Astronomers Detect a Mysterious Radio Signal Coming From the Direction of a Dead Star—And It Repeats Every 22 Minutes

Astronomers using the Murchison Widefield Array in Western Australia have detected a strange new object in the Milky Way: a source of radio waves that turns on for one minute every 22 minutes, emitting a burst of polarized radiation before falling silent again. The object, designated GPM J1839-2207, does not behave like a pulsar, a magnetar, or any other known class of neutron star. In fact, it does not behave like anything ever seen before.

"We have never seen anything like this," said Dr. Natasha Hurley-Walker, an astrophysicist at Curtin University who led the discovery team. "It is too slow to be a pulsar—pulsars spin in seconds or milliseconds, not minutes. It is too regular to be a flare star—flare stars are random. It is too bright to be a brown dwarf—brown dwarfs are faint in radio. It is a mystery object. Those are the best kind."

The signal was first detected in archival data from 2018. At the time, the team assumed it was terrestrial interference—a passing satellite, a malfunctioning GPS transmitter, a microwave oven at the observatory's cafeteria. But when they checked data from 2019, 2020, 2021, 2022, and 2023, the signal was there every time, always at the same frequency (180 megahertz), always for exactly one minute, always repeating every 22 minutes. Whatever it was, it had been blinking like a cosmic lighthouse for at least five years.

Follow-up observations using the Australian Square Kilometre Array Pathfinder (ASKAP) pinpointed the source's location: approximately 4,000 light-years away in the constellation Sagittarius, toward the galactic center. The distance measurement ruled out a human-made satellite. The object was in the Milky Way. It was real. It was unexplained.

"The first time we saw the signal in real time, I jumped out of my chair," Hurley-Walker said. "I literally jumped. I am not a jumper. But this was—you have to understand—every 22 minutes, on the dot, the signal appeared. You could set your watch by it. Nature does not do that. Nature is messy. This was not messy. This was clockwork."

The leading hypothesis is that GPM J1839-2207 is a type of neutron star called a magnetar, but one with an extremely long rotation period. Magnetars are neutron stars with magnetic fields a thousand times stronger than ordinary pulsars. They can emit radio bursts, but their bursts are usually erratic—bright for a few days, then silent for years. No magnetar has ever shown the perfect periodicity that GPM displays.

A competing hypothesis is that the object is a white dwarf, the Earth-sized remnant of a sun-like star after it exhausts its nuclear fuel. White dwarfs can rotate slowly—periods of minutes to hours are typical. But white dwarfs are not known to produce strong radio emission. If GPM is a white dwarf, it would be the first radio-luminous white dwarf ever discovered, a finding that would rewrite white dwarf physics.

A third, more exotic hypothesis is that GPM is a remnant of a collapsed dark matter halo, emitting radio waves as dark matter particles annihilate each other. Hurley-Walker considers this unlikely but cannot rule it out. "When you have a mystery object, you have to consider all possibilities," she said. "Including the weird ones. Especially the weird ones."

Skeptics have raised the possibility that the signal is an artifact of the data processing pipeline—a glitch that appears periodic by coincidence. The discovery team ran thousands of simulations and found that the probability of a random noise source producing this signal by chance is less than one in a million. The signal is real. The signal is unexplained.

Astronomers around the world are now pointing every available telescope at GPM. The Very Large Array in New Mexico has scheduled 40 hours of observing time. The Five-hundred-meter Aperture Spherical Telescope (FAST) in China has requested an emergency target-of-opportunity observation. The Hubble Space Telescope cannot see radio waves, but it is looking for an optical counterpart—a faint star or galaxy at the exact location of the radio source. So far, nothing has been found.

"It is possible that the object is invisible at optical wavelengths," Hurley-Walker said. "A cold, old neutron star would be too faint for Hubble to see at 4,000 light-years. A white dwarf would be visible, but it might be obscured by dust. We need more data. We need to observe the source at higher radio frequencies to see if the period changes. We need to observe it for another year to see if the period is actually slowing down, which would tell us we are watching a neutron star spin to a stop."

The discovery has captured the public imagination in a way few astronomical findings do. Social media is full of jokes about aliens, cosmic clocks, and "the universe's most punctual star." Hurley-Walker is more cautious. "It is never aliens," she said. "It is always a natural phenomenon we have not discovered yet. That is what makes it exciting. Not aliens. New physics. New physics is the real discovery. Aliens are just a distraction."

For now, GPM J1839-2207 continues to blink, every 22 minutes, every hour, every day, for at least five years and possibly for millions. Somewhere in the galaxy, a dead star is sending a message. We do not know what it is saying. But it is saying it on schedule.