A super-powerful cosmic laser may be the source of a mysterious signal that has puzzled astronomers for nearly 50 years. No, it’s not a warning shot from an alien civilization, but rather the work of a hydrogen cloud and a neutron star; the dense core of a massive star spewing its innards into space after a supernova.

A new hypothesis suggests that under certain conditions such an object could emit a signal similar to the one that astonished astronomer Jerry Ehman on August 15, 1977, and caused him to write the word “Wow!” on a printout.

The study, available as a preprint on arXiv, is currently in its final stages, but its results build on similar signals recently discovered and suggest that the team led by astrobiologist Abel Mendez of the University of Puerto Rico in Arecibo is on the right track.

“Our latest observations, made between February and May 2020, revealed similar narrowband signals near the hydrogen line, although less intense than the original Wow signal!” explains Mendez.

This original signal was detected by Ehman in data collected by Ohio State University’s Big Ear radio telescope. It lasted only 72 seconds and was an extremely powerful burst of radio waves near the 1420 MHz hydrogen line, the wavelength emitted by hydrogen when it changes spin.

Given that hydrogen is ubiquitous in the universe, some scientists think light at this frequency could serve as an easily recognizable landmark on the electromagnetic spectrum that technologically advanced aliens could use to signal their presence.

If the famous pulse was indeed a distant “hello,” it contained no resolvable modulation. The signal did not move like “Wow!”, making it an unlikely close friend, and it also never repeated as we might expect from an intentional transmission, ruling out repeated observations with advanced technology. Many natural objects emit radio waves at a wavelength of at most 1420 MHz, so while we can’t completely rule out aliens, it seems unlikely.

That left a natural explanation for the signal, and that’s what Mendez and his team tried to do. Specifically, they looked for bursts of short-band radio waves with a frequency similar to Wow! in archival data from the now-defunct Radio Emissions from Red Dwarf Stars (REDS) project at the Arecibo Observatory.

They found four records worthy of further study, coming from the neighborhood of a small red dwarf called Teagarden’s Star, located just 12.5 light-years away. Although weaker than the “Wow!” signal, they were similar enough to provide clues to the source of the famous signal.

The team’s analysis suggests that signals near Teegarden’s star are associated with interstellar clouds of cold hydrogen, suggesting a potential mechanism for the Wow! signal: A powerful burst of radiation passing through the cold hydrogen cloud excites the gas to produce a powerful pulse of radiation around 1420 MHz.

The final product will be a naturally occurring astrophysical microwave laser, or maser, that creates an intense concentration of narrow-band light in the hydrogen line that resembles a “Wow!” signal from a distance.

As a red dwarf, Teegarden’s star is unlikely to produce a flare powerful enough to produce a burst of radiation similar to the “Wow!” signal, but other cosmic objects may be up to the task. Neutron stars, known as magnetars, have extremely strong magnetic fields that can explode in explosions powerful enough to be detected in intergalactic space, and are thought to be responsible for mysterious signals known as fast radio bursts.

Another type of magnetic or neutron star, called a soft gamma-ray relay, could also be responsible. This is a type of star that periodically emits gamma rays or X-rays. In either case, the team’s model suggests that a pulse of light passes through the hydrogen, causing stimulated emission in the hydrogen band.

That’s a very nice explanation. Naturally, it would be very rare for objects and events to come together in this way; it would be even rarer for them to be directed in our direction. So the hypothesis not only explains the signal, it also explains why we only see it once.

“Our hypothesis explains all observed features of the Wow! signal, reveals another source of false positives in the search for technosignatures, and suggests that this signal is the first astronomical hydrogen line maser explosion recorded,” the researchers write in their draft paper.

“We will continue to work on the Arecibo Wow! project, exploring the extensive observations we have from Arecibo REDS. Future research will include archival data from the Arecibo Observatory.” The work, which has not yet been published in a journal, is available on arXiv.