Astronomers believe they have found a plausible explanation for the Wow signal!

An unusually bright burst of radio waves, dubbed the Wow! signal, discovered in the 1970s has since baffled astronomers, given the tantalizing possibility that it could come from an alien civilization trying to communicate with us. A team of astronomers thinks they have a better explanation, according to a preprint published on the physics site arXiv: clouds of atomic hydrogen that essentially act as a natural galactic maser, emitting a beam of intense microwave radiation when zapped by a flare from a passing magnetar.

As we’ve reported before, the Wow! signal was detected on August 18, 1977, by the Ohio State University radio observatory, known as “Big Ear.” Astronomy professor Jerry Ehman analyzed the Big Ear data in the form of printouts that, to the untrained eye, looked like a typewriter being run over by someone with a preference for smaller numbers. The numbers and letters in the Big Ear data essentially indicated the strength of the electromagnetic signal picked up by the telescope over time, starting with the ones digit and working up to the two-digit letters (A was 10, B was 11, and so on). Most of the page was covered in ones and twos, with a scattered six or seven.

But that day, Ehman discovered an anomaly: 6EQUJ5 (sometimes mistaken for a coded message in the radio signal). This signal had started at an intensity of six—already an outlier on the page—then climbed to E, then Q, peaked at U—the strongest signal Big Ear had ever seen—then faded again. Ehman circled the sequence in red pen and wrote “Wow!” next to it. The signal appeared to come from the constellation Sagittarius, and the entire signal lasted about 72 seconds. Alas, SETI researchers have never been able to detect the so-called “Wow!” signal again, despite numerous attempts with radio telescopes around the world.

One reason for the enthusiastic response is that such a signal had been proposed as a possible communication from extraterrestrial civilizations in a 1959 paper by Cornell University physicists Philip Morrison and Giuseppe Cocconi. Morrison and Cocconi believed that such a civilization might use the 1420 megahertz frequency naturally emitted by hydrogen, the most abundant element in the universe and, therefore, something that an extraterrestrial civilization would experience. In fact, the Big Ear had been reassigned to the SETI project in 1973 specifically to look for possible signals. Ehman himself was quite skeptical of the “could-be-aliens” hypothesis for several decades, although he admitted in a 2019 interview that “the Wow! signal certainly has the potential to be the first signal from extraterrestrial intelligence.”

Several other alternative hypotheses have been put forward. For example, Antonio Paris suggested in 2016 that the signal might have come from the hydrogen cloud surrounding a pair of comets, 266P/Christensen and 335P/Gibbs. This hypothesis, however, has been rejected by most astronomers, partly because comets do not emit strong waves at the relevant frequencies. Others have suggested that the signal was the result of interference from satellites orbiting Earth, or a signal from Earth reflected by space debris.

Master of space!

Astrobiologist Abel Mendez of the University of Puerto Rico at Arecibo and his coauthors believe they have found the strongest astrophysical explanation yet with their cosmic maser hypothesis. The team was actually looking for habitable exoplanets using signals from red dwarf stars. In some of the latest archival data collected from the Arecibo radio telescope (which collapsed in 2020), they noticed several signals that were remarkably similar to the Wow! signal in terms of frequency, but much less intense (bright).

Mendez admitted to Science News that he had always considered the Wow! signal to be a fluke—he certainly didn’t think it was aliens. But he realized that if the signals they were identifying had been brighter, even momentarily, they would look a lot like the Wow! signal. As for the mechanism that caused such brightness, Mendez et al. suggest that a magnetar (a highly magnetic neutron star) passing behind a cloud of atomic hydrogen could have flared up with enough energy to produce stimulated emission in the form of a tightly focused beam of microwave radiation—a cosmic maser. (Masers are related to lasers, except that they emit microwave radiation rather than visible radiation.)

Proving their working hypothesis will be much harder, although rare observations of such natural masers from hydrogen molecules have been made in space. But no one has ever spotted an atomic hydrogen cloud associated with a maser, and that is what would be needed to explain the intensity of the Wow! signal. That is why other astronomers are cautiously skeptical. “A magnetar will also produce (short) radio emissions. Is it really necessary for this complex maser phenomenon to also occur to explain the Wow! signal?” Michael Garrett of the University of Manchester told New Scientist. “I personally don’t think so. It makes a complicated story even more complicated.”

arXiv, 2024. DOI: 10.48550/arXiv.2408.08513 (About DOIs).