In 1977, astronomers received a strong, unusual radio signal coming from the direction of the constellation Sagittarius. Its frequency was that of neutral hydrogen, and astronomers had speculated that all ETIs attempting to communicate would naturally use this frequency. The signal, dubbed the Wow! Signal, has since become a myth in the SETI world.
But what was it?
Beginning in the 1970s, Ohio State University's Big Ear radio telescope was used in the university's Search for Extraterrestrial Intelligence (SETI) program, which ran from 1973 to 1995. This program is the longest-running SETI program in history.
In 1977, Big Ear discovered a strange signal that has taken on a life of its own: the Wow! signal. The Wow! signal was a strong narrow-band radio signal that was exactly near the frequency of neutral hydrogen. The Big Ear telescope is long gone, but efforts to find out what the signal is live on.
The signal lasted the entire 72-second window that Big Ear could observe it. A few days later, astronomer Jerry R. Ehman was looking at the data when he spotted the signal on a computer printout. Astronomers had never seen anything like it before, and he wrote “Wow!” next to it, and the name has stuck ever since.
The 1977 Wow! signal, discovered by astronomer Jerry R. Ehman. Image via Big Ear Radio Observatory and North American AstroPhysical Observatory (NAAPO).
The signal has another name: 6EQUJ5. This has been interpreted as a message hidden in the signal, but actually represents how the intensity of the signal has changed over time.
This image is a plot of the intensity of the Wow! signal over time. Image credit: By Maxrossomachin – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=16197844
The signal caused a great deal of excitement. Some thought it was of extraterrestrial origin, others thought it might be some kind of human interference, and still others thought it might be an unexplained natural phenomenon.
New research shows that there is a completely natural explanation for the Wow! signal.
The research paper is titled “Arecibo Wow! I: An Astrophysical Explanation for the Wow! Signal.” The lead author is Abel Méndez of the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo. The paper is available on the preprint server arxiv.org.
Arecibo Wow! is a new project based on an archival study of data from the now-defunct Arecibo radio telescope from 2017 to 2020. Arecibo's observations are similar to those from Big Ear, but are “more sensitive, have better temporal resolution, and include polarization measurements,” the authors say.
“Our recent observations, conducted between February and May 2020, have revealed similar narrowband signals near the hydrogen line, albeit less intense than the original Wow! signal,” said Méndez.
Arecibo discovered signals similar to the Wow! signal, but with some differences. They are far less intense and come from different locations. The authors say that these signals are easily explained by an astrophysical phenomenon, and the same is true for the original Wow! signal.
“We suspect that the Wow! signal was caused by a sudden brightening of the hydrogen line by stimulated emission from a strong, transient radiation source such as a magnetar flare or a soft gamma repeater (SGR),” the researchers write. Such events are rare and depend on precise conditions and orientations. They can cause hydrogen clouds to brighten significantly for seconds or even minutes.
This simple schematic shows how the Wow! signal was generated and detected. A radiation source such as a magnetar or a soft gamma repeater is located behind a cloud of cold, neutral hydrogen. The energy of the source stimulates the emission of the HI cloud, which abruptly brightens and is visible from Earth. Image credit: Méndez et al. 2024.
The researchers say that in 1977, Big Ear observed a temporary brightening of one of several H1 (neutral hydrogen) clouds in the telescope's line of sight. The 1977 signal was similar in many ways to what Arecibo saw. “The only difference between the signals observed in Arecibo and the Wow! signal is their brightness. It is precisely the similarity between these spectra that suggests a mechanism for the origin of the mysterious signal,” the authors write.
These signals are rare because the spatial alignment between source, cloud and observer is rare. The rarity of the alignment explains why discoveries are so rare.
The researchers were able to identify the clouds responsible for the signal, but not the source. Their results suggest that the source is much further away than the clouds that generate the hydrogen signal. “Given the detectability of the clouds demonstrated in our data, this finding could enable precise location of the signal origin and enable continuous monitoring of subsequent events,” the researchers explain.
The Wow! signal was originally interpreted by many as a technosignature. By explaining where the signal came from, this research reveals a new source of false news.
“Our hypothesis explains all observed properties of the Wow! signal, proposes a new source of false positives in the search for technosignatures, and suggests that the Wow! signal may be the first recorded event of an astronomical maser flare in the hydrogen line,” the authors explain in their conclusion.
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