Weird, repeating radio signal near the center of the Milky Way.

Astronomers have detected a strange and repetitive radio signal near the center of the Milky Way, and it does not resemble any other energy signature ever investigated.

According to a new paper accepted for publication in The Astrophysical Journal and posted on the preprint server arXiv, the energy source is extremely finicky, appearing bright in the radio spectrum for weeks at a time and then completely vanishing within a day. This behavior doesn't quite fit the profile of any known type of celestial body, the researchers wrote in their study, and thus may represent "a new class of objects being discovered through radio imaging."

The radio source, known as ASKAP J173608.2 321635, was detected with the Australian Square Kilometre Array Pathfinder (ASKAP), a distant Australian hinterland radio telescope. In an ASKAP survey taken between April 2019 and August 2020, the strange signal appeared 13 times, never lasting in the sky for more than a few weeks, the researchers wrote. This radio source is highly variable, appearing and disappearing without a foreseeable calendar, and does not appear in other radio telescope data before the ASKAP reading.

When the researchers tried to match the energy source with observations from other telescopes — including the Chandra X-ray Observatory and the Neil Gehrels Swift Observatory, as well as the Visible and Infrared Survey Telescope for Astronomy in Chile, which can pick up near-infrared wavelengths — the signal disappeared entirely. Without apparent emissions from any other part of the electromagnetic spectrum, ASKAP J173608.2 321635 is a radio phantom that seems to challenge the explanation.

Past studies have detected low-mass stars that periodically ignite with radio energy, but these flaring stars typically have X-ray counterparts, the researchers wrote. It renders a star source unlikely here.

Dead stars, such as pulsars and magnetars (two types of super-dense and collapsed stars), are also implausible explanations, the team wrote. While pulsars can project light beams of radio light beyond the Earth, they rotate with predictable periodicity, generally sweeping their lights beyond our telescopes on a scale of hours instead of weeks. Magnetars, on the other hand, still have a strong X-ray equivalent with each of their explosions – again, contrary to the behavior of ASKAP J173608.2 321635.

The closest match is a mysterious class of objects known as a galactic center radio transient (GCRT), a rapidly glowing radio source that brightens and decays near the Milky Way's center, usually over the course of a few hours. So far, only three GCRT have been confirmed, and all appear and disappear much more quickly than the new ASKAP object. However, the few known GCRT glow with similar brightness to the mysterious signal, and their radio flare-ups are never accompanied by X-rays.

If this new radio object is a GCRT, its properties extend the boundaries of what astronomers believed the GCRT could do, the researchers conclude. The next radio readings from the galactic center should help shed light on the mystery.