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Interesting Transient: A New Class of Object toward Galactic Center?

The 36 dish antennae at ASKAP — the Australian Square Kilometre Array Pathfinder in outback Western Australia — comprise an interferometer with a total collecting area of about 4,000 square meters. ASKAP has commanded attention as a technology demonstrator for the planned Square Kilometer Array, but today we’re looking at the discovery of a highly polarized, highly variable radio source labeled ASKAP J173608.2−321635, about 4 degrees from galactic center in the galactic plane.

According to Ziteng Wang, who is lead author of the study on this signal and a University of Sydney PhD student, the observations are strikingly different from other variable radio sources:

“The strangest property of this new signal is that it has a very high polarisation. This means its light oscillates in only one direction, but that direction rotates with time. 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.”

Variable celestial objects are common enough, from supernovae to pulsars, not to mention interesting sources like Fast Radio Bursts and, of course, the Cepheid variable stars that have played such a large role in astronomical history in helping us determine the scale of the universe. Any new variable source might be looked upon in light of such objects, perhaps as a type of flare star intermittently spewing out bursts of radiation. But none of these match the odd behavior of the new source. While J173608.2−321635 was found at ASKAP, Wang and team performed follow-up observations with the MeerKET telescope in South Africa.

So we have a source toward galactic center that is at first unseen, then brightens, fades, and reappears. Having detected six such signals from the source over nine months in 2020, the astronomers searched in vain for it in visible light, even as a search with the Parkes radio telescope turned up nothing. That’s when the team turned to MeerKAT, where it was once again detected. Tara Murphy, who is Wang’s PhD supervisor at Sydney, notes what happened next:

“Because the signal was intermittent, we observed it for 15 minutes every few weeks, hoping that we would see it again. 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.”

Image: The ASKAP telescope array. Credit: CSIRO.

Other low frequency transients from galactic center have been detected in recent years, including GCRT J1745-3009, which was quickly labeled a ‘burper’ by its discoverers due to its intermittent bursts after detection in 1998. Five bursts of equal brightness were noted, each about ten minutes in duration, and occurring every 77 minutes. No explanation has been agreed upon for that one either, although a pulsar, a neutron star pair, or a radio-emitting white dwarf have all been discussed in the literature.

For the ASKAP transient, the authors have considered pulsar scenarios, a transient magnetar, and “a low-mass star/substellar object with extremely low infrared luminosity,” with none of these providing a satisfactory answer. The suspicion grows that this is a new class of objects that future radio imaging surveys will observe as our capabilities improve. With the Square Kilometer Array coming online in the next decade, we are probably looking at a phenomenon that will generate a great deal of study and, doubtless, many more examples.

The paper is Wang et al., “Discovery of ASKAP J173608.2–321635 as a Highly Polarized Transient Point Source with the Australian SKA Pathfinder,” The Astrophysical Journal Vol. 920, No. 1 (12 October 2021), 45. Abstract.


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  • Henry Cordova October 15, 2021, 0:08

    So how bright is this thing? If other galaxies have similar transmitters in their nuclei could we detect them? That would help us determine if this is some kind of bizarre, unique, one-off phenomenon or if it is a normal feature of galactic nuclei. Then again, maybe it is not associated with the nucleus at all, it just lies coincidentally in that general direction. And what about the spectrum of the emission, is it narrow-band, black-body, or does it exhibit any other characteristics other than its polarization properties? Does that vary too?

    And just what is meant by a “steep-spectrum” radio source? I googled it but couldn’t follow the arguments, other than it is a characteristic of synchrotron radiation.

    Very strange.

    • Andrei October 16, 2021, 11:41

      Brightness. 5,0×10³¹ erg\ s−1 the highest received at the scope 5,6 mJy.
      Detection of similar signals from Andromeda and M33 do not seem likely over the distance of 2,5 – 2,7 million lightyears.
      While the authors dangle the possibility of this being a completely new type of object, this might be caused by a combination of some of the oddities we already know about in the galactic center such as the antimatter clouds etc.

  • Geoffrey Hillend October 16, 2021, 17:42

    I wonder what the radio frequency is? Maybe we can compare and contrast it with the Sagittarius A, pulsars, magnetars, etc.

