A mysterious signal that has continued for 35 years. Who is sending the unknown pulses?

Produced by: Science Popularization China

Author: Shu Bo (popular science creator)

Producer: China Science Expo

When people have a heart check, each heartbeat will be transformed into a similar broken line on the electrocardiogram, which records the electrical pulse signal emitted by the human body during the heartbeat. In fact, pulses are everywhere in the world. It is a physical process in which a short and strong signal appears first and then quickly fades away .

There are electrical pulses in our bodies, which are generated by the current between specific cells. When electrophysiological activities such as brain waves and heart waves occur, they will continuously send out corresponding pulse signals, which can be captured and recorded with professional instruments. In addition to our daily lives, various celestial bodies in the universe are performing various electromagnetic interactions, continuously sending out pulse signals and revealing their true colors in front of radio telescopes.

The most famous radio pulse celestial body is the pulsar , because it can always send out extremely fast pulse signals like a clock, and the fastest can send out once every few milliseconds. Recently, astronomers discovered an unknown pulse source that has lasted for 35 years. It flashes like a pulsar, but the frequency of the pulse is hundreds or thousands of times slower.

An electrocardiogram recording a pulse (Image source: Pixabay)

Pulse: Sweeped by a rapidly rotating neutron star

Before discussing the unidentified pulses, let us review the history of the discovery of pulsars.

In 1967, while analyzing data from a radio telescope, Jocelyn Bell, a doctoral student at Cambridge University, noticed a mysterious radio source that sent out pulse signals every 1.337 seconds, with extremely precise intervals . Astronomers couldn't help but wonder what could send such precise pulse signals? Could it be a broadcast from an extraterrestrial civilization?

In the science fiction works of the time, the little green aliens who drove UFOs to Earth attracted much attention. Bell and his mentor Hewish also dubiously named this radio source "LGM-1", which means "Little Green Man 1". But a few months later, Bell discovered a second radio source that emitted pulses, which showed that the mysterious pulses were not caused by aliens, because the signals emitted by different extraterrestrial civilizations on different planets should not all be the same. This should be caused by an unknown celestial body .

Josephine Bell (Image source: Wikipedia)

Astronomers remembered Baade and Zwicky's prediction in 1934: after a massive star burns out, it will be compressed by its own gravity into a dense celestial body composed of neutrons - a neutron star . A neutron star is only about 20 kilometers in diameter. Although it is only as big as a city, every cubic centimeter of matter on it weighs hundreds of millions of tons.

The huge amount of kinetic energy concentrated in such a small body gives the neutron star the ability to rotate at high speed , rotating once every few seconds. Neutron stars are also surrounded by strong magnetic fields , and particles emit strong radiation beams from the north and south poles of the magnetic poles. Moreover, the magnetic axis and rotation axis of the neutron star do not actually coincide , so the radiation beam will sweep across the sky as the neutron star rotates. As long as the angle is right, the radiation beam will sweep across the earth once every rotation of the neutron star, and the telescope will see a flash of pulses. This is the principle of how neutron stars become pulsars.

Mainstream theory holds that the pulses we see are radiation beams bursting out in the direction of the neutron star's magnetic axis (Image source: Beijing Planetarium)

What is the source of the mysterious pulse signal?

The theoretical model of pulsars seems perfect, but there is no perfect scientific theory in the world, and some new discoveries are challenging our cognition.

In July 2023, the journal Nature reported a strange new discovery: a strange "magnetar" numbered GPM J1839-10. This celestial body is located in the direction of Cygnus 15,000 light-years away, and it emits pulse signals regularly like a pulsar. However, compared with those pulsars with frequencies as fast as seconds, the pulse frequency of this celestial body is as slow as 22 minutes, and the emitted signal lasts for 5 minutes each time .

After consulting the archives, the researchers found that the pulse signal of the celestial body was recorded as early as 1988 and has continued for more than 30 years, but it has not been seriously examined by scientists until recently.

So what exactly is this celestial body?

