Zombie Star Pulses Every 7.9 Minutes, and a Sun-Like Star May Be Why

This visible star, not the white dwarf, is what Keck saw — the zombie star itself is too faint in visible light. The companion has an estimated mass about 94 percent of the Sun's and a surface temperature of roughly 5,500 Kelvin.

The researchers concluded that the system consists of a white dwarf and a Sun-like star orbiting each other roughly every 1.35 days. The eight-minute rhythm is the white dwarf's rotation; the 32.5-hour cycle marks the pair's full orbit.

"This is the first time the pulses are clear enough to see the white dwarf moving in its orbit around its companion, so we know that the pulsations are coming from a region close to the white dwarf, or possibly even from the white dwarf itself," said lead author Dr. Sam McSweeney, a commissioning scientist with the Square Kilometre Array Observatory.

Why the living star matters

The companion may be more than a bystander. Our Sun sits inside a corona, an atmosphere of extremely hot, electrically charged plasma shaped by magnetic fields that form loops, erupt in flares and hurl clouds of particles into space. The Sun-like companion probably hosts similar magnetic activity, and the white dwarf's radio pulses may pass through that environment on their way to Earth.

The team noticed that the polarization of the signals — the orientation of their electromagnetic waves — shifted in ways that depended on the stars' orbital positions. Magnetized plasma can twist polarization through a phenomenon called Faraday rotation, so each pulse acts like a probe sampling the outer atmosphere of a distant star.

"As a bonus, we also saw the white dwarf moving behind a region with a strong magnetic field, most likely a 'stellar prominence', which is a large loop of plasma erupting from a star's surface," said co-author Associate Professor Natasha Hurley-Walker. "It's amazing that by using radio wavelengths we can detect the storms on a star over 5000 light-years away."

What still puzzles researchers?

The central question remains open: what actually generates the pulses? J0715−1141 belongs to an emerging class called long-period transients, objects producing intense, often highly polarized radio signals at intervals from minutes to hours. Astronomers have found only about a dozen, and they differ from one another considerably.

Some resemble pulsars — rapidly spinning neutron stars whose radio beams sweep space like lighthouse beams — but many long-period transients pulse far more slowly. For white dwarfs, one proposal is that a spinning white dwarf's magnetic field interacting with material from its companion can accelerate charged particles and produce coherent radio emission. That idea is plausible but unproven.

The system's origin is also uncertain. The scientists suspect the white dwarf's progenitor once swelled into a giant star, engulfing its companion in a shared envelope of gas. As the stars interacted with that material, the orbit shrank dramatically; the envelope was eventually expelled, leaving the compact pair we see today.

More sensitive radio observations, particularly with the upcoming Square Kilometre Array telescopes, could reveal many more such systems. Each brief pulse may say as much about the living star beside the dead one as about the zombie star itself.

The study was published in Nature Astronomy.