Unidentified Gamma-Ray Sources as Ancient Pulsar Wind Nebulae

TL;DR

This paper models the evolution of pulsar wind nebulae (PWNe) over time, showing how their magnetic fields and emission properties change as pulsars spin down, affecting their visibility in gamma-ray and other wavelengths.

Contribution

It introduces a detailed MHD simulation of PWN evolution considering adiabatic and radiative losses, providing new insights into their changing emission characteristics over time.

Findings

Magnetic field strength decreases as t^{-1.3} over time.

Synchrotron flux diminishes while IC gamma-ray flux increases during PWN evolution.

PWN visibility in gamma rays is eventually terminated by losses.

Abstract

In this paper we explore the evolution of a PWN while the pulsar is spinning down. An MHD approach is used to simulate the evolution of a composite remnant. Particular attention is given to the adiabatic loss rate and evolution of the nebular field strength with time. By normalising a two component particle injection spectrum (which can reproduce the radio and X-ray components) at the pulsar wind termination shock to the time dependent spindown power, and keeping track with losses since pulsar/PWN/SNR birth, we show that the average field strength decreases with time as $t^{-1.3}$, so that the synchrotron flux decreases, whereas the IC gamma-ray flux increases, until most of the spindown power has been dumped into the PWN. Eventually adiabatic and IC losses will also terminate the TeV visibility and then eventually the GeV visibility.