Washington University. He holds twelve patents and is involved with the university’s efforts at increasing participation in science, technology, engineering, and math. The SETI methodology he explores today offers one way to narrow the observational arena to targets more likely to produce a result. Can spectacular astronomical phenomena serve as a potential marker that could lead us to a technosignature?
by Don Wilkins
Finite SETI search facilities searching a vast search volume must set priorities for exploration. Dr. Jill Tarter, Chair Emeritus for SETI Research, describes the search space as a “nine-dimensional haystack” composed of three spatial, one temporal (when the signal is active), two polarization, central frequency, sensitivity, and modulation dimensions. Methods to reduce the search space and prioritize targets are urgently needed.
One method for limiting the search volume is the SETI Ellipsoid, Figure 1, which is reproduced from a recent paper in The Astronomical Journal by lead author James R. A. Davenport (University of Washington: Seattle) and colleagues. 
Image: This is Figure 1 from the paper. Caption: Schematic diagram of the SETI Ellipsoid framework. A civilization (black dot) could synchronize a technosignature beacon with a noteworthy source event (green dot). The arrival time of these coordinated signals is defined by the time-evolving ellipsoid, whose foci are Earth and the source event. Stars outside the Ellipsoid (blue dot) may have transmitted signals in coordination with their observation of the source event, but those signals have not reached Earth yet. For stars far inside the Ellipsoid (pink dot), we have missed the opportunity to receive such coordinated signals. Credit: Davenport et al.
In this approach, an advanced civilization (black dot) synchronizes a technosignature beacon with a significant astronomical event (green dot). The astronomical event, in the example, is SN 1987A, a type II supernova in the Large Magellanic Cloud, a dwarf satellite galaxy of the Milky Way. The explosion occurred approximately 51.4 kiloparsecs (168,000 light-years) from the Sun.
Arrival time of the coordinated signals is defined by a time-evolving ellipsoid, with foci at Earth (or an observation station within the Solar System) and the source event. The synchronized signals arrive from an advanced civilization based on the distance to the Solar System or other system with a technological system (d1), and the distance from the advanced civilization to the astronomical event (d2). Signals from civilizations (blue dot) outside the Ellipsoid coordinated with the source event have not reached the Solar System. Stars inside the Ellipsoid (pink dot) but on line between the advanced civilization and the Solar System will not receive the signals intended for the Solar System. However, the advanced civilization could beam new signals to the pink star and form a new Ellipsoid.
The source event acts as a “Schelling Point” to facilitate communication between observers who have not coordinated the time or place of message exchanges. A Schelling point is a game theory concept which proposes links can be formed between two would-be communicators simply by using common references, in this case a supernova, to coordinate the time and place of communication. In addition to supernovae, source events include gamma-ray bursts, binary neutron star mergers, and galactic novae.
In conjunction with the natural event which attracts the attention of other civilizations, the advanced civilization broadcasts a technosignature signal unambiguously advertising its existence. The technosignature might, as an example, mimic a pulsar’s output: modulation, frequency, bandwidth, periods, and duty cycle.
The limiting factor in using the SETI Elliposoid to search for targets is the unavailability of precise distance measurements to nearby stars. The Gaia project remedies that problem. The mission’s two telescopes provide parallaxes, with precision 100 times better than its predecessors, for over 1.5 billion sources. Distance uncertainties are less than 10% for stars within several kiloparsecs of Earth. This precision directly translates into lower uncertainties on the timing for signal coordination along the SETI Ellipsoid.
“I think the technique is very straightforward. It’s dealing with triangles and ellipses, things that are like high-school geometry, which is sort of my speed,” James Davenport , University of Washington astronomer and lead author in the referenced papers, joked with GeekWire. “I like simple shapes and things I can calculate easily.” 
An advanced civilization identifies a prominent astronomical event, as an example, a supernova. It then determines which stars could harbor civilizations which could also observe the supernova and the advanced civilization’s star. An unambiguous beacon is transmitted to stars within the Ellipsoid. The volume devoted to beacon propagation is significantly reduced, which reduces power and cost, when compared to an omnidirectional beacon.
At the receiving end, the listeners would determine which stars could see the supernova and which would have time to send a signal to the listeners. The listening astronomers would benefit by limiting their search volume to stars which meet both criteria.
For example, astronomers on Earth only observed SN 1987A in 1987, thirty six years ago. If the advanced civilization beamed a signal at the Solar System a century ago, our astronomers would not have the necessary clue, the observation of SN 1987A, to select the advanced civilization’s star as the focus of a search. Assuming both civilizations are using SN 1987A as a coordination beacon, human astronomers should listen to targets within a hemisphere defined by a radius of thirty-six light-years.
The following is written with apologies to Albert Einstein. The advanced civilization could observe the motion of stars and predict when a star will come within the geometry defined by the Ellipsoid. In the case of the Earth and SN1987A, the advanced civilization could have begun transmissions thirty-six years ago.
The recently discovered SN 2023ixf in the spiral galaxy M101 could serve as one of the foci of an Ellipsoid. 108 stars within 0.1 light-year of the SN 2023ixf – Earth SETI Ellipsoid. 
Researchers propose to use the Allen Telescope Array (ATA), designed specifically for radio technosignature searches, to search this Ellipsoidal. The authors point out the utility of the approach and caution about its inherent anthropocentric biases:
“…there are numerous other conspicuous astronomical phenomena that have been suggested for use in developing the SETI Ellipsoid, including gamma-ray bursts (Corbet 1999), binary neutron star mergers (Seto 2019), and historical supernovae (Seto 2021). We cannot know what timescales or astrophysical processes would seem “conspicuous” to an extraterrestrial agent with likely a much longer baseline for scientific and technological discovery (e.g., Kipping et al. 2020; Balbi & Ćirković 2021). Therefore we acknowledge the potential for anthropogenic bias inherent in this choice, and instead focus on which phenomena may be well suited to our current observing capabilities.”
1. James R. A. Davenport , Bárbara Cabrales, Sofia Sheikh , Steve Croft , Andrew P. V. Siemion, Daniel Giles, and Ann Marie Cody, Searching the SETI Ellipsoid with Gaia, The Astronomical Journal, 164:117 (6pp), September 2022, https://doi.org/10.3847/1538-3881/ac82ea
2. Alan Boyle, How ‘Big Data’ could help SETI researchers intensify the search for alien civilizations, 22 June 2022, https://www.geekwire.com/2022/how-big-data-could-help-seti-researchers-intensify-the-search-for-alien-civilizations/
3. James R. A. Davenport, Sofia Z. Sheikh, Wael Farah, Andy Nilipour, B´arbara Cabrales, Steve Croft, Alexander W. Pollak, and Andrew P. V. Siemion, Real-Time Technosignature Strategies with SN2023ixf, Draft version June 7, 2023.