Extreme magnetic field: Astronomers have measured the strongest magnetic field in space for a neutron star in the Milky Way. So the most luminous pulsar has a magnetic field strength of 1.6 billion Tesla – that’s a new record. This magnetic force was measured through the subtle modulation of the X-rays emitted by this pulsar. The measurements also show that these stellar remnants have a highly magnetic field.
Magnetic fields are often created by moving electric charges. In the core of the earth, this drives the geodynamo of our planet, in technical equipment or laboratories it gives electromagnets their attraction. But their field strengths are mild compared to those achieved by the universe’s magnetic field generators: Super-accelerated streams of particles near black holes or certain neutron stars – called magnetars – can generate magnetic fields with millions of tesla.
A very bright pulsar
Now astronomers have discovered a new record holder among the world’s magnetic field generators. It is a neutron star that ejects a large amount of material from its partner star. This produces powerful X-rays. Named Swift J0243.6+6124, the binary star system was discovered in a 2017 burst of light and intense energy. Astronomers conclude that the neutron star is a very luminous pulsar.
It has long been suspected that luminous neutron stars must also have strong magnetic fields. “Various methods have already been used to determine the magnetic field of Swift J0243.6+6124. However, this produced conflicting results,” explains Ling-Da Kong from the Chinese Academy of Sciences in Beijing and his team. That’s why they’ve retargeted the 2020 pulsar with the new Chinese X-ray telescope Insight-HXMT.
The story line in the X-ray spectrum
The analysis revealed a prominent absorption line in the X-ray spectrum of pulsar radiation. This was in the energy range of 146 kiloelectron volts and showed typical signs of so-called cyclotron resonance scattering, as astronomers report. These lines form when electrons are scattered and accelerated in the magnetic field of a celestial body. If they then encounter X-rays, they absorb certain parts of this radiation.
The decisive point here is that the energy at which this absorption line occurs allows conclusions to be made about the strength of the magnetic field at the event site – and in the case of Swift J0243.6+6124 this is very large. “The energy of its cyclotron spectra is the highest ever measured from a neutron star,” write Kong and colleagues. It also surpasses the previous record holders by a long way.
The strongest magnetic field ever measured
But that means: The magnetic field surrounding this neutron star must also be very strong. According to their measurements, astronomers arrive at a field with a strength of 1.6 billion Tesla. “This is the strongest magnetic field ever measured directly using cyclotron sound scattering,” the team explains. This most luminous pulsar may have the strongest magnetic field so far known in space.
Closer analysis of the X-rays emitted by Swift J0243.6+6124 also suggests that this pulsar does not have a simple magnetic dipole field like Earth. Instead, its magnetic field appears to have a complex multipole structure, Kong and his colleagues report. Other neutron stars in such X-ray binaries may also have multiple magnetic fields.
If confirmed, this could also explain the contradictory measurement data on the magnetic field of many neutron stars: Because other measurement methods used in the past only recorded the dipole part of the magnetic field, they missed the full strength of the magnetic field. (Astrological Journal Letters, 2022; doi: 10.3847/2041-8213/ac7711)
Source: Headquarters of the Chinese Academy of Sciences