Two Dead Stars Are Circling Each Other Every Six Minutes, and the Orbit Is Shrinking Fast

An X-ray source called eRASSU J0608 flashes on a metronome: one bright pulse every 374 seconds, just over six minutes. That is not a rotation. It is two burned-out stars orbiting each other, and six minutes is an extraordinarily short time for a pair of objects each roughly the size of Earth to complete a lap.

A team led by Rahul Sharma reports in The Astrophysical Journal Letters that the pair is a double white dwarf — the dense remnants left when stars like the Sun exhaust their fuel and shed their outer layers. The researchers identified the system using data from NASA's Neutron Star Interior Composition Explorer, NICER, mounted on the International Space Station, and from the Einstein Probe X-ray observatory.

The important number is not the period but its change. Comparing the new observations against XMM-Newton data taken three years earlier, the team found the orbit measurably tightening. Two masses whipping around each other that quickly radiate energy as gravitational waves — ripples in spacetime — and losing that energy makes them fall closer together, which makes them orbit faster still. The measured decay rate is among the fastest recorded for any binary star system, which is another way of saying this is one of the strongest gravitational-wave emitters known.

The system's chirp mass, the combination of the two masses that sets how loud a gravitational-wave signal is, comes out at about 0.43 solar masses. That is modest as these things go, which makes the second missing number decisive.

Nobody knows how far away eRASSU J0608 is. X-ray data alone cannot fix a distance, and gravitational-wave amplitude falls off with the square of distance — double the distance and the signal drops to a quarter. A nearby system with this orbit would be a guaranteed detection for LISA, the European Space Agency's planned space-based gravitational-wave observatory, which is designed to hear exactly this frequency range and is due to launch in the 2030s. A distant one might be inaudible. The researchers hope to find a third stellar companion in the system whose light can be measured conventionally, which would pin the distance down.

Ground-based detectors such as LIGO and Virgo cannot help here. They listen at frequencies of tens to thousands of cycles per second, tuned to the final violent seconds of merging black holes and neutron stars. A six-minute orbit produces a signal thousands of times slower — a steady hum rather than a chirp, and one that only an instrument in space, with arms millions of kilometers long, can pick up.

If the distance turns out to be favorable, eRASSU J0608 becomes a verification binary: a source astronomers already know is there, at a known frequency, that LISA can be checked against the moment it switches on.