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

TL;DR

This study probes how non-thermal particles escape a bow-shock pulsar wind nebula by leveraging MeerKAT radio observations of the Lighthouse Nebula. The authors detect a long, structured radio tail and a set of transverse streaks, but find no radio counterpart to the known X-ray misaligned jet, interpreting the streaks as dynamically triggered, charge-independent particle release from the tail. Through spectral analysis and simple emission modeling, they infer a two-population synchrotron scenario in a magnetic field of tens of microgauss and estimate a substantial fraction of the pulsar spin-down power powering leptons, with a pair multiplicity in the range 10^4–10^6. The results imply long-range particle transport with relatively modest magnetic turbulence, informing models of cosmic-ray propagation near accelerators and the origin of extended gamma-ray halos and local positron flux contributions.

Abstract

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. We aim to get a better understanding of how the bulk of non-thermal particles are released into the interstellar medium. We focus 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. We analyze 10.5h of MeerKAT observations in the 0.9-1.7GHz band. MeerKAT observations reveal a highly structured synchrotron nebula downstream of pulsar PSR J1101-6101. A cometary tail is detected up to beyond 5pc from the pulsar, while a system of multiple transverse two-sided emission streaks is observed for the first time. No radio counterpart of the misaligned X-ray jet is seen. 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. Such a confinement may result from self-generated turbulence by resonant streaming instability, or be due to past activity of the progenitor star.

Figures (5)

• Figure 1: Intensity map of the central part of the region covered by our MeerKAT observations. The top panel displays a broad view dominated by a large and saturated source which is SNR MSH 11-61A. The bottom panel is a zoom on the Lighthouse Nebula. The green contours correspond to the X-ray photon flux measured with Chandra/ACIS in the $0.5-7$ keV band.
• Figure 2: Intensity map of a zoomed-in region centred on the Lighthouse Nebula. The position of PSR J1101-6101 is marked as a red star, and the contours correspond to emission at a significance of 3,5,7, and 10$\sigma$. The black dashed line gives the longitudinal axis of the nebula, the green and white dashed lines indicate conspicuous and tentative transverse emission streaks, respectively.
• Figure 3: Chart illustrating the regions used for flux and spectral index extraction. The blue boxes correspond to the north and south parts of the main streaks L1, L2, and L4. The blue dashed lines separate the tail into five regions: pulsar, head, shoulder, hip, and leg (from bottom-right to top-left).
• Figure 4: Radio intensity profiles of the Lighthouse Nebula. The top panel displays the intensity in the longitudinal direction along the tail, for the central axis and along two directions shifted sideways in the northwestern and southeastern directions. The bottom panel displays the intensity along the three main streaks.
• Figure 5: Spectral energy distribution function (SED) computed using different models for the particle injection spectrum: Model1 assumes a single particle population (and a radiation spectral index $\alpha=0.6$); Model2a/b assume two populations (radiation spectral indices $\alpha_L=0.0$ and $\alpha_H=0.6$), with a different position of the intrinsic and synchrotron break ($\gamma_b$ and $\gamma_s$) depending on the nebular magnetic field strength. The nebular magnetic field of each case is reported on the plot, with the same color coding as the curve it refers to. Observational bands are shown as gray shaded regions. Blue segments identify the spectral slopes in the observed windows.