The center of the Milky Way is a mysterious place. Astronomers think there’s a supermassive black hole there, though it could be dark matter instead. The region is densely packed with stars, dominated by red giants. And because of all the dust between Earth and the galactic center, we can’t see anything with visible light, ultraviolet light, or low-energy x-rays.
But we can detect radio waves, and there are some unexplained ones coming from the center of the galaxy, and adding to the mystery.
Astronomers have discovered a transient source of radio waves at the Milky Way’s center. The team of researchers presented their findings in a paper titled “Discovery of ASKAP J173608.2–321635 as a Highly Polarized Transient Point Source with the Australian SKA Pathfinder.” The lead author is Ziteng Wang, a Ph.D. student in the School of Physics at the University of Sydney. The paper is published in The Astrophysical Journal.
The team knew they’d found something remarkable. “Looking towards the centre of the Galaxy, we found ASKAP J173608.2-321635, named after its coordinates,” said co-author Professor Tara Murphy. “This object was unique in that it started out invisible, became bright, faded away and then reappeared. This behaviour was extraordinary.”
“The strangest property of this new signal is that it is has a very high polarisation. This means its light oscillates in only one direction, but that direction rotates with time,” said lead author Wang. “The brightness of the object also varies dramatically, by a factor of 100, and the signal switches on and off apparently at random. We’ve never seen anything like it.”
What is it? There are lots of different types of variable stars and objects in the sky. They emit variable light all across the spectrum.
Could it be a low-mass star or a substellar object? Could it be a pulsar or a transient magnetar? According to the authors, none of those possibilities matches the observations.
“At first we thought it could be a pulsar – a very dense type of spinning dead star – or else a type of star that emits huge solar flares. But the signals from this new source don’t match what we expect from these types of celestial objects,” Mr. Wang said. The object is highly polarized, just like a pulsar, but the team didn’t detect any pulsations in their data.
They also considered magnetars as the source, which are neutron stars with extreme magnetic fields. But the data didn’t match with what we know about magnetars either. “All radio magnetars show very high degrees of polarization, but their flat radio spectra, in contrast to what we see for ASKAP J173608.2?321635, makes a magnetar an unlikely interpretation,” they write in their paper.
This image from the study shows the location of the variable radio source and other objects in the galactic center. The yellow contours show the ASKAP detection, while the cyan contours show the MeerKAT detection. The best-fit positions from ASKAP and MeerKAT are shown as yellow + and cyan × symbols, respectively. Red inverted Y symbols show the sources from the VVV catalogue, a survey of variables in the infrared. The red Gemini star is a well-known source found with the Gemini Observatory. Image Credit: Wang et al, 2021
The team detected six radio signals from the object over the course of nine months. When they searched for the object in visible light, they didn’t find anything. So they decided to try detecting the object with another radio telescope in Australia, the Parkes Observatory. They found nothing.
Undeterred, the team performed follow-up observations with the MeerKAT radio telescope in South Africa, which is even more sensitive. They kept checking with the MeerKAT to see if the intermittent signal would show up again. “We then tried the more sensitive MeerKAT radio telescope in South Africa. Because the signal was intermittent, we observed it for 15 minutes every few weeks, hoping that we would see it again,” said Dr. Murphy.
This is what the center of the Milky Way looks like to the MeerKAT radio telescope in South Africa. Image Credit: By MeerKAT Public release photo – https://gizmodo.com/new-south-african-telescope-releases-epic-image-of-the-1827572028, Public Domain, https://commons.wikimedia.org/w/index.php?curid=70866309
They got lucky. The signal returned. But not in a way that they expected.
“Luckily, the signal returned, but we found that the behaviour of the source was dramatically different – the source disappeared in a single day, even though it had lasted for weeks in our previous ASKAP observations,” said Murphy.
Detecting the transient signal was a boost for the team, but it didn’t help them identify the nature of the source. The team thought it could be a type of object called a Galactic Center Radio Transient (GCRT). The new object was detected only four degrees from the galactic center. It shared some similarities with a GCRT, but the problem is astronomers don’t know exactly what a GCRT is, either.
“The information we do have has some parallels with another emerging class of mysterious objects known as Galactic Centre Radio Transients, including one dubbed the ‘cosmic burper’,” said Mr. Wang’s co-supervisor, Professor David Kaplan from the University of Wisconsin-Milwaukee.
“While our new object, ASKAP J173608.2-321635, does share some properties with GCRTs there are also differences. And we don’t really understand those sources, anyway, so this adds to the mystery.”
This is a composite infrared image of the Milky Way’s galactic center. It shows new details in complex structures in the hot ionized gas swirling around the central 300 light-years. Image Credit: By Hubble: NASA, ESA, and Q.D. Wang (University of Massachusetts, Amherst); Spitzer: NASA, Jet Propulsion Laboratory, and S. Stolovy (Spitzer Science Center/Caltech) – NASA Image of the Day, Public Domain, https://commons.wikimedia.org/w/index.php?curid=9378571
For now, the new object will remain a mystery. Future facilities will have more power and sensitivity. The Australian SKA itself is just one piece of the eventual Square Kilometer Array, an international radio interferometer that will be comprised of thousands of dishes around the world. It should be online in the next decade.
“Within the next decade, the transcontinental Square Kilometre Array (SKA) radio telescope will come online. It will be able to make sensitive maps of the sky every day,” Professor Murphy said. “We expect the power of this telescope will help us solve mysteries such as this latest discovery, but it will also open vast new swathes of the cosmos to exploration in the radio spectrum.”
Future studies will uncover more data on this new object and others like it. Will it turn out to be a Galactic Center Radio Transient?
“ASKAP J173608.2?321635 is further notable for its location toward the GC, although we do not yet know whether that is a coincidence or if that location is related to its nature: similar questions could be raised about the GCRT sources. Future comprehensive searches will quantify the exact number of such sources at different locations in the sky,” the authors write.