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

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.

Figures (15)

• Figure 1: Single-pulse relative energy (E/<E>) distributions of PSR J1820$-$0509 in the on-pulse and off-pulse windows. The blue curve represents the on-pulse region, while the red curve corresponds to the off-pulse region, with identical phase ranges selected for both. The inset shows a zoomed view of the off-pulse energy distribution. The on-pulse distribution exhibits a bimodal structure, corresponding to null pulses and normal emission, respectively.
• Figure 2: Average profile obtained by stacking all selected null pulses (blue curve). The red dashed line indicates the $3\sigma$ noise level of the off-pulse region. The result shows that the stacked signal is significantly above the noise, demonstrating the widespread presence of weak emission components within the nulls.
• Figure 3: Cumulative signal-to-noise ratio distribution of null pulses. Black points represent the on-pulse region, and blue points represent the off-pulse region. The red dashed line represents three times the RMS of $S/N_{\rm off}$, where the RMS is derived from the fitting curve shown in Figure \ref{['fig:4SN_RMS']}.The S/N sum in the on-pulse region continues to increase with the number of stacked pulses, providing further evidence that systematic weak emission is present within the nulls.
• Figure 4: The figure shows the root mean square (RMS) of $\mathrm{S/N}_{\mathrm{sum}}(k)$ as a function of the number of stacked pulses for different integration scales. Each data point is derived from 200 bootstrap realizations with replacement. The black points represent the on-pulse data, while the blue points correspond to the off-pulse data. The blue dashed line indicates the fitting result for the on-pulse data, and the red dashed line represents the fitting result for the off-pulse data. Both fitted trends decrease with increasing pulse number, indicating that the stacked null-pulse profile becomes progressively more stable as the sample size increases.
• Figure 5: Based on two independent observations, the single-pulse energy properties of PSR J1820$-$0509 are illustrated. The central panels show stacked sequences of consecutive pulses; the upper panels present the integrated pulse profiles constructed from all pulses; and the right panels display the single-pulse energy distributions. It is evident that the single-pulse energies exhibit a multi-peaked distribution rather than a smooth continuous variation.The left set of panels corresponds to the first observation, the middle set to the second observation, and the right panel shows an enlarged view of the region marked by the red solid line in the middle panel (pulse numbers 4000–4750), including bright, weak, and null emission states.This clearly demonstrates the multi-peaked nature of the single-pulse energy distribution, rather than a uniform or continuous variation.