Cosmic radio pulses that repeat every few minutes or hours, known as long-period transients, have baffled astronomers since their initial discovery in 2022. A groundbreaking new study, published in Nature Astronomy, may finally provide some much-needed clarity on these enigmatic signals.
Pulsars and the Puzzle of Slow Rotation
Radio astronomers are well-acquainted with pulsars, which are rapidly rotating neutron stars. From Earth, these objects appear to pulse as their powerful radio beams sweep past our telescopes, akin to a cosmic lighthouse. The slowest pulsars have rotation periods of just a few seconds. However, in recent years, long-period transients have been detected with periods ranging from 18 minutes to over six hours.
According to established physics, neutron stars should not be capable of emitting radio waves while spinning at such slow speeds. This discrepancy has led scientists to question whether there might be an issue with our current understanding of astrophysics. Yet, neutron stars are not the only compact stellar remnants in the universe, suggesting they may not be the source of these mysterious signals after all.
White Dwarf Pulsars: A New Explanation
The new research presents compelling evidence that the longest-lived long-period transient, GPM J1839-10, is actually a white dwarf star. White dwarfs are the remnants of dead stars, roughly the size of Earth but containing a mass equivalent to that of the Sun. While no isolated white dwarf has ever been observed emitting radio pulses, they possess the necessary components to do so when paired with an M-type dwarf star in a close binary system.
In fact, rapidly spinning "white dwarf pulsars" have been confirmed to exist, with the first one identified in 2016. This raises an intriguing question: could long-period transients be the slower relatives of these white dwarf pulsars? More than ten long-period transients have been discovered to date, but their distant locations and deep embedding within our galaxy have made it challenging to determine their true nature.
GPM J1839-10: A Uniquely Long-Lived Example
Discovered in 2023, GPM J1839-10 is a long-period transient with a 21-minute period. What sets it apart is its remarkable longevity; pulses have been found in archival data dating back to 1988, though they were only detected intermittently. Located 15,000 light-years away, this object is only visible in radio waves, prompting astronomers to conduct extensive observations.
Using a series of "round-the-world" observations with telescopes in Australia, South Africa, and the United States, researchers uncovered that the signal is not random. Instead, pulses arrive in groups of four or five, with pairs separated by two hours, and the entire pattern repeats every nine hours. This stable pattern strongly indicates that the signal originates from a binary system where two bodies orbit each other every nine hours.
The Heartbeat Pattern and Binary System Insights
Further analysis revealed that GPM J1839-10 is definitively a binary system. The peculiar "heartbeat" pattern of its pulses offers clues about its nature, which can only be deciphered through radio signals. Inspired by previous studies on white dwarf pulsars, scientists modelled GPM J1839-10 as a white dwarf generating a radio beam as its magnetic pole interacts with its companion's stellar wind.
This model accurately predicts the heartbeat pattern based on the varying alignment of the binary bodies with our line of sight throughout the orbit. Researchers have even been able to reconstruct the geometry of the system, including the distance between the stars and their masses. All of this evidence points to GPM J1839-10 being a white dwarf-M-dwarf binary.
Implications and Future Research
GPM J1839-10 holds the potential to serve as the missing link between long-period transients and white dwarf pulsars. Armed with this new model, other astronomers have already detected variability at the measured periods in high-precision optical data, despite not being able to distinguish the binary pair directly.
Ongoing research is focused on understanding the exact emission physics and how the broader range of long-period transient properties fit together. This discovery represents a crucial step forward in unravelling the mysteries of these cosmic signals and enhancing our comprehension of stellar remnants in the universe.
About the authors: Csanád Horváth is a PhD Candidate in Radio Astronomy at Curtin University, and Natasha Hurley-Walker is a Radio Astronomer at Curtin University. This article is republished from The Conversation under a Creative Commons license.
