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Theoretical modeling suggested kilonovas should produce tellurium, but the detection of a spectral line by the James Webb Space Telescope provided experimental evidence.

A spectral line is a dark or bright line within a continuous spectrum.

But the researchers didn’t observe phosphine’s spectral line.

An example of a distinct technological signal would be a spectral line with a puzzling radiation frequency that does not correspond to any known atomic or molecular transition.

An unidentified spectral line can be produced artificially by tunable lasers, like the free-electron lasers that our civilization developed to generate bright emission centered on a single frequency that could range from microwaves through terahertz radiation, to infrared, visible, ultraviolet or even x-rays.