A group of astronomers published in a report on January 26 about mysterious radio pulses that repeat periodically every 18.18 minutes from a source in space called GLEAM-X. The period between pulses drifts slowly over a time scale of about 30,000 years. Such a long period and such a slow drift have never been observed for radio pulsars.
The first radio pulsar was observed in 1967 as a series of pulses every 1.33 seconds. The signal, discovered by Jocelyn Bell Burnell, has been jokingly dubbed LGM for “little green men”, in reference to the remote possibility of an extraterrestrial technological origin. When other pulsars were discovered, it became apparent that they were spinning neutron stars. These compact stars, which bear about the mass of the Sun and are the size of a city like Boston, are remnants of the collapsed cores of massive stars. As first proposed in 1934, these collapses trigger supernova explosions that mark the death of these stars.
Pulsars are modeled as rotating magnets with their magnetic axis tilted relative to their axis of rotation. Their radio beams are aligned along their magnetic poles and sweep a circle across their sky – like a beacon. The beam appears as a pulse to a distant observer whose line of sight periodically aligns with it.
The rotating magnet loses energy and its rotation is gradually damped. This results in a slow increase in the period between pulses. The energy loss depends on the mass, radius, magnetic field strength and inclination angle of the neutron star. For all known pulsars, radio emission is a small fraction of the total energy loss rate, implying that most of the rotational energy of these giant flywheels is dissipated in forms other than radio waves. .
In the past 55 years of observing thousands of pulsars, no pulsar has exhibited a period as long as tens of minutes. If GLEAM-X were a neutron star, the model of the rotating magnet would predict a power a few thousand times lower than the observed power of the pulses emitted by it.
If not a neutron star, what could be the source of these pulses? Is it possible that this source deserves the LGM label? After all, a long period could signal a radio beam emitted from the surface of a rotating planet. In this case, a possible technological civilization could broadcast radio waves and create an artificial beacon of technological origin.
Not so fast. In a new paper with my colleague Dani Maoz from Tel-Aviv University, we have shown that by replacing a neutron star with the core of a Sun-like star that has just died, one can naturally obtain GLEAM-X. A flywheel 30,000 times larger than the compact scale of a neutron star carries nearly a billion times more rotational energy and can easily power the most radio observed.
We associated GLEAM-X with what astronomers call a “hot subdwarf.” We have discovered that this sub-dwarf actually has the most extreme properties that would occur naturally if it were rotated and fueled by the accretion of material from a companion star. Future observations can look for the subtle modulation of the pulse period due to variations in the travel time of light as GLEAM-X moves along its orbit with a companion. Indeed, in 2016, the first pulsating white dwarf was discovered with a companion star.
The bottom line is that GLEAM-X does not deserve the LGM label. Similar to the first pulsar ever discovered, it can be explained as a member of a class of remnants that are made naturally when stars end their lives. Much like celebrities in Hollywood, bright stars often end their short careers in explosive fireworks, while mainstream stars have longer careers and end up as cold leftovers.
Avi Loeb is the head of Harvard Galileo project, a systematic scientific search for evidence of extraterrestrial technological artifacts. Loeb is the founding director of Harvard’s Black Hole Initiative, the director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and he chairs the advisory board for the Breakthrough Starshot project. He is the author of “Extraterrestrial: the first sign of intelligent life beyond Earth.”