One of the most famous events in the history of astronomy is the Wow! Signal that was picked up by Ohio State University’s Big Ear radio telescope in August 1977. The signal entered folklore because it couldn’t be explained by any known natural phenomena and didn’t appear to have originated from Earth.

That left open an extraordinary possibility: that it had been produced by an extraterrestrial civilization. Indeed, Jerry Ehman, the astronomer who noticed the signal in the data, was so shocked by it that he wrote “Wow!” on the data print out, which is how it got its name.

Since then, various astronomers have trained their telescopes on the same part of the sky in the constellation of Sagittarius looking for a repeat or some clue to its origin. But none of these studies has found anything unusual; so the mystery has never been solved.

Enter Abel Mendez at the University of Puerto Rico and colleagues who think they have solved the puzzle and say they know where the Wow! signal came from. If they are correct, the signal is entirely natural but also an example of an entirely new class of false positives that may crop up in other searches for extraterrestrial civilizations.

ET Signals

The Big Ear radio telescope was built in 1963 and spent much of its life looking for signs of extraterrestrial life (it was decommissioned in 1998). Astronomers have long imagined that a good part of the radio spectrum to search is around 1411 to 1419 MHz, close to the 1420 MHz line produced when atoms of hydrogen transition from their first excited state to their ground state.

This region is relatively quiet and so eminently suitable for interstellar or intergalactic communication, which should look very different to the faint, wideband signals produced by most natural phenomenon. That’s why a strong, narrowband signal in Big Ear’s data in 1977 immediately stood out.

Although nobody has observed a similar signal from that region of the sky, Mendez and co reasoned that if it had a natural origin, similar signals might have been recorded in other regions by other telescopes with much greater sensitivity. So they began a systematic search through data gathered at the same frequency and in a similar way by the famous Arecibo Telescope in Puerto Rico, until its demise in 2020.

And they found something interesting. “We report the detection of narrowband signals near the hydrogen line similar to the Wow! Signal, although two-orders of magnitude less intense and in multiple locations,” say Mendez and co. “The only difference between the signals observed in Arecibo and the Wow! Signal is their brightness.”

That is an important clue to their origin, say the team. They say the signals are easily identifiable as those produced by clouds of cold hydrogen when stimulated by other energetic phenomena.

The Wow! Signal was much brighter, however. This could have been possible if the cloud of hydrogen was stimulated to the extent that it acted as a maser, the microwave equivalent of a laser based on the stimulated emission of radiation from hydrogen atoms.

In theory, a maser — a device that amplifies electromagnetic waves through stimulated emission — could generate signals that are billions of times stronger than those from ordinary hydrogen clouds and so could easily account for the intensity of the Wow! Signal.

There are one or two uncertainties in all this. The first is that astronomers have never seen a hydrogen maser in space, although physicists have made them on Earth. So if Mendez and co are correct, the Wow! Signal would be the first recorded observation of an astrophysical maser.

Magnetar Mystery

The second problem is that masers need an energy source. Mendez and co suggest it must have been the sudden brightening of a magnetar or soft gamma repeater behind the cloud. Neither of these types of objects had been discovered in 1977 and would probably have been too faint to pick up with equipment available at the time.

That’s interesting work that purports to solve the Wow! Signal mystery for the first time. “Our hypothesis accounts for all observed characteristics of the Wow! Signal, introduces another source of false positives in technosignature searches, and suggests that this signal represented the first recorded astronomical maser flare in the hydrogen line,” say Mendez and co.

The task now is to search for more evidence to back up this theory. The exact position of the Wow! Signal in the sky is not clear because of the broad spatial resolution of the original Big Ear Telescope. But Mendez and co suggest that if a hydrogen gas cloud was responsible, then this should be discoverable with today’s more advanced equipment. “Given the detectability of the clouds, as demonstrated in our data, this insight could enable precise localization of the signal’s origin and permit continuous monitoring for subsequent events,” they say.

Then there is the maser that provided the energy. That too could be detectable, even though it must be more distant.

The next step is surely to find this evidence. Mendez and co are now searching through more archived data from the Arecibo Telescope but there must be other potentially revealing databases elsewhere. Let the search begin!

Ref: Arecibo Wow! I: An Astrophysical Explanation for the Wow! Signal : arxiv.org/abs/2408. 08513