For some pulsars, radio emission occasionally shuts off during certain rotation periods—a phenomenon known as "pulsar nulling." When this silence recurs in a regular, repeating pattern, it is termed periodic nulling.

A new study on pulsar B0751+32 has used FAST to capture the time-variable nature of its periodic nulling, revealing that the nulling period varies significantly across different observing epochs and even evolves within a single session.

Earlier models suggested that periodic nulling might simply be a geometric effect, with our line of sight passing through gaps between sub-beams. However, the new findings challenge the traditional explanation. It suggests that the pulsar's magnetospheric physics is far more dynamic and complex than previously thought.

Led by Prof. YAN Wenming from the Xinjiang Astronomical Observatory (XAO) of the Chinese Academy of Sciences (CAS), the work has been published in the journal Monthly Notices of the Royal Astronomical Society.

In the study, the researchers used a mixture-model method to obtain a precise nulling fraction of 35.1 ± 0.6 percent. Pulse-energy analysis shows that both emission components remain stable at the onset of a burst state but gradually weaken toward its end, with the trailing component fading most severely.

This indicates that nulling is not a simple on-off switch but rather a gradual decay process, likely driven by the progressive depletion of magnetospheric particles or the slow breakdown of coherent emission conditions.

Previous studies had reported subpulse drifting in B0751+32, but the FAST observations detected no drifting whatsoever. According to the researchers, those earlier "drifting" signatures were probably misinterpretations of periodic nulling arising from insufficient sensitivity in the older data.

This research was supported by the National Key R&D Program of China and the National Natural Science Foundation of China.

A single-pulse stack of 600 successive pulses of PSR B0751+32. (Image by XAO)