With the support from the National Natural Science Foundation of China (Grant numbers 12225303 and 11988101), on June 21^st, 2023, Nature published the study from a team led by Peng Jiang and Jinlin Han from the National Astronomical Observatories, Chinese Academy of Sciences, and Bing Zhang from the University of Nevada, Las Vegas - "A Binary Pulsar in a 53-minute Orbit". Using the "Chinese sky-eye", the Five-hundred-meter Aperture Spherical radio Telescope (FAST), the team discovered a pulsar binary (namely PSR J1953+1844 or M71E) with the shortest known orbital period, filling in the missing link in the evolution of spider pulsar systems. The link of the paper is https://www.nature.com/articles/s41586-023-06308-w. Zhichen Pan and Jiguang Lu from the National Astronomical Observatories and Hailiang Chen from the Yunnan Astronomical Observatories are co-first authors of the paper.

Pulsars were discovered in 1967 by British astronomer Jocelyn Bell. Observations have shown that some pulsars are in binary systems. Astronomers predicted that if the two components of the binary pulsar are close, the pulsar will swallow up material from the companion star to spin faster. The evolution process always accompanies with the change of the orbital period. This behavior is similar to the natural behavior of female spiders who eat male spiders to support themselves after mating. So, astronomers named these objects after two types of spiders: redback and black widow. They are collectively known as spider pulsar.

Theory predicts an intermediate state in the evolution from redback to black widow. The orbital period of the pulsar in this state should be very short, and the two components is extremely close, resulting in many challenges during the observation. Thus, the evolution of spider pulsars from redback to black widow has not been fully confirmed. The research team used FAST to detect and monitor this binary pulsar with the shortest orbital period ever found. Based on the Roche lobe size and the mass-radius relation of low-mass stars, the companion mass is estimated to be about 0.07 solar mass. Accordingly, it was identified as a pulsar binary in the intermediate state of the evolution from the redback to black widow (Figure).

This discovery fills in the missing intermediate state of spider pulsar evolution, completes the evidence chain for the accretion evolution theory of pulsar binary systems, and has important significance for the pulsar population theory. In addition, the orbital inclination of the binary system is less than 10 degrees, and the probability to form this system is less than 1.5%. Such a system is extremely rare in observation, and worthy of attention for further researches. In this work, the observing and timing were taken for the binary in extremely compact orbit, and the high-precision timing results show the ultra-high sensitivity of FAST telescope and the scientific attitude of the staff to strive for excellence, proving that the research team has strong scientific research potential and is expected to achieve more scientific results in the future.

Figure: The position of M71E in the companion mass to orbital period diagram. Blue and red curves are for the theoretical evolution routines. M71E is in the inter-state during the evolution.