The 80s Called, They Want Their Ultra-Long-Period Radio Transient Back

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Title: Long-Period Radio Pulsars: Population Study in the Neutron Star and White Dwarf Rotating Dipole Scenarios
Authors: Nanda Rea et al.
First Author’s Institution:
Institute of Space Sciences (ICE-CSIC), Barcelona, Spain
Status: Published in ApJ

Earth is constantly bombarded by radio waves from all across the Milky Way. The shorter time-scale signals, known as “radio transients,” can sometimes be periodic. These periodic signals are often attributed to rotating neutron stars, which act as cosmic lighthouses, sending out radio waves that appear to us as “pulses” of emission. Normally, the slowest of these sources have pulses separated by milliseconds or seconds. However, the recent detection of two signals with astounding periods of 18 and 21 minutes has completely surpassed even the slowest signals previously detected. What could be causing these ultra-long-period radio signals, and what can they tell us about the populations of extreme compact objects they come from? The authors of today’s article investigate these very questions.

Time for a Mystery

GLEAM-X J1627–52 and GPM J1839–10 (astronomers love brevity) are the names given to these two ultra-long-period radio transients. Amazingly, we have archival data dating back to 1988 showing that GPM J1839–10 has been active for more than 30 years! It is still a great mystery what kind of object could produce such a long-period signal, but two kinds of stars might be the culprits.

White dwarfs, Earth-sized stars that form toward the end of the lifecycle of intermediate-mass stars like the Sun, are the first potential source of these signals. White dwarfs often have slow enough rotation periods to account for the ultra-long-period signals, but there is no known mechanism by which lone white dwarfs could produce bright enough radio signals. It is thought that a companion star could possibly enhance the white dwarf’s radio emission via the companion’s stellar wind. As the white dwarf’s emission beams cross through this stellar wind, the emission accelerates particles within the stellar wind, releasing radio waves as accelerated electrons interact with the white dwarf’s magnetic field.

Only two radio-emitting white dwarfs have ever been observed, and both were in binary systems. Based on the lack of an optical or infrared component, researchers thing that GLEAM-X J1627−52 is likely not in a main-sequence binary system similar to these radio-emitting white dwarfs. GLEAM-X J1627−52 could still have a low-mass companion, similar to the AR Scorpii system, which is a white dwarf pulsar binary containing a low-mass, red dwarf companion star (learn more about AR Scorpii in this Astrobite). GPM J1839–10 has not yet been constrained to be in a binary using optical and infrared observations.

Highly magnetized neutron stars known as magnetars are the second potential source. Magnetars are the extremely dense, extremely magnetic leftovers of stellar explosions. How dense and how magnetic, you might ask? Just one tablespoon of magnetar matter weighs as much as Mount Everest, and their magnetic fields, which are a thousand trillion times stronger than Earth’s magnetic field, make them the most magnetic objects known. Magnetars tap directly into their magnetic fields to fuel their powerful beams of radio waves.

Not So Perfect Sources

Despite the amazing properties of both white dwarfs and magnetars, the authors find several issues when trying to explain GLEAM-X J1627–52 and GPM J1839–10 using these types of objects. While observations from back in 2018 show that GLEAM-X J1627−52 does have a brightness and polarization similar to other radio magnetars, X-ray measurements put limits on the source that challenge a magnetar being responsible. The ultra-long period is also not in line with the rest of the neutron star population. On the other hand, white dwarfs are not known to produce the observed bright emission.

Crossing the Line

To figure out whether neutron stars or white dwarfs might be responsible for these long-period signals, the authors use two methods: death-line analyses and population-synthesis simulations.

“Death line” sounds like an ominous term, but it simply defines the threshold at which radio emitters no longer put out bright enough signals for us to detect with our telescopes, meaning they are effectively “dead.” The authors create a range of death lines (shown in Figure 1) based on models ranging from the simplest models available to more extreme models that incorporate complicated physics like twisted magnetic field lines.

plot of surface magnetic field at poles versus spin period

Figure 1: The two death “valleys” of the neutron star and white dwarf populations. An object that falls below the death line no longer emits strongly enough for detection using our current radio telescopes or no longer emits at all, hence why it is “dead.” The neutron star death lines are marked in red and white dwarf death lines are marked in blue. The bounds of the valleys come from the range of different models. For both populations, GPM J1839-10 falls below even the most extreme death line. [Rea et al. 2024]

Based on these death lines, the authors conclude that neutron stars could create the type of signal seen from GLEAM-X J1627−52, but not the signal seen from GPM J1839–10, since it falls below even the most extreme death line. The same is true for magnetized white dwarfs.

The second method is called population synthesis, which uses statistics from the already existing sample of neutron stars and white dwarfs to try and simulate the total population of these stars. The authors vary different parameters like magnetic field strength, birth rate, and the angle between the magnetic field axis and rotational axis to try and figure out if a special population of neutron stars or white dwarfs with these slow periods might exist in the galaxy.

The authors find that a large population of long-period radio emitters cannot be easily explained by neutron stars, even when assuming extremes like no magnetic field decay, stronger magnetic fields, etc. The white dwarf population synthesis shows that magnetized white dwarfs with long periods are more common, but isolated white dwarfs are not expected to be able to emit bright, coherent (constant phase) radio emission, based on the existing population.

This is likely just the first step in uncovering an entire unknown population of ultra-long-period radio transients. Many more sources like the two discussed in today’s article could be out there, but for now the authors recognize that the sample size is small. If either of the two sources, GLEAM-X J1627−52 or GPM J1839–10, is confirmed to be a neutron star or white dwarf in the future, it will tell us a lot about the physics of these extreme objects. It may even require a revision of our understanding of neutron stars or white dwarfs, both in terms of how exactly they emit radio waves, and what their populations look like in the galaxy!

Original astrobite edited by Maryum Sayeed and Annelia Anderson.

About the author, Magnus L’Argent:

Magnus is a first-year Master’s student and Trottier Fellow at McGill University. When not searching for new pulsars, fast radio bursts, and other radio transients, he enjoys going on hikes, reading sci-fi, and watching movies.