“We hypothesize that the Wow! Signal was caused by sudden brightening from stimulated emission of the hydrogen line due to a strong transient radiation source, such as a magnetar flare or a soft gamma repeater (SGR),” the researchers write. Those events are rare and rely on precise conditions and alignments. They can cause clouds of hydrogen to brighten considerably for seconds or even minutes.

The researchers say that what Big Ear saw in 1977 was the transient brightening of one of several H1 (neutral hydrogen) clouds in the telescope’s line of sight. The 1977 signal was similar to what Arecibo saw in many respects. “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 alignment explains why detections 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 more distant than the clouds that produce the hydrogen signal.