“Holy sharks Batman, it’s periodic!” I exclaimed to Slack.
We were all working remotely at the time, so it was the first lockdown in 2021 Perth. Astronomers are always looking for something to distract them from their existential dread.
In 2020, I led a project for Tyrone O’Doherty (undergraduate student): I looked for radio sources that were changing in a large-radio survey I’m leading.
He had discovered a very unusual source by the end of 2018. It was visible in data starting in early 2018 but had vanished within a few months. After the survey it was located in and its location, the source was named GLEAMX J162759.5-523504.
Radio transients are sources that appear and then disappear. They are often a sign of extreme Physics at Play.
The Mystery Begins
In my quest to find the source of this star, I began to investigate it earlier in the year. It was something I expected to know about.
I needed to determine how the source’s brightness and frequency relate to the electromagnetic spectrum in order to understand the physics. I compared observations taken at different frequencies of the same spot before and after detection and found it was not there.
It was disappointing, because spurious signals can sometimes crop up due to telescope calibration mistakes, Earth’s atmosphere reflecting TV signals, aircraft and satellites streaking overhead, etc.
I then looked at additional data. I made another observation 18 minutes later and found the source again in the exact same spot and at the exact same frequency. This was unlike anything astronomers have ever seen.
This was when I began to sweat profusely. A worldwide effort is underway to find repeating cosmic radio signals that are transmitted at one frequency. It is called the Search for Extra-Terrestrial Intelligence. This was the moment when we realized that there is truth to it all .
The Plot Thickens
I quickly downloaded additional data and posted updates to Slack. This source was extremely bright. This source was so bright that it outshone everything else.
Supermassive black hole jets of massive matter into space at almost the speed of light are the brightest radio sources. Were there any other radio sources brighter?
It was beginning to catch on with colleagues, who posted: “It’s repetition too slowly to be considered a pulsar. It’s too bright to be a flare star. What’s this? (alien emoji icon)?? ?”
I felt a relief within a matter of hours. I had detected the source at a large frequency range, so the power required to generate it could only have come from a natural source, not artificial (and certainly not aliens).
The radio waves repeat like clockwork three times an hour, just like Pulsars (highly magnetic rotating neutron star that emit radio waves from their poles). They would appear at a rate of about one tenth of a second, which I was able to predict.
I decided to look into our huge data archive. It contains 40 petabytes radio astronomy data that was collected by the Murchison Broadfield Array in Western Australia over its eight-year operation. I used powerful supercomputers to search hundreds of observations. 70 additional detections were made in the three-month period ending in 2018.
Radio transients are amazing because you can determine how far they are if there is enough frequency coverage. This is because radio frequencies that are lower in frequency arrive slightly later than those with higher frequencies depending on the amount of space they travel through.
Our new discovery is located approximately 4,000 light-years away, which is very far from our galactic backyard.
Also, we found that the radio pulses were nearly entirely polarized. This is usually a sign that the source of the radio pulses is a strong magnetic force. Also, the pulses changed in half a second. Therefore, the source must be smaller than half a light-second across. This is much smaller than our sun.
Everyone was thrilled to share the results with their colleagues around the globe, although no one knew what it was.
The jury is still out
Two main explanations were given for the compact, rotating and highly magnetic astrophysical object. One was a white dwarf or a neutron Star. These magnetic fields are created when stars die and collapsing, and can generate magnetic fields that are billions to quintillions of times stronger than the sun’s.
Although we haven’t found a neutron-star that behaves in this manner, theorists believe such objects, known as “ultralong period magnetars”, may exist. Despite this, it was impossible to imagine one being so bright.
We have never seen a radio source repeat every 20 minutes. Perhaps we didn’t look enough to see one before.
This source was my first attempt to understand it. I was influenced by my expectations. Transient radio sources can change rapidly like pulsars or slowly like the fading remnants from a supernova.
I was not looking for sources that would repeat at 18-minute intervals. This is a rare period for any object of known class. I wasn’t looking for something that would disappear after a few months. There was none.
Astronomers are building new telescopes to collect huge amounts of data. It’s important that we open our minds and search techniques to discover unexpected possibilities. If we choose to look, the universe holds many wonders.