In the southern sky, in the constellation Sign, an international team of researchers has come across a mysterious source of radio radiation: Every 21 minutes, the celestial object GPM J1839-10, located 15,000 light-years away, emits a radiation pulse that lasts between half a minute and five minutes. And, as research by scientists in astronomical data archives has shown, this has been the case for more than 35 years. This unusual object cannot be explained with previous ideas of pulsating radio sources – so-called pulsars – write the researchers in the science journal Nature.

In 1967, when the British astronomer Jocelyn Bell encountered radio pulses from space that repeated every 1.3 seconds, she initially thought it was a signal from extraterrestrial beings. Sky researchers now know of more than 3000 such pulsars with periods ranging from seconds down to milliseconds in our Milky Way. And the scientists have also found a natural explanation for the phenomenon: they are fast-rotating Neutron stars with a Strong Magnetic Field.

“The object challenges our understanding of neutron stars”

Neutron stars are extremely dense star corpses: they are only about ten kilometers across, but contain more mass than our sun. Many neutron stars have a strong Magnetic Field – and electrons accelerated by this magnetic field create the radio radiation that shoots out from the magnetic poles into space. However, since the north-south axis of the magnetic field does not coincide with the axis of rotation of the neutron star, this beam sweeps through space like a lighthouse. And whenever the beam hits the earth, the radio telescopes register a pulse from the object.

But not every neutron star is also a pulsar: the magnetic field has to be strong enough and the rotation fast enough. This results in a limit beyond which a neutron star should not emit any radio pulses. Astrophysicists call this limit the “death line” of the pulsars. With its extremely slow period of 21 minutes, GPM J1839-10 is not just a little, but well beyond that line of death.

“The object challenges our understanding of neutron stars,” admits Natasha Hurley-Walker of Curtin University in Australia. The astronomer and her colleagues were using a large telescope facility in Western Australia, the Murchison Widefield Array, to search for unusual, variable radio sources in the sky when she encountered GPM J1839-10.

At first, scientists thought it was a temporary phenomenon. But a search through the archives of other large radio telescopes yielded a surprising result: the object was also – previously unnoticed – hidden in the data of the Giant Metrewave Radio Telescope in India and the Very Large Array in the USA. The oldest data goes back to 1988 – and even then GPM J1839-10 emitted radio pulses with the same period.

But how can such a slowly rotating neutron star emit radio pulses – over decades? Perhaps, Hurley-Walker and her colleagues speculate, it is a magnetar – a neutron star with an extreme magnetic field thousands of times stronger than a normal pulsar. The idea is that this strong magnetic field could compensate for the slower rotation. However, no magnetar is known to rotate as slowly as GPM J1839-10. Magnetars also emit X-rays at the same time as radio emission – but the team has not been able to detect any X-rays from GPM J1839-10 so far.

So maybe GPM J1839-10 is not a neutron star at all? As a further explanation, the scientists are discussing a white dwarf star with a strong magnetic field. Since such objects are considerably larger than neutron stars, they can also produce radio emissions despite their slow rotation. So far, however, astronomers have not come across a white dwarf that shows a similarly strong radio emission as GPM J1839-10. The team is now specifically looking for other long-period pulsars in order to get to the bottom of the nature of this mysterious celestial object.