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“Holy sharks, Batman, it’s newspaper!”
I exclaimed in Slack.
It was Perth’s first lockdown of 2021 and we were all working from home. And when astronomers are looking for something to take their mind off the existential dread that lies ahead, there’s nothing better than a new cosmic mystery.
In 2020 I taught a college student, Tyrone O’Doherty, a fun project: look for radio sources that are changing in a big radio poll I’m conducting.
At the end of the year, it found a particularly unusual source that was visible in data from early 2018, but disappeared within a few months. The source was named GLEAM-X J162759.5-523504, after the survey in which it was found and its position.
The 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 began investigating the source, hoping it was something we knew about, something that would slowly change over months and perhaps point to an exploding star or a large collision in space.
To understand the physics, I wanted to measure how the brightness of the source is related to its frequency (in the electromagnetic spectrum). So I looked at observations of the same place, taken at different frequencies, before and after detection, and it wasn’t there.
I was disappointed, as spurious signals occasionally arise due to telescope calibration errors, the Earth’s ionosphere reflecting TV signals, or planes and satellites passing overhead.
So I looked at more data. And in an observation made 18 minutes later, there was the source again, in exactly the same place and at exactly the same frequency, as astronomers had never seen before.
At this point, I broke out in a cold sweat. There is a worldwide research effort looking for repeating cosmic radio signals transmitted on a single frequency. It’s called the Search for Extraterrestrial Intelligence. Was this the moment we finally found out that the truth is… out there?
The plot thickens
I quickly downloaded more data and posted updates on Slack. This fountain was incredibly bright. It was overshadowing everything else in the observation, which is nothing to sneeze at.
The brightest radio sources are supermassive black holes that hurl huge jets of matter into space at nearly the speed of light. What had we found that could be brighter than that?
Colleagues were beginning to take notice and posted:
It repeats too slowly to be a pulsar. But it’s too bright for a flare star. What’s this? (alien emoji icon)???
Within a few hours, I breathed a sigh of relief: I had detected the source over a wide range of frequencies, so the energy it would take to generate it could only come from a natural source; not artificial (and not alien)!
Like pulsars, highly magnetized spinning neutron stars that emit radio waves from their poles, the radio waves repeated like clockwork about three times an hour. In fact, he could predict when they would appear with an accuracy of one ten-thousandth 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, during its eight years of operation. Using powerful supercomputers, I searched hundreds of observations and collected 70 more detections over three months in 2018, but none before or since.
The amazing thing about radio transients is that if you have enough frequency coverage, you can figure out how far away they are. This is because the lower radio frequencies arrive a little later than the higher ones, depending on how far they have traveled.
Our new discovery is about 4,000 light-years away, very distant, but still in our galactic backyard.
We also found that the radio pulses were almost completely polarized. In astrophysics, this usually means that its source is a strong magnetic field. The pulses also changed shape in just half a second, so the source must be less than half a light-second wide, 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 highly magnetic, spinning, compact astrophysical object: a white dwarf or a neutron star. These remain after stars run out of fuel and collapse, generating magnetic fields billions to quintillions of times stronger than that of our Sun.
And while we’ve never found a neutron star that behaves in this way, theorists have predicted that such objects, called “ultra-long-period magnetars,” could exist. Still, no one expected that one could be so bright.
This is the first time we see a radio source that repeats every 20 minutes. But maybe the reason we never saw one before is because we weren’t looking.
When I first started trying to understand this source, I was biased by my expectations: transient radio sources change rapidly like pulsars or slowly like fading supernova remnants.
He wasn’t looking for sources that repeated at 18-minute intervals, an unusual period for any known class of object. He also wasn’t 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 is vital that we keep our minds and search techniques open to unexpected possibilities. The universe is full of wonders, we should only choose to look.
This article by Natasha Hurley-Walker, radio astronomer, Curtin University is republished from The Conversation under a Creative Commons license. Read the original article.