Picture of M71E (the pulsar binary on the right of the figure), FAST (on the bottom of the figure) and the globular cluster M71 (background) [IMAGE: SCIENCEAPE/CAS/NAOC]

The international academic journal Nature published an online Accelerated Article Preview on June 20, 2023, presenting the research results of a group led by scientists from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC). The research team used the Five-hundred-meter Aperture Spherical radio Telescope (FAST) to discover a pulsar binary named PSR J1953+1844 or M71E (Figure 1) with a 53-minute orbital period, filling a gap in our understanding of the evolution of spider pulsar systems.

The first pulsar was discovered in 1967 by English astronomer Jocelyn Bell. She showed it always rotated regularly and quickly, like a spinning top in the sky. The discovery was awarded the Nobel Prize in Physics, and over the more than fifty years since about 3,000 of these fascinating objects have been found.

Figure 1: Left panel: the red arrow indicates the location of M71E, next to the globular cluster M71; Right panel: average pulse profile and polarization position angle of M71E in FAST observations. [IMAGE: Left: Xing Gao; Right: Pan et al. 2023]

Some pulsars locate in binary systems, orbiting with their companion stars. If the two stars are close together, astronomers predicted that the pulsar will swallow up material from the companion star to keep spinning up. At the beginning, the companion star is heavy. As the pulsar eats this star, the two stars get closer and orbit each other with increasing rate. When the star gets lighter after mass loss, the pulsar can’t continue to plunder. Then the pulsar pushes the star away, and the orbiting speed becomes slower. This behavior is similar to that in which female spiders eat male spiders to feed themselves, thus astronomers named the objects in these two stages after two types of spiders: redback and black widow. They are collectively known as spider pulsars.

The evolution from the redback to the black widow takes a long time, up to hundreds of millions of years. Previously, only pulsar binary systems in two states, redback and black widow, had been detected, but no intermediate state between them had been found. The reason is that the orbital period of the intermediate pulsar predicted by this theory is very short and the distance between the two stars is very close, so the observation faces many challenges and the theory of the evolution of spider pulsar systems from redback to black widow has not been fully confirmed.

Now, the possibility of this evolutionary path is confirmed by FAST, the world’s largest and most sensitive radio telescope. The research team used FAST to detect the spider pulsar system in what is the shortest orbit ever discovered. The orbital period of this system was determined to be only 53 minutes after long-term observations. Based on the various indications in the observations, it is determined to be in the intermediate state on the evolution path from the redback to black widow system, filling in the missing link in the spider pulsar evolution theory (Figure 2). At the same time, the orbital of the binary is almost face-on; such a system is extremely rare. FAST found it in the vast sea of stars with extremely high detection capabilities, and its filling the gap in the evolution of spider pulsar systems reflects its unprecedented sensitivity.

Figure 2: The position of M71E on the orbital period-mass diagram of the companion star. Both the red and blue curves represent the paths of the theoretical simulations of binary star evolution. M71E locates at an intermediate state of evolution. [image: Pan et al. 2023]

The reviewers in the journal Nature described the result as a very interesting pulsar binary system. This discovery shortens the record for the shortest orbital period of a pulsar binary system by about 30 percent, and indicates a new and unknown process in the evolution of spider pulsars.

Co-first authors of the paper are Pan Zhichen and Lu Jiguang from the FAST group of NAOC, and Chen Hailiang from the Yunnan Astronomical Observatories. Corresponding authors are Jiang Peng and Han Jinlin, researchers from NAOC, and Zhang Bing, professor at the University of Nevada. This work has collaborators from the Guizhou Radio Astronomical Observatory of Guizhou University, Yunnan Astronomical Observatory, Shanghai Astronomical Observatory, National Time Service Center, Peking University, the University of Chinese Academy of Sciences, the Max Planck Institute in Germany, and the University of Nevada Las Vegas.

For more information, please contact:

Professor Xu Ang

Email: annxu@nao.cas.cn

National Astronomical Observatories,

Chinese Academy of Sciences

Source: National Astronomical Observatories,

Chinese Academy of Sciences