Giant Pulses with Nanosecond Time Resolution detected from the Crab Pulsar at 8.5 and 15.1 GHz

We present a study of shape, spectra and polarization properties of giant pulses (GPs) from the Crab pulsar at the very high frequencies of 8.5 and 15.1 GHz. Studies at 15.1 GHz were performed for the first time. Observations were conducted with the 100-m radio telescope in Effelsberg in Oct-Nov 2007 at the frequencies of 8.5 and 15.1 GHz as part of an extensive campaign of multi-station multi-frequency observations of the Crab pulsar. A selection of the strongest pulses was recorded with a new data acquisition system, based on a fast digital oscilloscope, providing nanosecond time resolution in two polarizations in a bandwidth of about 500 MHz. We analyzed the pulse shapes, polarisation and dynamic spectra of GPs as well as the cross-correlations between their LHC and RHC signals. No events were detected outside main pulse and interpulse windows. GP properties were found to be very different for GPs emitted at longitudes of the main pulse and the interpulse. Cross-correlations of the LHC and RHC signals show regular patterns in the frequency domain for the main pulse, but these are missing for the interpulse GPs. We consider consequences of application of the rotating vector model to explain the apparent smooth variation in the position angle of linear polarization for main pulse GPs. We also introduce a new scenario of GP generation as a direct consequence of the polar cap discharge. We find further evidence for strong nano-shot discharges in the magnetosphere of the Crab pulsar. The repetitive frequency spectrum seen in GPs at the main pulse phase is interpreted as a diffraction pattern of regular structures in the emission region. The interpulse GPs however have a spectrum that resembles that of amplitude modulated noise. Propagation effects may be the cause of the differences.

💡 Research Summary

The authors present the first systematic study of Crab pulsar giant pulses (GPs) at very high radio frequencies, specifically at 8.5 GHz and, for the first time, at 15.1 GHz. Observations were carried out with the 100‑m Effelsberg telescope during October–November 2007 as part of a broader multi‑station campaign. A newly developed data‑acquisition system based on a fast digital oscilloscope recorded the two circular polarisation channels (LHC and RHC) with a bandwidth of roughly 500 MHz and a sampling interval of 0.5 ns, providing true nanosecond time resolution.

From the recorded data the authors selected the strongest pulses and performed a comprehensive analysis of pulse morphology, dynamic spectra, polarisation properties, and cross‑correlations between the LHC and RHC streams. All detected GPs occurred only within the main‑pulse (MP) and interpulse (IP) phase windows; no emission was found elsewhere. The MP GPs displayed a complex, multi‑component structure in the time domain, consisting of sub‑nanosecond “nano‑shots” that together span a few tens of nanoseconds. Their dynamic spectra reveal a strikingly regular pattern of spectral lines spaced by roughly 30–40 MHz. Cross‑correlation of the two polarisation channels shows a clear, periodic frequency‑domain signature for MP GPs, indicating a coherent interference effect.

In contrast, IP GPs are much simpler in the time domain, appearing as single, short bursts. Their spectra are broadband and resemble amplitude‑modulated noise, lacking the regular line structure seen in MP GPs. The LHC–RHC cross‑correlation for IP GPs shows no systematic frequency pattern, suggesting that the emission is either intrinsically incoherent or heavily modified by propagation effects.

The authors interpret the regular spectral modulation of MP GPs as a diffraction pattern produced by quasi‑periodic structures in the emission region. Such a pattern implies that the radiating plasma contains regular spatial separations on the order of a few meters, acting like a diffraction grating for the high‑frequency radio waves. The smooth rotation of the linear polarisation position angle across the MP GP profile is consistent with the rotating‑vector model (RVM), supporting the idea that the emission region moves across the magnetic pole as the pulsar rotates.

To explain the stark differences between MP and IP GPs, the paper proposes an extended polar‑cap discharge scenario. In this picture, the MP emission originates from a relatively ordered cascade of discharge events on the polar cap, producing a series of nano‑shots that interfere constructively and generate the observed spectral comb. The IP emission, however, is thought to arise from a more chaotic discharge environment, possibly located higher in the magnetosphere, where the emitted radiation undergoes strong scattering or mode conversion in the surrounding plasma, thereby erasing any regular spectral imprint.

The comparison between 8.5 GHz and 15.1 GHz data shows that the spectral line spacing widens slightly at the higher frequency, and the degree of linear polarisation decreases while circular polarisation becomes more prominent. These trends are compatible with a shrinking effective size of the coherent emission region at higher frequencies and with increased propagation effects (e.g., birefringence, scattering) in the magnetospheric plasma.

Overall, the study demonstrates that giant pulses retain rich micro‑second and nanosecond structure even at frequencies well above 10 GHz, challenging the long‑standing view that GP activity diminishes sharply at high frequencies. The findings provide compelling evidence for nano‑scale discharge processes in the Crab pulsar magnetosphere and highlight the importance of ultra‑high‑time‑resolution, wide‑band observations for probing the fundamental physics of pulsar radio emission. Future work combining simultaneous multi‑frequency observations, detailed plasma propagation modelling, and higher‑sensitivity instrumentation is likely to refine the proposed models and deepen our understanding of the extreme plasma conditions near neutron star magnetic poles.