A compact pulsar wind nebula model of the gamma-ray loud binary LS I +61 303

TL;DR

This paper presents a detailed inhomogeneous spectral model of the gamma-ray loud binary LS I +61 303, explaining its broad-band emission from radio to TeV energies through pulsar wind and Be star wind interactions, with implications for variability and absorption.

Contribution

It introduces a compact pulsar wind nebula model incorporating wind clumpiness and magnetic inhomogeneities, providing detailed spectral calculations and constraints from recent Fermi observations.

Findings

The model reproduces the broad-band spectrum from radio to TeV energies.

It constrains the electron Lorentz factor to either 2×10^4 or 10^8 based on Fermi data.

Wind absorption and heating effects significantly influence observed emissions.

Abstract

We study a model of of LS I +61 303 in which its radio to TeV emission is due to interaction of a relativistic wind from a young pulsar with the wind from its companion Be star. We assume the fast polar wind is clumpy, which is typical for radiatively-driven winds. The clumpiness cause the two winds to mix. The relativistic electrons from the pulsar wind are retained in the moving clumps by inhomogeneities of the magnetic field, which explains the X-ray variability observed on time scales much shorter than the orbital period. We calculate detailed inhomogeneous spectral models reproducing the average broad-band spectrum from radio to TeVs. Given the uncertainties the form of the distribution of relativistic electrons, the X-ray spectrum could be dominated by either Compton or synchrotron emission. The recent Fermi observations constrain the high-energy cut-off in the electron distribution to be at the Lorentz factor of 2 10^4 or 10^8 in the former and latter model, respectively. We provide formulae comparing the losses of the relativistic electrons due to Compton, synchrotron and Coulomb processes vs. the distance from the Be star. We calculate the optical depth of the wind to free-free absorption, showing that it will suppress most of the radio emission within the orbit. We point out the importance of Compton and Coulomb heating of the stellar wind within and around the gamma-ray emitting region. Then, we find the most likely mechanism explaining the orbital modulation at TeV energies is anisotropy of emission.