Dr. LI Zhenwei and his collaborators, from Yunnan Observatories of the Chinese Academy of Sciences, reported the maximum accreted mass of recycled pulsars in The Astrophysical Journal on November 29.

Millisecond pulsars belong to the fundamental frontier of astronomy, and are one of the targeted sources of Five-hundred-meter Aperture Spherical radio Telescope (FAST). A pulsar is the remnant of a massive star, and a millisecond pulsar is a pulsar with spin period around several millisecond. The study of pulsar masses is of great importance for constraining equations of state of pulsars and understanding the mass gap between Pulsars and stellar-mass black holes. Pulsars in X-ray binaries increase in mass by accreting material from their companions (known as recycling process), and the uncertainties in the accretion process give challenge to study the pulsar mass at birth.

With considering the effect of spin evolution, the researchers investigated the maximum accreted mass of pulsars during the recycling process by modeling the binary evolution of X-ray binaries. They found the propeller effects may occur after the pulsar accretes a small part of material. At this moment, the matter is unable to be accreted onto the pulsar due to the centrifugal force exerted by the magnetosphere. Meanwhile, the centrifugal barrier exerts a propeller spin-down torque on the pulsar in that phase, and the pulsar can still accrete material in the subsequent evolution.

The researchers performed a large number of binary simulations, and found that the maximum accreted mass of pulsars is positively correlated to the initial pulsar mass. In other words, massive pulsars can accrete relatively more material than low-mass pulsars. On this basis, the relations between the maximum pulsar masses after the accretion and the mass transfer efficiencies and initial pulsar masses are given (as shown in Figure 1). The results addressed in this work can be used to estimate the minimum birth masses for the observed pulsars.

Contact:

LI Zhenwei, YNAO, CAS

lizw@ynao.ac.cn

Fig.1.The maximum pulsar masses after the accretion with given mass transfer efficiencies and initial pulsar masses. (Image by LI Zhenwei)