Abstract Using two observations from the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we performed a detailed single-pulse analysis of the high-nulling pulsar PSR J1820−0509. We find an exceptionally high nulling fraction of about 81.78 %, significantly exceeding previous results from Parkes observations. The single-pulse energy distribution shows a clear bimodal structure, consistent with classical nulling behavior. However, stacking the identified null pulses reveals a statistically significant residual profile above the noise level, indicating that these nulls correspond to a very weak emission state rather than a complete cessation of radio emission.In addition, PSR J1820−0509 exhibits clustered burst activities lasting several hundred rotation periods, with significant quasi-periodicities at characteristic timescales of 1191 ± 81 and 590 ± 15 pulse periods in the two observations. Based on the temporal clustering and integrated profile morphology, we identify three distinct emission modes (Modes A, B, and C) and a pseudo-null state (Mode D). These modes show systematic differences in pulse morphology, polarization, and energy statistics. The pulse width–energy relations for all detectable modes display significant transitions between low- and high-energy regimes. The energy distributions of Modes A and C are well described by lognormal functions, while Mode B follows a composite distribution of Gaussian and lognormal components. These results suggest that the radio emission is governed by multiple quasi-stable magnetospheric states. The detection of weak emission during pseudo-nulls, together with systematic mode differences, supports the interpretation that pulsar nulling reflects transitions between different magnetospheric activity levels rather than a complete shutdown of emission.
Tu et al. (Sun,) studied this question.