What suddenly gets bright in the sky, sends out random radio signals and then disappears for months?
It’s a question that has stumped astronomers since the discovery of a mysterious radio signal coming from near the centre of our galaxy in January last year.The signal is so strange it may be coming from a new type of celestial object, an international team of astronomers report in The Astrophysical Journal.
The team first picked up the signal while scanning the sky with the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope in outback Western Australia.
The signal appeared another four times in a row in just a couple of weeks, said study co-author Tara Murphy of the University of Sydney.Then the signal from the source dubbed ASKAP J173608.2−321635 disappeared, reappearing a couple of times a few months later.
“Sometimes it seems to stay on, detectable for days or weeks at a time, and then other times it can come on and off in a single day, which is extremely fast for an astronomical object,” Professor Murphy said.
Not only is the timing random, but the signal can vary in strength, becoming 100 times brighter in the radio spectrum.Even more weirdly, the radio waves are aligned in one direction that rotates as the signal travels through the universe towards us.
“That rules out almost all astronomical objects we know of,” Professor Murphy said.
But that doesn’t mean we’ve suddenly detected aliens.In the months following the initial discovery, the team led by PhD student Ziteng Wang explored a number of options trying to work out what the source was.
“It’s a bit like a detective story trying to rule out all these different possibilities,” Professor Murphy said.
They narrowed it down to three.
The first option was a pulsar — the fast-spinning heart of a dead star, which regularly send out very fast pulses of energy like clockwork.
The team turned to the Parkes Radio Telescope, which is famous for detecting pulsars, but found nothing that could be the source.
So then they went to the MeerKAT radio telescope in South Africa, which can detect not only pulses, but take images of signals.
They saw nothing for three months, but in February this year they detected a single signal that was almost as strong as the original signal, before fading away.Even though the signal had some of the properties of a pulsar, there were no telltale fast pulses.
Space telescopes also found no objects emitting light in the X-ray and infrared ranges in the area that would indicate the source was a pulsar or another type of dead, fast-spinning star known as a magnetar.
The second possibility the team explored was whether the signal could have come from a massive flare from a star.
“This object was so bright that if it was a star, we should be able to see it in visible light,” Professor Murphy said.
“But … we didn’t see it at all, it was completely invisible.
“So then we’ve got this situation where we’ve ruled out the two most likely explanations.”The only other possibility is that it belongs to a rag-tag group of rare objects known as galactic centre radio transients, one of which is dubbed the “cosmic burper”.
“It could be that we’ve discovered one of these, so in a way that’s exciting, because there are very few of them known, but also frustrating because we don’t actually know what galactic centre radio transients actually are,” Professor Murphy said.
Every single galactic centre radio transient that has been discovered is slightly different; while some emit regular pulses of radio waves, others don’t.
“All of [this object’s] properties are slightly different to all of the ones that are known, but the thing is they are all different to each other,” Professor Murphy said.
So although they are all lumped in the same category, we don’t know enough about them to tell if they are related at all.
“They all might just be unknown variable polarised objects near the centre of the galaxy,” Professor Murphy said.Transient objects are very hard to spot because they only turn on for a short amount of time, said Gemma Anderson, an astrophysicist at Curtin University and the International Centre for Radio Astronomy Research, who was not involved in the study.
“You want to hope your telescope is pointing at the right part of the sky when it is turning on, because it may not appear again for weeks, months or ever,” Dr Anderson said.
And unlike transient objects that emit pulses of energy in the X-ray, optical or infrared wavelengths, objects that only emit radio waves are very difficult to detect.But the advent of radio telescopes such as ASKAP Pathfinder and MeerKAT have enabled us to peer further into the universe than ever before.
“ASKAP is particularly powerful because it looks at such a large part of the sky in one go,” Dr Anderson said.”This transient that this team has found is the tip of the iceberg.”
The ASKAP and MeerKAT telescopes are the first stages of the world’s largest radio telescope, the Square Kilometre Array (SKA).
When the SKA comes on board, it may be possible to find even fainter transient objects “of which there could be tens, hundreds, thousands of them in our galaxy that have remained hidden,” Dr Anderson said.
Professor Murphy said she hoped the team would find more objects like this with the ASKAP telescope.
“Then by building up a statistical number of them, we’ll be able to work out what they are.
“That’s how things often happen in astronomy; you find one rare thing, then you find more like it and eventually you can actually understand what’s going on.”