In 2022, a new pulsating radio source in the Milky-Way galaxy was discovered by Natasha Hurley-Walker and collaborators from Curtin University in Australia, based on archival data taken by the Murchison Widefield Array (MWA) at low radio frequencies. The analysis revealed a periodic, low-frequency radio transient which pulses every 18.18 minutes. This unusually slow pulsation was never observed before for pulsars associated with neutron stars — the collapse products of massive stars which possess nuclear density, the radius of a big city (about 12 kilometers) and a mass up to twice that of the Sun. The sporadic emission from this unusually slow pulsator, called GLEAM-X J1627−5, was highly linearly polarized, bright, persisted often for up to a minute in a given pulse and was visible across a broad frequency range. The pulse profiles evolved on timescales of hours. The discovery paper in Nature magazine suggested at the end of its abstract that this source “could be an ultra-long-period magnetar.” Since then, a few additional pulsators like it were discovered.
Shortly after the discovery of GLEAM-X J1627−5, I published a paper with my brilliant colleague Dani Maoz, suggesting that the enigmatic pulsar is most likely a proto white dwarf, commonly called a hot subdwarf. We showed that a magnetic dipole model explains the observed period and period-derivative for a highly magnetized (by a hundred million gauss), hot subdwarf of half a solar mass and a third of the solar radius with an age of about thirty thousand years. We concluded that the subdwarf spin is close to its breakup speed and its spin-down luminosity is near its maximum possible value, the so-called Eddington limit, likely the result of accretion from a companion star.
Our paper pointed another star, AR Scorpii, which was discovered earlier as a 2-minute period pulsar in a 3.5-hour orbit with an M-dwarf companion star, in which the pulsar is powered by the spin-down of a white dwarf with a magnetic field below a hundred million gauss. By analogy to this system, we suggested that the spin-down luminosity could easily power the radio emission from GLEAM-X J1627−5.
The inferred spin-down time suggested an age consistent with a stellar remnant that has yet to cool to a white dwarf. Hot subdwarfs with rotation periods similar to that of GLEAM-X J1627−5 are known to exist. The breakup period of such subdwarfs is comparable to the observed period of GLEAM-X J1627−5, making the rotation physically plausible for a subdwarf that was spun up close to its maximum possible spin by accretion from a companion star. We also argued that the sporadic appearance of the observed pulses from the source suggests the presence of a companion star, which endowed the white dwarf pulsar with its rapid spin and high magnetic field. In AR Scorpii, the companion M-type star is not in synchronous orbit with the white dwarf spin. We argued that a similar configuration in GLEAM-X J1627−5 would produce modulation in the pulse arrival time owing to the light travel time, which in principle may be detectable.
In a new paper published last week, Natasha Hurley-Walker and collaborators report the discovery of another long-period radio transient, labelled GLEAM-X J0704−37, with an optical companion consistent with a cool main-sequence star of spectral type M3. The radio periodicity corresponding to the longest period ever found, 2.9 hours, was again discovered in the archival MWA low-frequency data. Observations from the MeerKAT radio observatory with high time resolution show that pulsations from the source display complex temporal structure, implying pulsar-like emission due to a strong magnetic field as suggested in my paper with Dani on GLEAM-X J1627−5. The timing residuals also show tentative evidence of a six-year modulation. The concluding sentence in the abstract of this paper states that a magnetar interpretation is excluded, and the system is likely to be associated with a binary star system composed of an M-dwarf and a white dwarf.
There is no greater reward to a theoretical astrophysicist like myself than to find out that theoretical analysis published several years ago is validated as a likely explanation of a new class of astrophysical sources. These slow pulsators are likely associated with the remnants of sun-like stars. Once these stars consume their nuclear fuel, their core collapses to become a metallic sphere which carries about 60% of a solar mass and steadily cools and condenses to the size of the Earth.
Since billions of sun-like stars formed 7.6–9.2 billion years before the Sun, the graveyard of the Milky-Way is full of white dwarfs. Some of these remnants are accompanied by a common long-lived, low-mass star (M-dwarf) as a close-in companion which damps mass on their surface, spins them up and makes them appear as long-period pulsators like GLEAM-X J1627−5. Many others, like the Sun, do not have such a companion. They will fade away in loneliness, similar to some of the old people we all know.
It must be remarkable for an advanced civilization to reside on a habitable planet near the M-dwarf companion of a pulsating white dwarf of this class. Rather than use atomic clocks, all meetings on this planet can be synchronized to the giant clock in their sky.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
