“Damn sharks, Batman, it’s periodic!” I exclaimed on Slack.
It was the first lockdown of 2021 in Perth, and we were all working from home. And when astronomers are looking for something to distract from the looming existential dread, there’s nothing better than a new cosmic mystery.
In 2020, I gave undergrad Tyrone O’Doherty a fun project: researching changing radio sources in a large radio survey I’m leading.
By the end of the year, he had found a particularly unusual source that was visible in data from early 2018, but disappeared within months. The source was named GLEAM-X J162759.5-523504, after the survey in which it was found and its position.
Sources that appear and disappear are called “radio transients” and are usually a sign of extreme physics at play.
The mystery begins
Earlier this year I started investigating the source, expecting it to be something we knew about – something that would slowly change over the months and possibly indicate an exploded star or a large collision in the space.
To understand the physics, I wanted to measure the relationship between the brightness of the source and its frequency (in the electromagnetic spectrum). So I looked at observations of the same location, taken at different frequencies, before and after detection, and it wasn’t there.
I was disappointed, as spurious signals sometimes appear due to errors in the calibration of the telescope, the earth’s ionosphere reflecting television signals, or airplanes and satellites passing overhead.
So I looked at more data. And in an observation taken 18 minutes later, the source was there again, in exactly the same place and at exactly the same frequency – like nothing astronomers had ever seen before.
At this point, I broke into a cold sweat. There is a worldwide research effort looking for repeated cosmic radio signals transmitted at a single frequency. This is called the search for extraterrestrial intelligence. Was this the moment we finally found out that the truth is… the low?
The plot thickens
I quickly uploaded more data and posted updates to Slack. This source was incredibly brilliant. It eclipsed everything else in the observation, which is nothing to sniff at.
The brightest radio sources are supermassive black holes shooting huge jets of matter out into space at near the speed of light. What did we find more brilliant than that?
Colleagues were beginning to take notice, posting:
It repeats too slowly to be a pulsar. But it’s too bright for a blazing star. What is that? (alien emoji icon) ???
Within hours, I breathed a sigh of relief: I had detected the source over a wide range of frequencies, so the power it would take to generate it could only come from a natural source; not artificial (and not extraterrestrial)!
Much like pulsars – highly magnetized spinning neutron stars that emit radio waves from their poles – radio waves repeat like a clock about three times an hour. In fact, I could predict when they would appear with an accuracy of ten thousandths of a second.
So I turned to our huge data archive: 40 petabytes of radio astronomy data recorded by the Murchison Widefield Array in Western Australia, over its eight years of operation. Using powerful supercomputers, I searched for hundreds of sightings and found another 70 detections over three months in 2018, but none before or after.
The amazing thing about radio transients is that if you have enough frequency coverage, you can determine how far away they are. This is because the lower radio frequencies arrive slightly later than the higher frequencies, depending on the space travelled.
Our new discovery is about 4,000 light years away – very far away, but still in our galactic backyard.
We also found that the radio pulses were almost completely polarized. In astrophysics, this usually means that their source is a strong magnetic field. The pulses also changed shape in just half a second, so the source must have been less than half a second in diameter, much smaller than our Sun.
Sharing the result with colleagues around the world, everyone was excited, but no one knew for sure what it was.
The jury is still out
There were two main explanations for this compact, rotating and highly magnetic astrophysical object: a white dwarf or a neutron star. These remain after stars have run out of fuel and collapsed, generating billions to quintillions of magnetic fields stronger than those of our Sun.
And while we’ve never found a neutron star that behaves quite this way, theorists have predicted that such objects, called “ultra-long-period magnetars,” might exist. Even so, no one expected that one could be so brilliant.
This is the first time we see a radio source that repeats itself every 20 minutes. But maybe the reason we haven’t seen one before is that we weren’t looking.
When I first started trying to understand this source, I was skewed by my expectations: transient radio sources change either rapidly like pulsars or slowly like fading supernova remnants.
I wasn’t looking for sources repeating at 18 minute intervals – an unusual period for any known class of object. Nor was I looking for something that would show up for a few months and then disappear forever. No one was.
As astronomers build new telescopes that will collect vast amounts of data, it’s critical that we keep our minds and research techniques open to unexpected possibilities. The universe is full of wonders, you just have to choose to look.
This article is republished from The Conversation under a Creative Commons license. Read the original article.