Strange radio waves hint at the existence of a new stellar object

While many objects in the vastness of space shine or radiate with more or less constant intensity, astronomers have come to discover more and more objects that flash, blip, burp, and otherwise appear, disappear, and reappear in X-ray, visual, or radio frequencies.

Many such time-domain objects, as astronomers called them, are well known: Pulsars and neutron stars with regular, periodic signals, and the intermittent flash of a dying star, dashed on the rim of a black hole.

But some cosmic signals remain mysterious, including a radio signal described by an international team of researchers in a paper published Wednesday in the Astrophysical Journal.

The mysterious signal is named ASKAP J173608.2-321635. It was found by the Australian Square Kilometre Array Pathfinder (ASKAP), and appears to be emanating from near the center of our galaxy.

While more research is necessary to know for sure, the researchers were unable to match the behavior of ASKAP J173608.2-321635 to any known celestial object, raising the possibility that they may have discovered a new class of object in space.

What’s new — The researchers found a unique and unusual radio signal emanating from near the center of our galaxy’s center. Initially invisible, it became very bright (in the radio spectrum), faded away, and then reappeared, switching off and on apparently at random. It could persist for weeks, and then fade away in a day.

When visible, ASKAP J173608.2-321635 is doubly strange in that its signal is highly polarized, meaning the waves oscillate in only one direction. But with the direction of that oscillation also shifts over time.

Adding to the mystery the researchers failed to detect ASKAP J173608.2-321635 in the infrared or X-ray spectrums, including in archived surveys of the region of the galaxy where they detected the object’s radio signal. Whatever generates the signal is cool and dim enough to go unnoticed except in the radio spectrum, while simultaneously turning on, growing bright in that spectrum, and then disappearing at random.

None of those strange elements are in themselves unique, according to James Cordes, a Cornell professor of astronomy who was not involved in the study. But all those strange elements taken together, he tells Inverse, present an intriguing puzzle for astronomers.

“It has characteristics that you can find in other objects,” Cordes says. “It's just that some of those characteristics, you don't see them all together like in this one.”

How they did it— The researchers first detected the signal as part of the Australian Square Kilometre Array Pathfinder Variables and Slow Transients (ASKAP VAST) survey, detecting the signal six times between January and September 2020.

The Australian Square Kilometre Array Pathfinder has a large field of view designed to scan large swathes of the sky for transient signals that come and go as ASKAP J173608.2-321635 does, even if such signals are not typically so unusual.

The researchers then used the MeerKAT — originally known as the Karoo Array Telescope — radio telescope array in South Africa to confirm the signal, watching those coordinates from November 2020 to February 2021.

Why it matters — If ASKAP J173608.2-321635 cannot be explained by known stellar objects, it could be evidence of something entirely new, which would be very exciting for astronomers like Cordes.

“I think a lot of us would like them to be entirely new things,” he says. “But you really have to sort of rule out the knowns, and that's what they spent a lot of time doing in this paper.”

The researchers rejected the possibility that ASKAP J173608.2-321635 is a star, for instance, based on its low infrared and X-ray luminosity, among other properties.

They also monitored the signal for signs of radio pulses like those generated by pulsars, usually neutron stars or white dwarf stars that emit periodic powerful beams of electromagnetic signals from their poles. But ASKAP J173608.2-321635 did not show signs of being a pulsar either.

It’s possible, the researchers note in the paper, that is ASKAP J173608.2-321635 could be a pulsar, but that scattering of its signal in the interstellar medium or some other process prevented the team from detecting the distinctive pulsations of that class of object.

Cordes believes ASKAP J173608.2-321635 could turn out to be some sort of brown dwarf star, small stellar bodies between a planet and a star in mass that are relatively cool and dim.

Then again, ASKAP J173608.2-321635 could be something else entirely.

What’s next?— For the time being, ASKAP J173608.2-321635 cannot be identified either way. There’s simply not enough data yet, and Cordes says he hopes the study team or others will find the telescope time to “monitor the hell out this” and learn more.

But he also believes ASKAP J173608.2-321635 could be the first of many such discoveries.

The large singular radio telescopes such as the now-defunct dish at Arecibo in Puerto Rico provided great sensitivity, but a very narrow field of view. When searching for transient signals, Cordes says, astronomers need wide views of the sky, the sort provided by large arrays of many antennas as with the Australian Square Kilometre Array Pathfinder.

“They can get reasonably good sensitivity, but also a huge field of view,” he says. “So you're sampling a lot of the sky all at once.”

That’s a philosophy animating even singular telescope projects going forward, Cordes says. The Vera C. Rubin observatory, previously known as the Large Synoptic Survey Telescope, will scan the entire night sky over and over again when it begins operating in a year or so, with the aim of capturing all the different kinds of changing or intermittent mysterious in the sky.

“You name it, anything that goes burp in the night is going to be found by the LSST,” Cordes says. “I would say [ASKAP J173608.2-321635] is just one case on the leading edge of what's going to be an explosion of variable things in the sky.”

ABSTRACT: We report the discovery of a highly-polarized, highly-variable, steep-spectrum radio source, ASKAP J173608.2−321635, located ∼4◦ from the Galactic center in the Galactic plane. The source was detected six times between 2020 January and 2020 September as part of the Australian Square Kilometre Array Pathfinder Variables and Slow Transients (ASKAP VAST) survey at 888MHz. It exhibited a high degree (∼ 25%) of circular polarization when it was visible. We monitored the source with the MeerKAT telescope from 2020 November to 2021 February on a 2–4 week cadence. The source was not detected with MeerKAT before 2021 February 07 when it appeared and reached a peak flux density of 5.6mJy. The source was still highly circularly polarized, but also showed up to 80% linear polarization, and then faded rapidly with a timescale of one day. The rotation measure of the source varied significantly, from −11.8 ± 0.8 rad m−2 to −64.0 ± 1.5 rad m−2 , over three days. No X-ray counterpart was found in follow-up Swift or Chandra observations about a week after the first MeerKAT detection, with upper limits of ∼ 5.0 × 1031 erg s−1 (0.3–8 keV, assuming a distance ∼ 10 kpc). No counterpart is seen in new or archival near-infrared observations down to J = 20.8 mag. We discuss possible identifications for ASKAP J173608.2−321635 including a low-mass star/substellar object with extremely low infrared luminosity, a pulsar with scatter-broadened pulses, a transient magnetar, or a Galactic Center Radio Transient: none of these fully explains the observations, which suggests that ASKAP J173608.2−321635 may represent part of a new class of objects being discovered through radio imaging surveys.