Radio waves

What causes the mysterious radio waves coming from the heart of the Milky Way?

Artist’s impression of the oscillating and variable radio signal ASKAP J173608.2-321635 arriving at Earth from the center of the Milky Way. Credit: Sebastian Zentilomo/University of Sydney

The center of the Milky Way is a mysterious place. Astronomers believe there is a supermassive black hole there, though it could be dark matter instead. The region is densely populated 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 unexplained ones coming from the center of the galaxy, adding to the mystery.

Astronomers have discovered a transient source of radio waves in the center of the Milky Way. The research team 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, who holds a Ph.D. student at the University of Sydney School of Physics. The paper is published in The Astrophysical Journal.

The team knew they had found something remarkable. “Looking towards the center 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 was initially invisible, became glowing, faded, then reappeared. This behavior was extraordinary.

“The strangest property of this new signal is that it has a very high bias. This means that its light oscillates in one direction, but this direction rotates over time,” said lead author Wang. “The brightness of the object also varies considerably, by a factor of 100, and the signal turns on and off seemingly randomly. We have never seen anything like this.”

What is that? There are many different types of stars and variable objects in the sky. They emit variable light 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 these possibilities correspond to the observations.

“At first we thought it might be a pulsar – a very dense type of spinning dead star – or 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,” Wang said. The object is highly polarized, just like a pulsar, but the team did not detect any pulsation in its data.

They also considered magnetars, which are neutron stars with extreme magnetic fields, as a source. But the data also didn’t match what we know about magnetars. “All radio magnetars show very high degrees of polarization, but their flat radio spectra, unlike what we see for ASKAP J173608.2?321635, make a magnetar an unlikely interpretation,” they write in their paper.

ASKAP detection

This survey image shows the location of the variable radio source and other objects in the galactic center. Yellow outlines show ASKAP detection, while cyan outlines show MeerKAT detection. The best-fit positions of ASKAP and MeerKAT are represented by yellow + and cyan × symbols respectively. Red inverted Y symbols indicate sources for the VVV Catalog, a survey of variables in the infrared. The red star Gemini is a well-known source found with the Gemini Observatory. 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 found nothing. So they decided to try to detect the object with another radio telescope in Australia, the Parkes Observatory. They found nothing.

Undeterred, the team made follow-up observations with the even more sensitive MeerKAT radio telescope in South Africa. They continued to check with the MeerKAT to see if the intermittent signal would reappear. “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 we would see it again,” Dr Murphy said.

Center of the Milky Way

This image of the central part of the Milky Way shows a region of 1000 x 500 light years and was taken with the MeerKAT telescope stationed in South Africa, a system made up of 64 radio antennas. © SARAO

They were lucky. The signal has returned. But not in the way they expected.

“Fortunately, the signal returned, but we found that the behavior of the source was dramatically different – ​​the source disappeared in a single day, even though it had lasted for weeks during our previous ASKAP observations,” Murphy said.

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 might be a type of object called Galactic Center Radio Transient (GCRT). The new object was detected just four degrees from the galactic center. It shared some similarities with a GCRT, but the problem is that astronomers don’t know exactly what a GCRT is either.

“The information we have has parallels with another emerging class of mysterious objects known as Galactic Center Radio Transients, including one dubbed the ‘cosmic burper,'” said Mr. Wang’s co-director, the Professor David Kaplan of the University of Wisconsin. Milwaukee.

“Although our new object, ASKAP J173608.2-321635, shares some properties with GCRTs, there are also differences. And we don’t really understand those sources, anyway, so that adds to the mystery.

Milky Way Infrared Color Composite

This is a composite infrared image of the galactic center of the Milky Way. It shows new details in complex structures in the hot ionized gas swirling around the central 300 light-years. Credit: Hubble: NASA, ESA and QD Wang (University of Massachusetts, Amherst); Spitzer: NASA, Jet Propulsion Laboratory and S. Stolovy (Spitzer Science Center/Caltech)

For now, the new object will remain a mystery. Future installations will have more power and sensitivity. The Australian SKA itself is just one element of the eventual Square Kilometer Array, an international radio interferometer that will include thousands of dishes around the world. It should be online within the next decade.

“Within the next decade, the Square Kilometer Array (SKA) transcontinental radio telescope will be commissioned. It will be able to make sensitive maps of the sky every day,” Prof Murphy said. “We hope the power of this telescope will help us solve mysteries like this latest discovery, but it will also open vast new swaths 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 radio transient from the galactic center?

“ASKAP J173608.2?321635 is further notable for its location towards the GC, although we do not yet know if this is a coincidence or if this location is related to its nature: similar questions could be raised about GCRT sources. Future comprehensive research will quantify the exact number of these sources at different locations in the sky,” the authors write.

Originally published on Universe today.

To learn more about this discovery:

Reference: “Discovery of ASKAP J173608.2-321635 as a highly polarized transient point source with the Australian SKA Pathfinder” by Ziteng Wang, David L. Kaplan, Tara Murphy, Emil Lenc, Shi Dai, Ewan Barr, Dougal Dobie , BM Gaensler, George Heald, James K. Leung, Andrew O’Brien, Sergio Pintaldi, Joshua Pritchard, Nanda Rea, Gregory R. Sivakoff, BW Stappers, Adam Stewart, E. Tremou, Yuanming Wang, Patrick A. Woudt and Andrew Zic, October 12, 2021, The Astrophysical Journal.
DOI: 10.3847/1538-4357/ac2360