Single-Pulse Study of the Pseudo-nulling Pulsar PSR J1820-0509 Based on FAST Observations

TL;DR

This study uses FAST telescope data to analyze the single-pulse behavior of pulsar PSR J1820-0509, revealing high nulling fraction, multiple emission modes, and evidence of magnetospheric state transitions.

Contribution

It provides the first detailed single-pulse analysis of PSR J1820-0509, identifying distinct emission modes and weak residual emission during nulls, suggesting magnetospheric state changes.

Findings

Nulling fraction approximately 81.78%

Identification of three emission modes and a pseudo-null state

Detection of quasi-periodic burst activities and mode-dependent pulse properties

Abstract

Using two observations obtained with the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we present a detailed single-pulse analysis of the high-nulling pulsar PSR J1820-0509. We measure an exceptionally high nulling fraction of approximately 81.78%, significantly exceeding previous estimates from Parkes observations. The single-pulse energy distribution exhibits 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 the nulls correspond to a very weak emission state rather than a complete cessation of radio emission. The pulsar shows clustered burst activities spanning several hundred rotation periods, with prominent quasi-periodicities at 1191 +/- 81 and 590 +/- 15 pulse periods in the two observations. Based on temporal clustering and integrated profile morphology, we identify three distinct emission modes (A, B, and C) and a pseudo-null state (D). These modes exhibit systematic differences in pulse morphology, polarization, and energy statistics. The pulse width-energy relations reveal clear 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 Gaussian-lognormal distribution. These results suggest that the radio emission of PSR J1820-0509 is governed by multiple quasi-stable magnetospheric states. The presence of weak emission during pseudo-nulls, together with systematic mode-dependent variations, supports the interpretation that pulsar nulling reflects transitions between different magnetospheric activity levels rather than a complete shutdown of emission.