Cosmic radio pulses repeating every few minutes or hours, known as long-period transients, have puzzled astronomers since their discovery in 2022. A new study, published in Nature Astronomy, now offers a compelling explanation.
Astronomers are well acquainted with pulsars — rapidly rotating neutron stars whose powerful radio beams sweep past Earth like cosmic lighthouses. Even the slowest known pulsars rotate once every few seconds. By contrast, long-period transients repeat on timescales ranging from 18 minutes to more than six hours.
From everything known about neutron stars, they should not be able to emit radio waves while spinning so slowly. This raised a stark question: was something missing from existing physics, or were astronomers looking at the wrong kind of object?
Neutron stars are not the only compact stellar remnants. The new study presents evidence that the longest-lived long-period transient, known as GPM J1839-10, is not a neutron star at all, but a white dwarf. Crucially, it appears to be producing powerful radio beams with the help of a stellar companion — suggesting other long-period transients may do the same.
White dwarfs are the remnants of dead stars, similar in mass to the Sun but compressed into a sphere roughly the size of Earth. On their own, white dwarfs have never been observed to emit radio pulses. However, when paired with an M-type dwarf — a common star about half the Sun’s mass — in a close binary system, the conditions can change.
Astronomers already know that such systems exist. Rapidly spinning “white dwarf pulsars” were confirmed observationally in 2016. This naturally raises the possibility that long-period transients are simply their slower cousins.
More than ten long-period transients have been discovered so far, but most are distant and deeply embedded in the Milky Way, making them difficult to study. Only in 2025 were two identified as white dwarf–M-dwarf binaries — an unexpected result that left open key questions about how they produce radio emission.
The breakthrough came with GPM J1839-10, discovered in 2023. It has a period of 21 minutes and is uniquely long-lived. Archival data revealed pulses dating back to 1988, although they appeared intermittently. Located 15,000 light-years away, it is detectable only at radio wavelengths.
To probe its behaviour, astronomers carried out “round-the-world” observations using three radio telescopes: Australia’s ASKAP, South Africa’s MeerKAT, and the Karl G Jansky Very Large Array in the US. As Earth rotated, each telescope handed off observations to the next.
The signal turned out to be anything but random. Pulses arrived in groups of four or five, with pairs of groups separated by two hours. The pattern repeated every nine hours — a level of stability that points to a binary system orbiting on that timescale. Using this, researchers refined the orbital period to a precision of 0.2 seconds and inferred the masses of the two objects, consistent with a white dwarf–M-dwarf binary.
Modelling the system showed that the radio beam is likely produced when the white dwarf’s magnetic pole sweeps through its companion’s stellar wind, creating a distinctive “heartbeat” pattern in radio data. This makes GPM J1839-10 a strong candidate for the missing link between long-period transients and white dwarf pulsars.
Research into the exact emission physics is ongoing, but this finding marks a crucial step towards understanding one of radio astronomy’s most intriguing mysteries.
The Conversation