One of the most famous events in astronomical history was the Wow! signal, captured by Ohio State University’s Big Ear radio telescope in August 1977. The signal entered folklore because it could not be explained by any known natural phenomenon and did not appear to originate from Earth.
It opened up a huge possibility: it could have been created by an alien civilization. In fact, astronomer Jerry Eman, who discovered the signal in the data, was so shocked by it that he wrote “Wow!” on the data printout, which is how it got its name.
Since then, various astronomers have pointed their telescopes at the same part of the sky in the constellation Sagittarius, looking for clues to the repetition or origin. However, none of these studies have found anything unusual, so the mystery has never been solved.
Abel Mendez and colleagues at the University of Puerto Rico think they have solved the puzzle and say, wow! we know where the signal came from. If they are right, the signal is completely natural, but it is also an example of a completely new kind of false positive that can occur in other searches for extraterrestrial civilizations.
ET signal
The Big Ear radio telescope was built in 1963 and spent most of its time searching for signs of extraterrestrial life (it was decommissioned in 1998). Astronomers have long thought that a large portion of the radio spectrum to be searched is between 1411 and 1419 MHz, near the 1420 MHz line produced when hydrogen atoms transition from their first excited state to their ground state.
This region should be relatively quiet and well suited to interstellar or intergalactic communication, and should be very different from the faint, broadband signals produced by most natural phenomena. So the strong, narrowband signal was immediately noticeable in the 1977 Big Ear data.
No one had ever observed a similar signal in that region of the sky, but Mendez and his colleagues reasoned that if it had a natural origin, other, much more sensitive telescopes would have recorded similar signals in other areas. So they began systematically searching data collected by the famous Arecibo telescope in Puerto Rico at the same frequency and in a similar manner until it disappeared in 2020.
And they found something interesting. “We report the detection of a narrowband signal near the hydrogen lines that is similar to the Wow! signal, but is two orders of magnitude less intense and located in more places,” Mendez and colleagues said. “The only difference between the Arecibo signal and the Wow! signal is brightness.”
That’s a key clue to their origins, the team says. They say the signals are easily identifiable as being generated by cold hydrogen clouds when stimulated by other energetic events.
But wow! The signal was much brighter, if only because the hydrogen cloud was excited and acting as a maser. A maser is the microwave equivalent of a laser based on the stimulated emission of hydrogen atoms.
In theory, a maser, a device that amplifies electromagnetic waves through stimulated emission, could produce a signal billions of times stronger than the signal from a typical hydrogen cloud, which could easily explain the strength of the “Wow!” signal.
There are one or two uncertainties in all of this. The first is that astronomers have never seen a hydrogen maser in space. But physicists have created one on Earth. So if Mendez and colleagues are right, the wow! signal would be the first recorded observation of an astrophysical maser.
Magneta Mystery
The second problem is that the maser needs a source of energy. Mendez and colleagues suggest that the sudden brightening could have been a magnetar or soft gamma repeater behind the cloud. This type of object was not discovered until 1977 and would have been too faint to be detected by the equipment available at the time.
It is an exciting piece of work that claims to solve the mystery of the Wow! signal for the first time. “Our hypothesis explains all the observed features of the Wow! signal, introduces another source of false positives in the search for technical signatures, and suggests that this signal represents the first astronomical major flare ever recorded in the hydrogen line,” Mendez and colleagues said.
The challenge now is to find more evidence to support this theory. Because of the large spatial resolution of the original Big Ear telescope, the exact location of the Wow! signal in the sky is unclear. But Mendez and colleagues suggest that if a hydrogen gas cloud is the cause, it could be detected with today’s more advanced instruments. “Given the detectability of the cloud, as demonstrated by our data, this insight could help pinpoint the origin of the signal and enable ongoing monitoring for subsequent events,” they say.
And there are masers that provide energy. Those too can be detected, but they have to be further away.
The next step is certainly to find this evidence. Mendez and colleagues are currently searching through more archived data from the Arecibo telescope, but there are likely other databases out there that could potentially be uncovered. Let’s start searching!
Reference: Arecibo Wow! I: Astrophysical explanation for the Wow signal: arxiv.org/abs/2408.08513
