According to the website of the National Astronomical Observatory of the Chinese Academy of Sciences on October 9th, recently, Researcher Han Jinlin from the National Astronomical Observatory, along with researchers from several domestic institutions, combined data from FAST's radio observations with multi-band astronomical data from international optical and gamma-ray telescopes. They revealed that the pulsar J0435+3233 discovered by FAST is the first known primanova triple system still in the evolutionary stage. This is also the second confirmed pulsar triple system. The research findings were published in the Astrophysical Journal Letters on October 9th, 2026, according to Beijing time.
It is reported that the pulsar J0435+3233 was first discovered by the drift scanning team of the National Astronomical Observatory on June 8, 2020, through observations with FAST. Its rotation period is only 3.2 milliseconds. The follow-up observation team from the Xinjiang Astronomical Observatory continued the monitoring using FAST for a total of five years, successfully obtaining 192 measurement results. By analyzing the data from FAST, they found that this pulsar orbits around a white dwarf star, moving along a circular orbit around their common center of mass with an period of 8 days. They also discovered that the period of this pulsar increases by 1 nanosecond each year, which is twice as much as that of similar pulsars. This discovery shocked scientists around the world. The results of this study were published in the journal Nature Astronomy in April 2026.
Researcher Han Jinlin from the National Astronomical Observatory, along with researchers from five other scientific institutions, immediately took over the second phase of exploration. They independently analyzed open archives data from China's FAST radio band, the Gaia satellite, 2MASS, and PanSTARRS in the infrared and optical bands. Combining this with the 160 valid gamma photon data accumulated by the Fermi satellite over 18 years (where only one photon was detected per month, making the signals extremely rare), they accurately identified the optical companion star of the pulsar, confirming that it belonged to a triple system. The second companion star of the pulsar is a solar-like star that is still in the process of evolving. It orbits the pulsar-white dwarf binary system with an eccentricity of 0.6, on a cycle of 73.5 years. It is this star, serving as the triple companion, that caused the abnormal increase in the rotation period of the pulsar, as detected by FAST earlier.
Since the third celestial body in this triple star system is a evolving star, this triple star system is extremely unique. According to theories of stellar evolution, this triple star system originated from three "twin stars" formed within the same cloud. The largest of these stars had a mass about twenty times that of the Sun when it first formed. It has now evolved to its final state and has become a pulsar. The second-largest star, which had an initial mass several times that of the Sun, has also evolved to its final state and now serves as a white dwarf companion to the pulsar. The smallest star, with a mass of only one solar mass, is located far away from the other two stars and has not been affected by their early supernova explosions. It is currently in a stable state of evolution. Over time, this star will eventually die out, and the triple star system will become a long-term stable triple star system consisting of a pulsar and two white dwarfs. Prior to this, the only known triple star system with a pulsar that has been discovered by humans was found by an American team using the GBT survey telescope. This system consisted of a pulsar and two white dwarfs, and all three stars had evolved to their final states. Astronomers can use various observational methods, such as radio, optical, and gamma-ray observations, to deeply study the physical properties and dynamical coupling laws of this newly discovered triple star system, as well as its evolutionary trajectory and entire dynamic process.
Meanwhile, a European team composed of scientists from Germany, the Netherlands, France, and the United Kingdom also independently analyzed similar archival data and reached the same conclusions. Since the Chinese team took into full consideration the gravitational interactions within the triple-star system and various relativistic effects, they determined the geometric configurations of the triple stars' orbits and the orbital parameters of the triple stars with a precision that is one order of magnitude better than that of the European team. In addition, they combined data from FAST radio telescopes with data from Fermi satellites to accurately measure the masses of the celestial bodies in the triple-star system.
This significant discovery is a landmark achievement resulting from the collaborative efforts of multiple research teams from China and abroad, involving the integration of data from various spectral bands. Starting with FAST’s initial detection of the unique pulsar J0435+3233, followed by five years of continuous monitoring by domestic teams to accurately identify the characteristics of the binary system. Then, a team from China and Europe independently utilized top-tier observational data from radio, optical, and gamma rays to cross-verify the data. Ultimately, it was determined that this pulsar belongs to a primordial triple star system still in the process of evolution. This discovery fully demonstrates the spirit of open collaboration and joint efforts in scientific exploration. This primordial triple star system exhibits clear and significant gravitational interactions between the stars. It serves as an excellent natural observational platform for verifying fundamental gravitational theories, such as the strong equivalence principle. It provides a valuable sample with great scientific value, enabling humanity to delve deeper into the complex evolutionary mechanisms of triple star systems and explore astrophysical laws under extreme conditions.
This work has been supported by the national key research project, the natural science foundation project, and the establishment project of the Chinese Academy of Sciences.
