An unusual radio signal in space was discovered last year; it appeared and disappeared every 18 minutes.
Even though astronomers anticipate detecting periodic radio transmissions in space, these signals typically blink on and off at far higher rates. The majority of recurring signals come from pulsars, which are neutron stars that spin and generate powerful beams like lighthouses, causing them to blink on and off as they move in orbit around the Earth.
As a pulsar age, its radio emission slows down until it ceases to emit any radio waves at all. A magnetar, a type of pulsar with extraordinarily complex and powerful magnetic fields that may sustain radio wave production for months at a time, is the most likely explanation for our pulsar’s abnormally slow rotation rate.
Unfortunately, we were able to identify the origin using data from 2018. It was 2020 when we reviewed the data and found what we assumed was a magnetar, but by then it had stopped emitting radio waves. Our magnetar theory couldn’t be put to the test without more information.
Nothing new under the sun
The cosmos we live in is huge, and every new phenomenon we’ve found has been seen before. We were confident that if we looked again, this time with well-planned observations, we would identify yet another long-period radio source.
So, we scanned the Milky Way every three nights over the course of several months using the Murchison Widefield Array radio telescope in Western Australia.
There wasn’t much of a wait. Almost immediately after we began our search, we discovered a second source in a different region of the sky, repeating once every 22 minutes.
Finally, the time had come that we had all been anticipating. We made as many observations as we could using every available radio, X-ray, and optical telescope because we knew it probably wouldn’t be active for long. The duration of each pulse was five minutes, and there were 17-minute breaks in between them. Compared to pulsars, our item spun 1,000 times slower.
Hiding in plain sight
The real shocker occurred when we looked through the earliest radio observations of this sector of the sky. The longest data set is stored at the Very Large Array in New Mexico, USA. Every year we looked at, we identified pulses from the source, with the earliest one coming from an observation in 1988.
A single tile of the hundreds that make up the Murchison Widefield Array, a radio telescope in outback Western Australia. Credit: Natasha Hurley-Walker
Our 30-year observation period allowed us to pinpoint the exact timing of the pulses. They are being generated by the source at a constant rate of 1,318.1957 sec (give or take a hundredth of a millisecond).
Our current understanding is that the source must be slowing down in order to produce radio waves. The data, however, suggests that this is not the case.
In our Nature piece, we demonstrate that even for highly complicated models of neutron star magnetic fields, the source is still “below the death line,” the theoretical limit of how neutron stars create radio waves. Furthermore, the radio emission should only be detectable for a few months to years, not 33 years, and counting if the source is a magnetar.
As a result, we compounded the original issue by trying to fix it. These weird radio sources keep repeating; what are they?
What about ET?
It’s tempting to look to alien technology as a possible solution right now. The same thing happened when pulsars were initially discovered; the team led by astronomer Jocelyn Bell Burnell dubbed the first pulsar “LGM 1,” short for “Little Green Men 1.”
When Bell and her team uncovered additional discoveries, however, they realized that it was not aliens. It’s quite improbable that identical signals are emanating from all across the sky at the same time.
The pulses had no information, simply “noise” at all frequencies like those from natural radio sources, therefore they were similar to those from our source. To top it all off, you’d need a neutron star’s worth of energy to send out a signal at every possible frequency.
It’s easy to fall into this trap when faced with a novel phenomenon, but it’s not very convincing. It does not inspire us to maintain our active imaginations, keen senses, and experimental spirit. The “aliens of the gaps” theory, as I like to call it.
Observation of pulses from the first pulsar to be discovered, CP 1919. The chart recorder shows regular deflections every 1.3 seconds. Credit: Jocelyn Bell Burnell and Anthony Hewish
Observations of this source are possible from anywhere in the world because it is still functioning. Perhaps this new cosmic riddle can be solved with inventive follow-up observations and additional investigation.
