Using the world’s most advanced radio telescopes, astronomers have discovered a spinning dead star so rare, strange and unique that they have dubbed it a “cosmic unicorn.” The unique properties of this object, CHIME J1634+44, challenge our current understanding of spinning dead stars and their environments.
CHIME J1634+44, also known as ILT J163430+445010 (J1634+44), is part of a class of objects called Long Period Radio Transients (LPTs). LPTs are a newly found and mysterious type of celestial body that emits bursts of radio waves that repeat on timescales of minutes to hours. That’s significantly longer than the emission of standard pulsars, or rapidly spinning neutron star stellar remains that sweep beams of radiation across the cosmos as they spin.
But as strange as all LPTs are, CHIME J1634+44 still stands out. Not only is it the brightest LPT ever seen, but it is also the most polarized. Additionally, its pulses of radiation seem highly choreographed. And what really stands out about CHIME J1634+44 is the fact that it is the only LPT astronomers have ever seen whose spin is speeding up.
“You could call CHIME J1634+44 a ‘unicorn’ even among other LPTs. The bursts seem to repeat either every 14 minutes or 841 seconds — but there is a distinct secondary period of 4206 seconds, or 70 minutes, which is exactly five times longer,” team leader Fengqiu Adam Dong, a Jansky Fellow at the Green Bank Observatory (GBO), said in a statement. “We think both are real, and this is likely a system with something orbiting a neutron star.”
The team discovered the unusual traits of CHIME J1634+44 using ground-based instruments including the Green Bank Telescope, the Very Large Array (VLA), the Canadian Hydrogen Intensity Mapping Experiment (CHIME) Fast Radio Burst and Pulsar Project, the NASA-operated space-based observatory, and the Neil Gehrels Swift Observatory (Swift). The object was, in fact, simultaneously discovered by a separate team of astronomers at ASTRON, the Netherlands Institute for Radio Astronomy, using the LOFAR (Low Frequency Array) radio telescope.
While the team led by Dong believes a stellar remnant at the heart of CHIME J1634+44 is a neutron star, the ASTRON team, captained by astronomer Sanne Bloot, refers to it as J1634+44 and think it is a white dwarf. What both teams agree on, though, is just how strange this LPT is.
This unicorn is speeding up by feeding on a star
Both white dwarfs and neutron stars are dead stars created when stars of differing masses run out of the fuel supplies they need for nuclear fusion at their cores. Once that fuel is over, the stars can no longer support themselves against their own immense gravities.
Neutron stars are stellar remnants that form when massive stars, with masses at least eight times that of the sun, reach the end of their lives and collapse. Smaller stars closer in mass to the sun leave behind a slightly less extreme stellar remnant called a “white dwarf.”
Though most of the mass of these dying massive stars is shed in supernova explosions, the cores of the stars maintain a mass between one and two times that of the sun. This is crushed down to a width of around 12 miles (20 kilometers), creating matter so dense that if a teaspoon of neutron star “stuff” were scooped out and brought to Earth, it would weigh 10 million tons (equal to stacking 85,000 blue whales on a teaspoon).
This collapse has another extreme consequence. The dying star maintains its angular momentum, meaning that when its radius is rapidly reduced during collapse, it speeds up greatly. Though the collapse of white dwarfs is less extreme, it also causes an increase in spin speed due to the conservation of angular momentum.
An Earth-based example of this is an ice skater pulling in their arms to increase the speed of their spin.
What this means is some young neutron stars can spin as fast as 700 times every second. However, as neutron stars and white dwarfs age, they should slow down as they lose energy. That’s why no matter what CHIME J1634+44 is, the fact that it is speeding up its spin is very strange.
There is a way neutron stars or white dwarfs can increase their spin speed, or “spin up” after their birth. It depends on whether they have a close companion star.
As such, the new study’s team suspects CHIME J1634+44 may actually be composed of two stellar objects orbiting each other in a tight binary format. The ASTRON team proposes that this companion is either another stellar remnant (like a white dwarf or neutron star) or is a “failed star” brown dwarf — a body that forms like a star but fails to gather enough mass to trigger the nuclear fusion that defines what a star is.
As these bodies swirl around each other, they would emit ripples in spacetime called gravitational waves. This carries away angular momentum and causes the two stellar bodies to move closer together. This would cause the period of the binary to appear as if it is shortening. This type of orbital tightening has been witnessed before by astronomers in white dwarf binaries.
CHIME J1634+44 gets stranger, however.
Its radio bursts are 100% circularly polarized. This means the electromagnetic waves escaping J1634+44 rotate in a circle (like a corkscrew) as they propagate.
Thus, the electromagnetic radiation escaping CHIME J1634+44 twists around in a perfect spiral as it moves away from its source. Not only is that extremely rare, but it is something that has never been seen in bursts of radiation from either neutron stars or white dwarfs.
That implies the radio wave blasts of CHIME J1634+44 are being generated in a way that is unique for this dead star.
Astronomers have a mystery on their hands with this dead star
What is also weird about these pulses is the fact that they arrive in pairs, but only when the dead star in the CHIME J1634+44 binary has spun several times without emitting a burst.
“The time between pulse pairs seems to follow a choreographed pattern,” team member and ASTRON astronomer Harish Vedantham said in a statement. “We think the pattern holds crucial information about how the companion triggers the white dwarf to emit radio waves.
“Continued monitoring should help us decode this behavior, but for now, we have a real mystery on our hands.”
The research conducted by these astronomers not only reveals more about neutron stars, the universe’s most extreme stellar objects, but also hints at an exciting new phase for radio astronomy.
“The discovery of CHIME J1634+44 expands the known population of LPTs and challenges existing models of neutron stars and white dwarfs, suggesting there may be many more such objects awaiting discovery,” Dong concluded.
Both teams’ research was published on Thursday (July 17) in the journal Astronomy & Astrophysics.