A steady pulse of bright energy has been emanating from the outskirts of the Milky Way for the past 10 years, occurring every three hours and lasting for about a minute. Astronomers believe they’ve identified the source of the signal, but this discovery brought with it a new mystery—one they now claim to have solved as well.
A team of researchers from the Curtin node of the International Centre for Radio Astronomy Research (ICRAR) first stumbled upon the radio signal while going through archival data from the Murchison Widefield Array (MWA), a radio telescope located in Western Australia. The energy pulse is the longest-period radio transient ever detected, with most signals appearing on timescales between tens to thousands of seconds.
Long-period radio transients are relatively new, with several having been discovered recently. Celestial objects with a changing magnetic field, like the Sun or Jupiter, can produce radio waves. The ones on shorter time-scales are known as radio transients—sudden, short-lived bursts of energetic emissions that are often produced by rotating neutron stars (the collapsed core of a dead star).
“The long-period transients are very exciting, and for astronomers to understand what they are, we need an optical image,” Natasha Hurley-Walker, associate professor at ICRAR, and lead author of a paper on the discovery published in The Astrophysical Journal Letters, said in a statement. “However, when you look toward them, there are so many stars lying in the way that it’s like 2001: A Space Odyssey. ‘My god, it’s full of stars!’.”
Luckily, the newly discovered radio transient, GLEAM-X J0704-37, was not hiding behind stars. Instead it was discovered on the outskirts of the Milky Way around 5,000 light-years away in the Puppis constellation, a region that’s a little less congested than the rest of the galaxy. “Our new discovery lies far off the Galactic Plane, so there are only a handful of stars nearby,” Hurley-Walker added.
With a clear view of the signal, the team behind the discovery used the MeerKAT telescope in South Africa to pinpoint the location of the radio waves to one specific star. Using another telescope, the SOAR observatory in Chile, the researchers then measured the star’s spectrum, determining that it is an M dwarf star, also known as a red dwarf star.
Although the team solved one mystery by locating the source of the signal, another mystery now lay ahead. “An M dwarf alone couldn’t generate the amount of energy we’re seeing,” Hurley-Walker said. “The M dwarfs are low-mass stars that have a mere fraction of the Sun’s mass and luminosity. They constitute 70 per cent of the stars in the Milky Way, but not one of them is visible to the naked eye.”
Instead, the data suggested that the M dwarf was in cahoots with another type of star, both working together to produce the repeating radio transient. The M dwarf is likely in a binary system with a white dwarf—the remains of a star that has exhausted its nuclear fuel and shed its outer layers. “Together, they power radio emission,” Hurley-Walker said.
Although the archival data of MWA showed that the radio transient has been active for 10 years, it may have been emitting bursts of energy for even longer that had gone undiscovered. The team behind the discovery want to carry out follow-up observations of GLEAM-X J0704-37, as well as dig through the data to possibly find more long-period radio transients. There could be a lot more weird cosmic sources pulsing with energy across the universe.