Radio streaks in the Lighthouse Nebula discovered with MeerKAT -- Particles escaping from the tail and illuminating the ambient magnetic field

The gist: MeerKAT observations reveal highly structured radio emission in the Lighthouse Nebula, including new transverse two-sided emission streaks, which are interpreted as energetic leptons escaping from the tail into the ambient magnetic field

Context and Aims

Bow-shock pulsar wind nebulae are valuable sources to investigate the dynamics of relativistic pulsar winds and the mechanisms by which they are converted into cosmic-ray leptons at the highest energies. The Lighthouse Nebula is one such object, famous for the high velocity of its pulsar and a long misaligned X-ray jet that is understood as a specific escape channel for the most energetic particles. The aim of this work is to get a better understanding of how the bulk of non-thermal particles are released into the interstellar medium, focusing on GHz radio observations which probe lower-energy particles that are dominant in number and long-lived, thus offering a picture of how escape proceeds in the long run.

Key Observations from MeerKAT Data

The MeerKAT observations revealed a highly structured synchrotron nebula downstream of pulsar PSR J1101-6101. Specifically, the study detected a cometary tail extending up to beyond 5pc from the pulsar, along with a system of multiple transverse two-sided emission streaks observed for the first time. The analysis identified several distinct streaks, labeled L0 through L5, with corresponding angular positions and lengths measured from the pulsar. The intensity profiles along these main streaks showed very similar intensities on either side of the tail despite their different distances from the pulsar.

Interpretation of Radio Streaks

The radio streaks are interpreted as the occasional charge-independent release of energetic leptons from the tail into the surrounding medium, as a result of dynamical instabilities and reconfiguration in the downstream flow. The intensity layout suggests that most of the particle content of the nebula is discharged into the ambient medium within several parsec. Once escaped, particles light up the ambient magnetic field, which appears to have a coherence length of at least a few parsec. The length and persistence of the streaks indicate a low level of magnetic turbulence, possibly slightly enhanced with respect to average cosmic-ray transport conditions in the Galaxy.

Comparison with X-ray Features

The radio streaks are not counterparts of the misaligned X-ray jet because if they came from the same particles, it would require an energy loss that is too great for radio emission at current timescales. The nature of the particles responsible for these two kinds of structures (X-ray filament and radio streaks) is different. The X-ray feature is characterized by a strong asymmetry connected with the charge-separated escape, whereas the radio streaks appear double sided with almost the same extension of the two parts.

Particle Populations and Magnetic Fields

The observed broadband synchrotron emission requires about 20−40% of the pulsar spin-down power to be converted into relativistic leptons and involves pair multiplicity in the 10 4 − 10 6 range. The magnetic field in the PWN is estimated to be below ∼ 100 µG, consistent with equipartition values derived from both radio and X-ray emission. The required particle injection rate suggests a pulsar pair multiplicity (κ) in the range of 10 4 − 10 6, which is in agreement with theoretical expectations.

Physical Origin of Escape

The most realistic origin for the radio streaks is that of dynamically-induced features, where the tail becomes dynamically unstable at a certain distance from the pulsar. This instability breaks the confinement of particles, allowing them to propagate in the ambient medium. The longitudinal extension of the streaks cannot be interpreted as an effect of synchrotron cooling, as this would require a magnetic field strength that is not realistic. Instead, particle propagation is governed by diffusion, which implies a perturbation of the magnetic field by δB/B ∼ 10-3−10-2 on a scale RL(EMeerKAT).

Conclusion

The analysis confirms that the transverse radio streaks are interpreted as the occasional release of leptons from the tail into the surrounding medium, triggered by dynamical instabilities. The observed intensities along the tail and in the streaks suggest that most of the particle content is discharged in the environment within 2 − 3 pc. Once set free, particles light up the ambient magnetic field, revealing a magnetic field with a coherence length of at least a few parsec. The length and persistence of the streaks imply a degree of magnetic turbulence that is consistent with, or slightly enhanced compared to, the galactic average at the relevant scales. Deep radio observations are encouraged to improve our understanding of nonthermal particle transport in these complex objects.

1. textbf Discovery of charge-independent particle release mechanism in PWNe (Section 4): The improved system can model the occasional charge-independent release of energetic leptons from the tail into the surrounding medium which is interpreted as a result of dynamical instabilities and reconfiguration in the downstream flow. This allows for better simulation of particle escape dynamics beyond simple advection models.

2. textbf Discrimination between X-ray jets and radio filaments (Section 3.1): The system can differentiate between the physical origins by checking if radio emission is a relic trace of the same feature or if it arises from dynamically-induced features, specifically noting that in this scenario, no charge separation of the escaping particles is expected.

3. textbf Multi-population spectral modeling for PWNe (Section 3.2): The AI can fit available data using models that assume the injection of two different particle populations, both described as power-law spectral distributions, allowing it to determine parameters like the nebular magnetic field and the injection break γB and synchrotron break γs.

4. textbf Magnetic field modification prediction (Section 3.3): The improved system can estimate the required magnetic field perturbation by relating longitudinal extension to diffusion: li ≃ √D∆ti, which allows it to predict a perturbation of the magnetic field by δB/B ≃ √cRL(EMeerKAT)/(3D) ∼ 10−3−10−2 on a scale RL(EMeerKAT).

5. textbf Particle injection rate and pulsar pair multiplicity estimation (Section 3.2): The system can compute the pulsar pair multiplicity (κ) required to support the interpretation by comparing theoretical pair production rates with observed fluxes, yielding estimates in the range N ≃ 2×1047−6×1048 particles and a resulting multiplicity of κ ≃ N˙ /N˙GJ ∼ 104 − 106.

Bow-shock pulsar wind nebulae are valuable sources for investigating the dynamics of relativistic pulsar winds and the mechanisms by which they produce cosmic-ray leptons at the highest energies. The Lighthouse Nebula is one such object, famous for the high velocity of its pulsar and its long, misaligned X-ray jet, which may provide a specific escape channel for the most energetic particles. We aim to understand how the bulk of non-thermal particles is released into the ISM. We focus on GHz radio observations, which probe lower-energy particles that are dominant in number and long-lived, thus providing a picture of how escape proceeds over long timescales. We analysed MeerKAT and ASKAP observations at 1.28GHz and 0.94GHz, respectively. MeerKAT observations reveal a highly structured synchrotron nebula downstream of the pulsar PSRJ1101-6101. A cometary tail extends beyond 5pc from the pulsar, while we detect multiple two-sided transverse emission streaks for the first time. We detect no radio counterpart of the misaligned X-ray jet, but the spectral index map shows an apparent asymmetry, with the steepest values near the base of the jet. We interpret the radio streaks as the occasional, charge-independent release of energetic leptons from the tail into the surrounding medium, resulting from dynamical instabilities and reconfiguration in the downstream flow. Most of the particle content of the nebula is discharged into the ambient medium within several pc. Escaped particles light up the ambient magnetic field, which appears to have a coherence length of at least a few pc. The length and persistence of the streaks indicate a low level of magnetic turbulence, possibly slightly enhanced with respect to average cosmic-ray transport conditions in the Galaxy. This may result from self-generated turbulence caused by a resonant streaming instability or from past activity of the progenitor star.