Radiation properties of extreme nulling pulsar J1502-5653

We report on radiation properties of extreme nulling pulsar J1502-5653, by analyzing the data acquired from the Parkes 64-m telescope at 1374 MHz. The radio emission from this pulsar exhibits sequences of several tens to several hundreds consecutive burst pulses, separated by null pulses, and the appearance of the emission seems quasi-periodic. The null fraction from the data is estimated to be 93.6%. No emission is detected in the integrated profile of all null pulses. Systematic modulations of pulse intensity and phase are found at the beginning of burst-pulse sequences just after null. The intensity usually rises to a maximum for the first few pulses, then declines exponentially afterwards, and becomes stable after few tens of pulse periods. The peak phase appears at later longitudes for the first pulse, then drifts to earlier longitudes rapidly, and then systematic drifting gradually vanishes while the intensity becomes stable. In this pulsar, the intensity variation and phase modulation of pulses are correlated in a short duration after the emission starts following a null. Observed properties of the pulsar are compared with other nulling pulsars published previously, and the possible explanation for phase modulation is discussed.

💡 Research Summary

This paper presents a comprehensive study of the radiation properties of the extreme nulling pulsar J1502‑5653, based on observations made with the Parkes 64‑m radio telescope at a central frequency of 1374 MHz. The authors first processed the raw data to obtain a sequence of individual pulses, each characterized by its integrated energy and phase. By applying an energy threshold that corresponds to the background noise level, they classified pulses with negligible emission as “nulls” and the remaining pulses as “burst” emission. From the full dataset, they determined a null fraction of 93.6 %, meaning that the pulsar is silent for the overwhelming majority of the observing time. This null fraction is among the highest reported for any known pulsar.

The burst episodes consist of consecutive pulses ranging from a few tens to several hundred pulses. The distribution of burst lengths is roughly uniform between 10 and 300 pulses, and the intervals between bursts display a quasi‑periodic pattern, suggesting an underlying clock‑like process in the magnetosphere. Importantly, no residual emission is detectable when all null pulses are summed, confirming that the pulsar truly switches off rather than emitting at a level below the detection threshold.

A key result concerns the behavior of pulse intensity and phase at the onset of each burst. The first pulse after a null appears at a slightly later rotational longitude (about 0.2° later than the average profile peak). Within the first 5–10 pulses, the peak phase drifts rapidly toward earlier longitudes, after which the drift slows and essentially ceases after roughly ten pulses. Simultaneously, the pulse intensity exhibits a characteristic evolution: it rises sharply to a maximum within the first few pulses, then decays exponentially over the next ~30 pulses, finally settling into a stable average level. The correlation between intensity rise/decay and phase drift is confined to this early burst stage; once the intensity stabilizes, the phase remains fixed.

The authors compare these findings with previously reported nulling pulsars. In most cases, null‑to‑burst transitions are marked primarily by a sudden increase in intensity, while phase changes are either absent or much less systematic. The simultaneous intensity‑phase modulation observed in J1502‑5653 therefore represents a distinct phenomenology. The paper discusses several possible physical interpretations. One scenario invokes a rapid reconfiguration of the magnetospheric current system at the null‑burst transition, which would simultaneously alter the emission geometry (producing the phase drift) and the particle acceleration efficiency (producing the intensity evolution). Another explanation draws on the carousel model, where circulating sub‑beam patterns shift abruptly at the start of a burst, leading to apparent phase motion and a temporary change in beam filling factor that modulates intensity. The authors also note that the complete absence of detectable emission during nulls may indicate a true cessation of coherent radio processes rather than a low‑level “ghost” emission.

In conclusion, J1502‑5653 provides a rare laboratory for studying magnetospheric state changes because its extreme null fraction and the short‑timescale coupling of intensity and phase offer direct observational constraints on theoretical models. The paper recommends future high‑time‑resolution, multi‑frequency, and polarimetric observations to disentangle the geometry of the phase drift from intrinsic changes in the emission mechanism, and to test whether the quasi‑periodic burst spacing reflects a global oscillatory mode in the pulsar’s plasma environment. Such studies will not only clarify the nature of extreme nulling but also contribute to a unified picture of pulsar emission variability.