In early 2020, we detected an unusual radio signal coming from somewhere near the center of our galaxy. The signal turns on and off, becoming 100 times faster and slower over time.
What’s more, the signal had an unusual “circular polarization” in radio waves, meaning that the electric field in radio waves rotates around as the waves travel through space.
We first observed the signal using the Australian Square Kilometer Array Pathfinder Telescope (ASKAP), then followed it with other telescopes around the world and in space. Despite our best efforts, we are still unable to figure out what generated these mysterious radio waves.
A strange signal from the heart of the galaxy
We will be surveying the sky with ASKAP, called the Variable and Slow Transients (VAST) survey, during 2020 and 2021 looking for unusual new objects.
Most of the things that astronomers see in outer space are fairly stable and do not change much on the human time scale. That’s why objects that change (known as variables) or appear and disappear (known as transgenders) are very interesting.
Usually associated with some of the most energetic and violent events in the universe, such as the death of massive stars. The past decade has seen thousands of transistors discovered in optical and X-ray wavelengths, but radio wavelengths remain largely untested.
When we looked to the center of our galaxy (the Milky Way), we found a source we called ASKAP J173608.2-321635 (this catchy name comes from its coordinates in the sky). This object was unique in that it began to disappear, brightened, faded and then reappeared. This behavior was extraordinary.
The signal was circularly polarized, changing over time. Our eyes can’t differentiate between polarized and unpolarized light, but ASKAP has the Polaroid equivalent of sunglasses for radio waves.
Polarized radio sources are extremely rare: we can find less than ten circularly polarized sources out of thousands. Almost all of them are sources that we understand well, such as pulsars (fast-spinning, highly magnetized remnants of exploding stars) or highly magnetized red dwarf stars.
finding more evidence
Investigating a new celestial object is like detective work. We need evidence to determine what it is.
Based on our ASKAP data, we thought the new object might be a pulsar or a bright star: both types of objects can be polarized, and have changes in brightness. However, we needed to find more clues.
We next observed the source with the Parkes Radio Telescope in New South Wales to determine whether it was a pulsar. However, nothing came out of these comments.
Next we tried the more sensitive Meerkat radio telescope in South Africa. Because the signal was intermittent, we watched it every few weeks for 15 minutes, hoping to see it again. Fortunately, the signal returned, but the behavior of the source was now dramatically different. The source disappeared over the course of a day, even though it had gone on for weeks in our previous ASKAP comments.
It’s always a good idea to investigate from multiple perspectives. Telescopes operating at other wavelengths can serve as another pair of eyes to help us find new clues.
After locating Meerkat, we searched for the source of X-ray (using the space-based Neil Gehrels Swift Observatory and Chandra X-ray Observatory) and infrared (using the Gemini Telescope in Chile). However, we didn’t see anything.
still a mystery
We’ve seen this strange object on three continents and in space at many wavelengths using telescopes. What can we say about what it really is?
Could it be a star? This seems impossible because stars also emit most of their light in the optical and infrared (like the Sun), but we detect nothing at these wavelengths.
Could it be Pulsar? Like our signal, pulsars produce polarized radio waves and can vary dramatically in brightness. But pulsars are characterized by fast pulses ranging from milliseconds to seconds, and we didn’t detect these with Parks or Meerkat.
Read more: Fifty years ago Jocelyn Bell discovered pulsars and changed our view of the universe
Is the source’s proximity to the center of our galaxy a clue? Over the past 15 years, several interesting radio sources have been discovered toward the galactic center (including one called the “Cosmic Burper”). We don’t know what they are, but they are imaginatively called Galactic Center Radio Transients (GCRTs).
Are they related to ASKAP J173608.2-321635? There are some similarities, but there are also differences. And even known GCRTs exhibit heterogeneity, and may not share a common origin. So our sign is still a mystery.
We will continue to explore this source in new ways. It is the first of several unusual transient sources that we expect to find with the powerful ASKAP array, and it signals the future of radio astronomy.
Read more: ‘WTF?’: Newly discovered ghostly circle in the sky can’t be explained by current theories, and astronomers are excited