Study of Four Young TeV Pulsar Wind Nebulae with a Spectral Evolution Model

TL;DR

This study models the spectral evolution of four young TeV pulsar wind nebulae, revealing that their gamma-ray emission is mainly due to inverse Compton scattering and that their particle injection characteristics are consistent across different objects.

Contribution

It applies a spectral evolution model to multiple PWNe, providing insights into their magnetic fields, particle distributions, and energy injection processes, extending previous work on the Crab Nebula.

Findings

Gamma-ray emission dominated by inverse Compton scattering off infrared photons.

Magnetic energy is a small fraction of total energy injected.

Broken power-law particle injection models fit observed spectra well.

Abstract

We study four young Pulsar Wind Nebulae (PWNe) detected in TeV gamma-rays, G21.5-0.9, G54.1+0.3, Kes 75, and G0.9+0.1, using the spectral evolution model developed and applied to the Crab Nebula in our previous work. We model the evolution of magnetic field and particle distribution function inside a uniformly expanding PWN considering a time-dependent injection from the pulsar and radiative and adiabatic losses. Considering uncertainties in the interstellar radiation field (ISRF) and their distance, we study two cases for each PWN. Because TeV PWNe have a large TeV gamma-rays to X-rays flux ratio, the magnetic energy of the PWNe accounts for only a small fraction of the total energy injected (typically a few x 10^{-3}). The gamma-ray emission is dominated by inverse Compton scattering off the infrared photons of the ISRF. A broken power-law distribution function for the injected particles reproduces the observed spectrum well, except for G0.9+0.1. For G0.9+0.1, we do not need a low energy counterpart because adiabatic losses alone are enough to reproduce the radio observations. High energy power-law indices at injection are similar (2.5 -- 2.6), while low energy power-law indices range from 1.0 to 1.6. The lower limit of the particle injection rate indicates that the pair multiplicity is larger than 10^4. The corresponding upper limit of the bulk Lorentz factor of the pulsar winds is close to the break energy of the broken power-law injection, except for Kes 75.