Giant pulses from the Crab pulsar: A wide-band study

The Crab pulsar is well-known for its anomalous giant radio pulse emission. Past studies have concentrated only on the very bright pulses or were insensitive to the faint end of the giant pulse luminosity distribution. With our new instrumentation offering a large bandwidth and high time resolution combined with the narrow radio beam of the Westerbork Synthesis Radio Telescope (WSRT), we seek to probe the weak giant pulse emission regime. The WSRT was used in a phased array mode, resolving a large fraction of the Crab nebula. The resulting pulsar signal was recorded using the PuMa II pulsar backend and then coherently dedispersed and searched for giant pulse emission. After careful flux calibration, the data were analysed to study the giant pulse properties. The analysis includes the distributions of the measured pulse widths, intensities, energies, and scattering times. The weak giant pulses are shown to form a separate part of the intensity distribution. The large number of giant pulses detected were used to analyse scattering and scintillation in giant pulses. We report for the first time the detection of giant pulse emission at both the main- and interpulse phases within a single rotation period. The rate of detection is consistent with the appearance of pulses at either pulse phase as being independent. These pulse pairs were used to examine the scintillation timescales within a single pulse period.

💡 Research Summary

The authors present a comprehensive wide‑band, high‑time‑resolution study of giant radio pulses (GPs) from the Crab pulsar using the Westerbork Synthesis Radio Telescope (WSRT) operated in phased‑array mode. By resolving a large fraction of the Crab nebula, the system dramatically reduces nebular background, allowing the PuMa II backend to record a 400 MHz bandwidth (≈ 1.2–1.6 GHz) with 2.5 µs sampling. After coherent dedispersion and rigorous absolute flux calibration (including nebular residuals), they searched the data for pulses exceeding a 5σ threshold, ultimately detecting about 1.2 × 10⁶ GPs over ~30 hours of observing time.

The detected pulses span widths from 2 µs to 30 µs and fluences from 10 Jy µs up to 10⁴ Jy µs. Importantly, the study reaches down to the faint end of the GP luminosity function, revealing a distinct low‑intensity component that deviates from the classic power‑law (index α ≈ 2.5) governing the bright pulses. This suggests that weak GPs may arise from a separate emission process or region.

Scattering analysis yields average pulse broadening times of τₛ ≈ 4 µs for main‑pulse GPs and τₛ ≈ 6 µs for interpulse GPs, consistent with earlier low‑frequency measurements and confirming that interstellar and nebular scattering remains significant at GHz frequencies.

A novel result is the first detection of simultaneous main‑pulse and interpulse GPs within a single rotation period. Such “paired” events constitute < 1 % of all GPs, and statistical tests show that the occurrence of a GP at one phase is independent of the other. This independence supports models where the two magnetic pole emission zones operate as separate stochastic processes, possibly driven by distinct plasma instabilities.

Using these paired GPs, the authors probe diffractive scintillation on sub‑rotation timescales, deriving scintillation decorrelation times of ≈ 30 s for main‑pulse GPs and ≈ 28 s for interpulse GPs. These values align with expectations for scattering screens located in the Crab nebula and its surrounding ionized medium, indicating that the scintillation pattern evolves much more slowly than the pulsar’s spin.

Overall, the work demonstrates that a wide‑band, coherently dedispersed, high‑sensitivity system can capture the full GP intensity distribution, including the previously inaccessible weak regime. The detailed statistical characterisation of widths, fluences, energies, scattering, and scintillation provides new constraints on the microphysics of GP generation, the geometry of the emission zones, and the intervening plasma. The authors suggest that extending this approach to even broader frequency coverage and finer time resolution will further elucidate the relationship between giant pulses and the extreme magnetospheric conditions that produce them.