    • Henry Cordova October 16, 2021, 21:47

      Thanks, gentlemen.

      According to the ApJ abstract, the original survey was carried out at 888 MHz, but it didn’t say what the bandwidth of the signal or the receiver was.

      According to my calculations, that freq works out to a wavelength of about 34 cm, certainly in the Waterhole, microwave region. Why they picked that particular line to listen at I do not know.

      • Jeff Wright October 20, 2021, 4:22

        Of more interest to me is this ‘tunnel’ surrounding the solar system. Could this and our heliopause weaken any signals from ET…allowing only the blaring loud noise of the cosmos itself.?

      • Geoffrey Hillend October 20, 2021, 17:46

        That’s a pretty high frequency. We know that the micro wave radio frequency is caused by the synchrotron radiation of fast moving electrons accelerated in the magnetic fields of stars. I thought 888 MHz mighty be the magnetic field of a supermassive black hole probably accelerated gas at a supermassive black hole. The problem with that idea s it would have to have a constant radio emission. This would also include a “a low-mass star/substellar object with extremely low infrared luminosity,” unless it has a body orbiting it obstructing the radio waves, but even that should have a constant pattern unless something is blocking it. It can’t be easily explained. We do know that it an ET message since there is now information being sent.

      • Geoffrey Hillend October 20, 2021, 17:48

        Excuse me. It is NOT an ET message because it does not contains any information.

  • kzb October 16, 2021, 21:02

    Is this not some gravitational lensing phenomenon?
    It seems to have things in common with the “extreme scattering events” although I am no expert.

  • Robin Datta October 16, 2021, 23:27

    Could a message be encoded into the formatting of some such transmission? The NSA should routinely look into such phenomena if it is not already doing so, on the off-chance that one such episode may provide return plane tickets to Stockholm.

  • David Herne October 18, 2021, 3:18

    A note for the curious – ASKAP is indeed a SKA pre-cursor instrument. How then does MEERKAT fit in? The SKA fully realised would actually comprise three observatories, each with collecting areas of 1 square kilometre. The first, a low-frequency component operating in the range 80 – 300 Mhz. Next, a mid-frequency component operating in the hundreds of Mhz to low-Ghz range and finally a high-frequency component operating at Ghz frequencies. The high-frequency component might never be built, however, the low and mid frequency components are under active development. ASKAP is a mid-frequency pre-cursor, MEERKAT is the full mid-frequency observatory. Imagine then what that instrument should accomplish considering the discoveries being made with the 32 dish ASKAP. The low-frequency component also enjoys the benefit of a pre-cursor instrument, the Murchison Wide-field Array (MWA) right next door. This instrument has been under development on the site since 2007. The SKA low-frequency component will be built on the MWA.

    Investments in astronomy pay off handsomely.

  • Mike Serfas October 21, 2021, 13:39

    It’s best always to have the Arxiv link: https://arxiv.org/pdf/2109.00652.pdf I’m no expert, and the first thing that crosses my mind is a black hole, given the galactic center and small time scales are involved. It seems like there could be a mechanism for separating circularly polarized components of light. ( https://arxiv.org/pdf/1205.4479.pdf ). Why didn’t they think that was a possibility?

  • Mike Serfas October 24, 2021, 11:03

    We’re going to need an article on this paper: https://www.nature.com/articles/s41467-021-26037-w (See intro: https://phys.org/news/2021-10-skyrmions.html ) There have been some other odd ideas for transmitting information in light, for example associating photons into higher order structures ( http://www.newsweek.com/photons-light-physics-808862 ). Maxwell’s equations seem to be getting new solutions all the time, and at least the Wikipedia article on them claims they’re actually approximate in the first place (?!)

    We can’t expect aliens to send signals in some archaic code that takes an entire planet’s energy supply if a few hundred years of extra research could have us receiving entire topological packages, complete with checksums, encoded in self-contained “crystals” of photons and skyrmions.

    • Adam Crowl October 28, 2021, 19:12

      Fascinating thought is whether it can form structures in raw materials at the other end. A different way to do Panspermia?

  • ljk January 27, 2022, 16:02


    January 27, 2022