Demystify the origins of the very low-mass Black Widow pulsar

Recently, Guo Yunlang, a doctoral student at the Yunnan Astronomical Observatory of the Chinese Academy of Sciences, and Wang Bo, a researcher, have made new progress in the field of millisecond pulsars, explaining the origin of the very low-mass black widow pulsar. The International Scientific Journal, THE ROYAL ASTRONOMICAL Society (MNRAS), recently published the results online under the title “Formation of black widows through ultra-compact X-ray binaries with He star companions”.

Millisecond pulsars are a class of high-speed spinning neutron stars with a rotation period of less than 30 milliseconds and are generally thought to have originated from low-mass X-ray binary stars. During the evolution of X-ray binary stars , neutron stars acquire angular momentum by accreting the material of their companion stars , thereby accelerating rotation. The study of millisecond-pulsed binary stars is of great scientific significance. Short-orbital millisecond-pulsed binary stars are important sources of low-frequency gravitational waves. In addition, such objects can also be used to reveal the equations of state of neutron stars, as well as physical processes such as the decay of the magnetic field on the surface of neutron stars during rotational acceleration.

Observations have found some special radio mask millisecond pulse binary stars with an orbital period of about 1 day. Their companion stars are stripped of their material by the pulsar’s pulsar’s pulsar radiation, as if spiders were eating away at their mates (see Figure 1). So scientists named these pulsars after black widows and red backs. The main difference between Black Widow and Red-backed Pulsars is the mass of their companions: Black Widow’s companion masses are typically less than 0.5 solar masses, while red-backed companions are between 0.1 and 1 solar masses.

The latest observations suggest that Black Widow pulsars can be divided into two categories, one with a companion mass greater than 0.01 times the mass of the Sun (see Figure 2, Region A), and the other with a very low companion mass, i.e. less than 0.01 times the mass of the Sun (see Figure 2, Region B). Previous scholars have proposed several models to explain the origin of the Black Widow pulsar, but the origin of the Black Widow pulsar with very low companion mass has been unclear. Researcher Wang Bo et al. proposed a neutron star + helium star model that forms extremely dense X-ray binary stars in 2021. Based on this model, the project team further considered the evaporation process of pulsars by pulsars. Studies have shown that due to the stripping of the companion material by pulsar radiation, a rapid decrease in the mass of the companion star can be explained, which can explain the formation of the very low-mass black widow pulsar (see Figure 2). In addition , the mass of the companion star can drop to one-thousandth of the mass of the Sun, or even one hundredth of a thousandth of the mass of the Sun, within the Hubble time scale. When the mass of the companion in a millisecond pulsar binary is low enough, the companion is tidally disintegrated, leaving an isolated millisecond pulsar. Thus, the work also provides a possible formation channel for isolated millisecond pulsars.

Figure 1. Schematic diagram of the pulsar radiation evaporation companion of a millisecond pulsar

(Image source:

Figure 2.Comparison of observational samples and theoretical simulations, where the dark black sample in region (B) is an extremely low-mass black widow pulsar. The pentagram represents the moment when the companion’s mass decreases to one hundred thousandth of the mass of the Sun

This achievement was supported by the Western Light Cross Team of the Chinese Academy of Sciences and the National Natural Science Foundation of China. (Source: Yunnan Astronomical Observatory, Chinese Academy of Sciences)

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