by Astronomers have discovered a highly magnetic dead star in the Milky Way, called a “magnetar,” that briefly behaves like a pulsar, a type of rapidly spinning neutron star. The magnetar donned its pulsar disguise after emitting a powerful burst of radiation that was originally discovered in 2020.
The emission is an example of a fast radio burst (FRB), a mysterious burst of energy whose origin and source are not yet fully understood. While most FRBs have been attributed to sources outside the Milky Way, this one, designated FRB 20200428, originates from our galaxy, making it the first “galactic FRB” ever observed. FRB 20200428 was associated with the strongly magnetic neutron star, or “magnetar,” SGR J1935+2154, located about 30,000 light-years away and orbiting the supermassive black hole at the heart of the Milky Way, Sagittarius A* (Sgr A*).
This led many researchers to theorize that FRBs discovered outside the Milky Way also originate from magnetars. The problem was that definitive proof of this connection was lacking. An international team of researchers continued to observe SGR J1935+2154 to track down this “smoking gun” and discovered the magnetar behaving like a rapidly rotating neutron star, or “pulsar,” as it entered a brief “radio pulsar phase” five months after the launch of FRB 20200428.
To study this magnetar in the Milky Way, the team used the Five-hundred-meter Aperture Spherical Radio Telescope (FAST) in China, which first discovered FRB in 20200428. This giant radio telescope has a long history and allows researchers to search for FRBs.
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What was interesting about the late pulsar phase of SGR J1935+2154 is that FAST detected it differently than the magnetar’s FRB explosion. This suggested to the team that these two phenomena have different origins.
“FAST recorded 795 pulses from the source within 16.5 hours over 13 days,” said team leader Weiwei Zhu of the National Astronomical Observatory of China (NAOC) in a statement. These pulses show different observational properties than the [FRB] Eruptions observed from the source.”
This difference in “emission modes” between the FRB and the pulses could help astronomers better understand what triggers FRBs both within the Milky Way and at vast cosmological distances. It could also reveal more about neutron stars in their various guises.
Magnetar killed the radio star
Like all neutron stars, magnetars and pulsars form when a star at least eight times as massive as the Sun exhausts its supply of fuel for nuclear fusion, cutting off the outward flow of energy that protects the star against the inward pressure of its own gravity.
Without this outward radiation pressure, the star’s core collapses. This sends shock waves through the outer material and triggers a massive supernova explosion. As a result of this catastrophic explosion, the star’s outer layers are blasted away, leaving behind a rapidly collapsing stellar core with one to two times the mass of the Sun. The result is a neutron star that dramatically decreases in diameter.
All neutron stars are estimated to be about 20 kilometers in diameter, meaning they could easily fit into some major cities here on Earth. However, the effects of this would be anything but pleasant.
Neutron stars contain up to twice the mass of the Sun in a sphere as wide as Manhattan Island is long. They are made of a unique form of incredibly dense matter that is rich in neutrons. Neutrons are particles that are normally bound with protons in the nuclei of atoms. If a sugar cube-sized sample of this matter were taken from a neutron star and brought to Earth, it would weigh an incredible 1 billion tons. That’s more than twice the weight of the entire human population, which is estimated at 390 million tons.
However, this is not the only extreme property of a neutron star.
Due to the conservation of angular momentum, the rotation speed of a newly formed neutron star increases massively if its diameter decreases rapidly.
A very earthly example of this is ice skating. When a skater wants to increase the speed of his rotation, he pulls his arms in. To slow down the rotation, he stretches his arms out again.
Pulsars are neutron stars that can spin so fast that they complete hundreds of revolutions per second. In fact, the fastest-spinning pulsar ever discovered is PSR J1748-2446ad, which spins 716 times per second. A pulsar also emits rays from its poles. That is, as it rotates, this type of neutron star sweeps rays across the universe like a cosmic lighthouse.
The reduction in the width of neutron stars also causes the progenitor star’s magnetic field lines to be squeezed together. The closer the magnetic field lines are to each other, the stronger the magnetic field lines become. This means that neutron stars have the strongest magnetic fields in the universe, with some reaching over 1 billion Tesla. For comparison, the strongest magnetic fields generated here on Earth are around 1,500 Tesla. The neutron stars with the strongest magnetic fields are called magnetars.
To be clear: all magnetars are neutron stars and all pulsars are neutron stars, but magnetars are different from pulsars because they normally Magnetars lack the radio wave beams from their poles that make them pulsate. However, magnetars are not emission-free; they have long been cited as a source of FRBs.
When a magnetar imitates a pulsar
Radio pulses like those the team discovered during the late pulsar phase of SGR J1935+2154 are similar to FRBs, but the latter emissions are tens of billions of times brighter. They are also common in pulsars, as mentioned above, but not so much in magnetars. Most magnetars do not emit radio wave pulses, possibly because their strong magnetic fields prevent them from doing so.
Nevertheless, some magnetars can briefly become pulsars after a period of intense activity. This appears to be what Zhang and his team observed in SGR J1935+2154.
“Like the pulses in radio pulsars, the magnetar pulses are emitted within a narrow phase window within the period,” Zhang explained. “This is the well-known ‘lighthouse’ effect, namely that the emission beam crosses the line of sight once per period and only for a short time interval in each period. You can then observe the pulsed radio emission.”
When FRB 20200428 was discovered from the same magnetar in April 2020, this FRB and several less energetic outbursts that followed it were coincidental, meaning they were not part of the precise frequency pulse window of the pulsar phase of SGR J1935+2154.
“This strongly suggests that pulses and bursts originate from different locations within the magnetar’s magnetosphere, possibly indicating different emission mechanisms between pulses and bursts,” Zhang continued.
One possible outcome of this study and its follow-up is a better understanding of why some FRBs recur but most do not.
FAST has detected thousands of repeated FRBs from the same sources, probably magnetars. However, unlike pulsar pulses, these repeated FRBs lacked a clear pattern or “periodicity.” This has cast doubt on the idea that FRBs originate from magnetars.
This doubt could be dispelled by this research.
“Our discovery that bursts are typically generated in random phases provides a natural explanation for the lack of detection of the periodicity of repeating FRBs,” Zhang concluded. “For unknown reasons, bursts from a magnetar are typically emitted in all directions, making it impossible to identify periods of FRB sources.”
The team’s research was published in the journal Science Advances on July 28.
