Pulsating White Dwarf

SETI Live recently featured a discussion with Dr. Simon Steel and PhD researcher Sanne Bloot of the Netherlands Institute for Radio Astronomy (ASTRON), lead author of a recent study published in Astronomy & Astrophysics. Bloot presented the discovery of a pulsating white dwarf — a rare stellar remnant that produced periodic bursts of radio emission. This finding, made with the LOw Frequency ARray (LOFAR) radio telescope, represents a new frontier in the study of compact stellar systems.

LOFAR and Low-Frequency Radio Astronomy

LOFAR is a low-frequency radio astronomy array that operates in the range of 15–170 megahertz (MHz). This is the lowest frequency range accessible from Earth before the ionosphere absorbs incoming radio waves.

Unlike traditional radio surveys that integrate hours of observation into a single image, this study segmented the data into shorter intervals. This method enables the detection of short-lived signals that would otherwise disappear when averaged over time.

A Transient Radio Source

During one particular set of observations, a source of radio waves appeared unexpectedly. It was not present for most of the observation but flared for roughly 10 seconds before disappearing. Over the course of eight hours, this source appeared and disappeared five times at intervals of approximately 800 seconds.

Transient sources (objects that seem to appear and disappear when observed at a specific wavelength) are difficult to confirm. Many fast radio bursts (FRBs) can be attributed to terrestrial interference, such as cell phones or electronic devices near radio telescopes. However, this object was verified across multiple observations and independent datasets, confirming it as a non-terrestrial source.

Optical Counterpart and White Dwarf Identification

Additionally, archival optical data revealed a faint source at the same location. Its color profile suggested the source was a white dwarf, the dense remnant left behind when a medium-mass star exhausts its nuclear fuel. White dwarfs are roughly Earth-sized but extremely hot and dense, with masses up to 1.4 times that of the Sun (the Chandrasekhar limit).

The detected white dwarf was exceptionally strong in blue wavelengths, indicating a high surface temperature. Based on luminosity estimates, the star’s distance from Earth was calculated at approximately 1–4 kiloparsecs (3,000–14,000 light-years).

Long-Period Transients

This object belongs to a new class of astrophysical sources called long-period transients. These objects are compact stellar systems that produce periodic radio pulses over timescales of minutes to hours.

Historically, periodic radio sources have been associated with pulsars, rapidly rotating neutron stars. However, pulsars emit at much shorter timescales, seconds or less. The mechanism driving long-period transients remains under investigation.

To date, only about ten such systems have been identified. Among them, four have been linked to white dwarfs. This suggested that white dwarfs, possibly when interacting with nearby companions, could produce powerful bursts of radio emission.

Companion Star and Interaction Mechanisms

In those other known systems, the pulsating white dwarfs were accompanied by a secondary star. The interaction between the white dwarf’s strong magnetic field and the companion appeared to generate the radio pulses.

In this case, no companion has been directly observed in optical or infrared surveys. If present, the companion may be a low-mass star or a brown dwarf, an object too small to sustain hydrogen fusion, often referred to as a “failed star.”

Evidence for a hidden companion came from the pulse pattern itself. The radio beam did not appear with every rotation, but in a consistent two-out-of-five cycle. This suggests that the emission beam was being influenced, or “pulled,” by the gravitational or magnetic presence of a nearby body.

Furthermore, the white dwarf’s rotation appears to be accelerating. Normally, stellar remnants slow down over time as they radiate energy due to the conservation of angular momentum. Accretion — the collection of material from a companion star onto a white dwarf — could provide the extra angular momentum needed to speed up this latest object’s rotation.

Scientific Significance

The discovery has two key implications:

• It expands the known diversity of radio-emitting stellar remnants beyond neutron stars
• It suggests that binary interactions between white dwarfs and companions may generate unique classes of astrophysical transients.

While only a handful of these objects have been cataloged, the development of more refined search methods, such as short-interval imaging, may reveal many more.

This discovery underscores the importance of innovative observational strategies in radio astronomy and challenges existing models of compact object emission, thereby opening new avenues for studying stellar remnants.

Long-period transients represent one of the most intriguing classes of astrophysical sources discovered in the past decade. As LOFAR and future low-frequency instruments continue to survey the sky, more such systems will likely emerge, allowing astronomers to refine models of magnetic interactions, accretion, and stellar evolution.

Watch the full conversation with Sanne Bloot on SETI Live. Read the paper and press release on this pulsating white dwarf.