STUDY OF FOUR YOUNG TeV PULSAR WIND NEBULAE WITH A SPECTRAL EVOLUTION MODEL

Abstract

We study four young pulsar wind nebulae (PWNe) detected in TeV γ -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 the magnetic field and the 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 γ -ray to X-ray flux ratio, the magnetic energy of the PWNe accounts for only a small fraction of the total energy injected (typically a few ×10 −3 ). The γ -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. The initial rotational energy and the magnetic energy of the pulsars seem anticorrelated, although the statistics are poor.

NuSTARSTUDY OF HARD X-RAY MORPHOLOGY AND SPECTROSCOPY OF PWN G21.5–0.9

“…The errors are 90% confidence level. An X-ray spectral break at ∼12 keV was first suggested by Tanaka & Takahara (2011). They extrapolated the spectral fits from the hard and soft X-ray bands reported by Tsujimoto et al (2011), and noted that a break at ∼12 keV was likely.…”

“…While this could in fact cause a sharp transition between photon indices, it would cause spectral hardening with energy, not the softening seen by NuSTAR. Finally, spectral breaks are often attributed to synchrotron cooling, as proposed by Tanaka & Takahara (2011). However, the break energy of 9 keV would require an unreasonably low magnetic field strength of ∼6 μG, compared to the ∼300 μG derived from equipartition arguments (Camilo et al 2006).…”

“…Specifically, Tanaka & Takahara (2011) presented synchrotron emission from radio to GeV energies by modeling the electron spectrum emanating from the pulsar of G21.5−0.9. They incorporated adiabatic and energy losses in a one-zone model.…”

Numerical simulations of composite supernova remnants for smallσpulsar wind nebulae

“…This is possible since the magnetic field (in the present) model does not influence the dynamics of the PWN. For the simulations, the initial luminosity ranges between L 0 = 10 37 −10 40 erg s −1 , and the spin-down time-scale between τ = 500−5000 yr. Based on observations, e.g., Zhang et al (2008); Fang & Zhang (2010); Tanaka & Takahara (2011) have derived similar L 0 and τ values for a number of PWNe. Core-collapse supernovae (where pulsars are produced as a byproduct) are expected to take place for stars with masses 8 M (e.g.…”

“…Zhang et al 2008;Fang & Zhang 2010;Tanaka & Takahara 2011). Additionally, the radial profiles of the magnetic field and flow velocity can also be incorporated into spatially-dependent transport models that describe the evolution of the non-thermal particle spectrum in the nebula (Vorster & Moraal 2013).…”