In early 2020, we detected an uncommon radio sign coming from someplace close to the centre of our galaxy. The sign blinked on and off, rising 100 instances brighter and dimmer over time.
What’s extra, the radio waves within the sign had an unusual “round polarisation”, which implies the electrical subject within the radio waves spirals round because the waves journey by house.
We first noticed the sign utilizing the Australian Sq. Kilometre Array Pathfinder Telescope (ASKAP), then adopted up with different telescopes all over the world and in house. Regardless of our greatest efforts, we’re nonetheless unable to work out precisely what produced these mysterious radio waves.
An odd sign from the guts of the Milky Manner
Now we have been surveying the sky with ASKAP all through 2020 and 2021 in quest of uncommon new objects, in a challenge referred to as the Variables and Sluggish Transients (VAST) survey.
Most issues astronomers see in outer house are pretty steady and don’t change a lot on human time scales. That’s why objects that do change (generally known as variables) or seem and disappear (generally known as transients) are so fascinating.
Transients are often related with a few of the most energetic and violent occasions within the Universe, such because the demise of huge stars. The previous decade has seen 1000’s of transients found at optical and X-ray wavelengths, however radio wavelengths are largely untapped.
After we regarded in direction of the centre of our galaxy (the Milky Manner), we discovered a supply we referred to as ASKAP J173608.2-321635 (this catchy title comes from its coordinates within the sky). This object was distinctive in that it began out invisible, grew to become vivid, pale away, after which reappeared. This behaviour was extraordinary.
In addition to altering over time, the sign was circularly polarised. Our eyes can’t distinguish between polarised and unpolarised mild, however ASKAP has the equal of polaroid sun shades for radio waves.
Polarised radio sources are extraordinarily uncommon: we would discover fewer than ten circularly polarised sources out of 1000’s. Virtually all of them are sources we perceive properly, similar to pulsars (the quickly rotating, extremely magnetised remnants of exploded stars) or extremely magnetised pink dwarf stars.
Discovering extra proof
Investigating a brand new astronomical object is a bit like a detective job. We’d like proof to find out what it’s.
Primarily based on our ASKAP information, we thought the brand new object may be a pulsar or a flaring star: each forms of object will be polarised, and alter in brightness. Nevertheless, we would have liked to search out extra clues.
We subsequent noticed the supply with the Parkes radio telescope in New South Wales to resolve whether or not it was a pulsar. Nevertheless, these observations yielded nothing.
We then tried the extra delicate MeerKAT radio telescope in South Africa. As a result of the sign was intermittent, we noticed it for quarter-hour each few weeks, hoping we might see it once more. Fortunately, the sign returned, however the behaviour of the supply was now dramatically totally different. The supply disappeared in the midst of a single day, despite the fact that it had lasted for weeks in our earlier ASKAP observations.
It’s all the time a good suggestion to research from a number of views. Telescopes working at different wavelengths can function one other pair of eyes to assist us discover new clues.
After the MeerKAT detection, we looked for the supply in X-rays (utilizing the space-based Neil Gehrels Swift Observatory and Chandra X-ray Observatory) and infrared (utilizing the Gemini telescope in Chile). Nevertheless, we noticed nothing.
Nonetheless a thriller
Now we have noticed this unusual object at a number of wavelengths utilizing telescopes on three continents and in house. What can we are saying about what it truly is?
Can it’s a star? It appears unlikely as a result of stars additionally emit a lot of their mild within the optical and infrared (just like the Solar), however we detect nothing at these wavelengths.
Can it’s a pulsar? Like our sign, pulsars produce polarised radio waves and may range dramatically in brightness. However the attribute of pulsars is speedy pulses betweem milliseconds to seconds lengthy, and we didn’t detect these with Parkes or MeerKAT.
Fifty years in the past Jocelyn Bell found pulsars and altered our view of the universe
Is the supply’s proximity to the centre of our galaxy a clue? Over the previous 15 years, plenty of intriguing radio sources have been found towards the Galactic centre (together with one dubbed the “cosmic burper”). We don’t know what they’re, however they’re imaginatively referred to as Galactic Middle Radio Transients (GCRTs).
Are they associated to ASKAP J173608.2-321635? There are some similarities, however there are additionally variations. And even the identified GCRTs exhibit variety, and will not share a standard origin. So our sign continues to be a thriller.
We are going to preserve observing this supply in new methods. It’s simply the primary of many uncommon transient sources that we look forward to finding with the highly effective ASKAP array, and it provides a touch of the way forward for radio astronomy.
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