One of the most famous events in the history of astronomy is the “Wow! Signal” captured by Ohio State University’s Big Ear radio telescope in August 1977. The signal became legendary because it could not be explained by any known natural phenomenon and did not appear to originate from Earth.
This left open the startling possibility that it was generated by an extraterrestrial civilization. In fact, when astronomer Jerry Ehman noticed a signal in the data, he was so struck by it that he wrote “Wow!” on a printout of the data, hence the name of the signal.
Since then, various astronomers have aimed their telescopes at the same part of Sagittarius, searching for a repeat of the phenomenon or for clues as to its origin, but these studies have not found anything unusual, and the mystery remains unsolved.
Abel Mendez of the University of Puerto Rico and his colleagues believe they have solved the mystery and claim to know the source of the Wow! signal. If they’re right, the signal is entirely natural, but it also represents an entirely new kind of false positive that could emerge in the search for extraterrestrial civilizations.
ET signal
The Big Ear radio telescope was built in 1963 and spent most of its life searching for signs of extraterrestrial life (it was decommissioned in 1998). Astronomers have long imagined that the bulk of the radio spectrum to be explored was around 1411-1419 MHz, which is close to the 1420 MHz line produced when hydrogen atoms go from their first excited state to their ground state.
This region is relatively quiet and extremely suitable for interstellar or intergalactic communication, which should be quite different from the weak broadband signals produced by most natural phenomena, so the strong narrowband signals stood out immediately in the 1977 Big Year data.
No one had observed a similar signal from that region, but Mendez and his colleagues reasoned that if it was of natural origin, similar signals might have been recorded in other regions by other, much more sensitive telescopes. So they began a systematic review of data collected at the same frequency and in a similar way from the famous Arecibo telescope in Puerto Rico until it was decommissioned in 2020.
And they found something intriguing: “We report the detection of a narrowband signal near the hydrogen line that is similar to the Wow! signal, but two orders of magnitude lower in intensity, and detected at multiple locations,” Mendes and his colleagues say. “The only difference between the signal observed at Arecibo and the Wow! signal is its brightness.”
This is a key clue to its origin, the team say: the signal is easily identifiable as one produced by a cloud of cold hydrogen stimulated by other energetic phenomena.
But the Wow! signal was much brighter, which could have been possible if the hydrogen cloud had been stimulated to the point where it acted as a maser, a microwave equivalent of a laser based on the stimulated emission of radiation from hydrogen atoms.
In theory, masers (devices that amplify electromagnetic waves by stimulated emission of radiation) could easily explain the strength of the “Wow! signal,” as they could generate a signal billions of times stronger than that emitted by a typical hydrogen cloud.
There are one or two uncertainties in all this. First, although physicists have made hydrogen masers on Earth, astronomers have never seen one in space. So if Mendes and his colleagues are right, the Wow! signal would be the first recorded observation of an astrophysical maser.
The Mystery of Magnetars
The second problem is that masers need an energy source. Mendes and his colleagues suggest that the masers may be the result of a 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 have been too faint to detect with the instruments of the time.
This is an intriguing study that attempts to unravel the mystery of the WOW! signal for the first time: “Our hypothesis explains all the observed features of the WOW! signal, presents a new source of false positives in technosignature searches, and suggests that this signal is the first recorded astronomical maser flare in hydrogen lines,” say Mendes and his colleagues.
The challenge now is to find more evidence to support this theory. Due to the wide spatial resolution of the original Big Ear telescope, the exact location of the Wow! signal is unknown; however, Mendes et al. suggest that if a hydrogen gas cloud was responsible, it should be possible to spot it with today’s more advanced instruments. “Given the detectability of the cloud, as demonstrated by our data, this insight may allow us to pinpoint the source of the signal and potentially enable ongoing monitoring of subsequent events,” they say.
And then there are the masers that we’ve provided the energy for, which would be even further away, but could potentially be detected.
The next step will undoubtedly be to find this evidence. Mendez and his team are currently searching further archived data from the Arecibo Telescope, but there are surely other databases out there that could reveal more. Let the search begin!
Reference: Arecibo Wow! I: Astrophysical explanation of the Wow! signal: arxiv.org/abs/2408.08513
