CSIRO/David McClenaghan

Unexplained radio waves aren't uncommon at radio telescopes. But while it's probable that some of these signals are from outer space, the source of a mysterious type of radio signal known as perytons is something a little closer to home.

These signals are very similar to the dispersion of an astrophysical pulse through tenuous cold plasma -- what is known as a "fast radio burst." These FRBs are unexplained to this day... but the origin of the peryton has now been revealed.

"Until now, the physical origin of the dispersion-mimicking perytons had remained a mystery. We have identified strong out-of-band emission at 2.3-2.5 GHz associated with several peryton events," wrote Emily Petroff and her team at Australia's Parkes Observatory in a paper uploaded to arXiv.

"Subsequent tests revealed that a peryton can be generated at 1.4 GHz when a microwave oven door is opened prematurely and the telescope is at an appropriate relative angle."

This means that if you open a microwave door before it has finished microwaving, it releases a short but strong radio signal. When a radio telescope is angled towards the direction of that microwave, it will pick up that signal -- which bears a strong resemblance to a FRB.

There is one key difference, though: perytons are visible over a wide field-of-view, indicating that they are in the near field (close to us); FRBs, on the other hand, emanate from a single source. Moreover, Petroff noted, they predominantly occurred on weekdays and during office hours.

In order to discover the origin of perytons, Petroff and her team installed a real-time radio interference monitor on the site.

Sure enough, in January, the monitor picked up three signals at the same time as the telescope registered perytons. As they occurred in the frequency range emitted by microwave ovens, the team decided to test the observatory's units -- and found they could produce perytons by opening the oven door while the microwave oven was running.

"The two ovens responsible for most or all of the observed perytons are from the same manufacturer (Matsushita/National) and are both in excess of 27 years of age though still working reliably. Our tests point clearly to the magnetron itself as the source of the perytons since these are not detected unless the oven door is opened," Petroff wrote.

"Further, our analysis of the peryton cluster of 23rd June 1998 implies the perytons are a transient phenomenon that occurs only when the magnetron is switched off. That we have observed perytons from at least two ovens over 17 years suggests that they are not the product of an unusual failure or fault but are inherent to, and long-lived in, at least some common types of oven."

The good news is that this study was able to determine that perytons and FRBs are distinct from one another -- and that the microwaves at the Parkes Observatory could not have been responsible for FRB 010724, the first FRB ever detected by the telescope and the first known FRB.

The bad news is that perhaps it's time for the observatory to update its kitchen equipment.