The first guess is that it is a neutron star that rotates so slowly that it breaks the record. This is because after a neutron star rotates for a long time, its energy will gradually be lost , and it will rotate slower and slower. However, as the energy decreases, the radiation beam generated by the neutron star's magnetic field will also gradually weaken. According to calculations, a neutron star that rotates once every 22 minutes is too old to emit pulses, so GPM J1839-10 should not be an old neutron star .

Neutron stars can even emit pulses less than a second (Image credit: Australian National Telescope Agency)

Astronomers thought of a second possibility: magnetars . Magnetars are rare neutron stars with magnetic properties thousands of times stronger than ordinary neutron stars . Only about 20 of them have been discovered so far. Magnetars rotate relatively slowly, taking several seconds to complete a rotation. The environment on them is not very stable and they often explode, producing strong X-rays and radio pulses. If X-rays are found around GPM J1839-10, wouldn't that mean it is a magnetar that rotates particularly slowly? But to everyone's disappointment, no trace of high-energy rays was found around it.

Schematic diagram of magnetar (Image source: Astronomy magazine official website)

Could it be that GPM J1839-10 is not a neutron star, but another celestial body?

The mystery is still unfolding: White dwarfs, or an unsolved mystery?

The third hypothesis is very bold: the mysterious celestial body may be a white dwarf .

White dwarfs are dense celestial bodies formed when smaller stars go out . Their diameter is hundreds of times larger than that of neutron stars and is about the same size as the Earth. You may ask, aren't pulsars all neutron stars? Can white dwarfs also emit pulse signals?

That’s right, humans have actually discovered two white dwarfs disguised as pulsars: AR Scorpii , discovered in 2016, emits a pulse every 1.97 minutes; and J1912-4410 , discovered in 2023, emits a pulse every 5 minutes.

AR Scorpius (Image source: Wikipedia)

The two white dwarfs form a binary star with the red dwarf next to them, rotating around each other every few minutes. The particles flying out of the white dwarf's magnetic field at high speed are thrown towards the red dwarf next to it, forming a strong pulse. Such white dwarfs only produce pulses every few minutes , which is very close to 22 minutes compared to the second-level frequency of neutron stars. So are they the source of the mysterious pulses?

Unfortunately, the white dwarf speculation still does not match reality. Because white dwarfs release less energy , the brightness of the AR Scorpii star just mentioned is only one thousandth of that of GPM J1839-10.

If this celestial body is neither a magnetar, a neutron star, nor a white dwarf, then what is it?

History seems to have returned to 1967 when humans once regarded pulsars as "little green man signals". Today, many people also regard GPM J1839-10 as a possible "alien signal". However, the team of scientists who made this discovery also discovered another similar mysterious celestial body in 2022, numbered GLEAM-X J162759.5-523504.3, which emits a pulse signal lasting about 1 minute every 18 minutes and 11 seconds, but this signal disappeared after two months.

Two different extraterrestrial civilizations will not send out the same signal. This is the same reason why "LGM-1" is not a "little green man". The two mysterious pulse signals that have appeared recently are also caused by the same celestial body, not by aliens. It seems that there is a celestial body in the universe that emits a pulse every 20 minutes or so. It may be a new type of neutron star with super slow rotation, or it may be a brand new unknown celestial body. It is a naturally formed astronomical phenomenon, and we just need new observations and theories to explain it.

Conclusion

Historically, astronomers have discovered more and more similar celestial bodies since the discovery of pulsars in 1967. As data accumulates, their secrets are finally revealed. Today, if we want to find out what GPM J1839-10 is, the best way may be to conduct more detailed and longer-term observations of the sky and develop new theories. When we discover more similar radio sources, the mystery will not be far away.

References:

[1] Hurley-Walker, N., et al. "A long-period radio transient active for three decades." Nature 619.7970 (2023): 487-490.

[2] Kaspi, Victoria M. "Slow-beating radio waves from a long-lived source." (2023): 472-473.

[3] A rare, mysterious radio source is stumping astronomers, Astronomy Magazine.

[4] Bizarre 'slow' neutron star challenges our theories about dead stars.

[5] Something in space has been lighting up every 20 minutes since 1988, ArcTechnica.